Chapter III: THE SOUTHERN ZONE.--Characterized by absence of Cyprinidae and (17)

The egg drawer (E, fig. 2) lies beneath the warm-water tank (T), and
above this is a nursery (N). The egg drawer is ventilated by two
tubular shafts (V), of which only one is represented in the
illustration; the tubes are about 2½ in. in diameter, and each one is
fitted at its upper end, where it opens into the nursery, with a
swing-valve (V´) which turns upon a horizontal axis (A), in its turn
connected, by means of cranks (C) and shafts (S), with the heat
regulating apparatus (R). A space of about 2 in. between the top of
the incubating drawer and the warm-water tank is necessary for the
insertion of this apparatus. The water in the tank (T) is heated by
means of the boiler (B); the tank and boiler are connected by the two
pipes (I) and (O), of which one is the inlet and the other the outlet
channel. The boiler consists of an inner (I´) and an outer (O)
division in communication with each other below. The latter is
cylindrical in form, while the outer wall of the former is cylindrical
and its inner wall conical. The conical wall of the inner boiler is
the surface which is heated by the lamp (L). The arrangement of the
inlet and outlet tubes is important. In the illustration, for the sake
of clearness, they are represented as one above the other. In reality
they lie in the same plane, and the fork (F) of the inlet pipe
similarly lies in the horizontal plane and not vertically as
represented. The inlet pipe not only differs from the outlet pipe in
the possession of a forked end, but it is carried to the farther end
(not shown in the diagram) of the water tank, while the outlet pipe
opens from about the middle of the tank. The inlet pipe is connected
with the inner portion of the boiler and the outlet one with the outer
portion. The result of this adjustment of the parts is that the warmer
water of the inner boiler, being specifically lighter than the cooler
water of the outer boiler, rises up and passes through the inlet pipe
(I) and is discharged into the tank through the two divergent orifices
of the fork (F). Here the water strikes the side wall of the farther
end of the tank and is reflected back along the back and front walls
towards the nearer side. Hence it is again reflected, but in the
opposite direction, and now forms a central current, which is directed
towards the centrally situated orifice of the outlet tube (O). Through
this it passes to the outer boiler, and sinking towards the bottom,
reaches the base of the inner boiler. Here it becomes heated and
lighter and consequently rises to the top, and once more passes
through the inlet pipe to the water tank. The warm water thus travels
round the outer walls of the tank and the cooled water is conducted
away along the middle portion. A more equable distribution of
temperature over the roof of the incubating chamber is thus ensured
than would be the case if the heated water were discharged either into
the centre or at any other single point only of the tank.

To a very large extent, the efficiency of this apparatus depends upon
the approximately perfect performance of the lamp. A good, steadily
burning one should be employed, and only the best oil used; for,
should the wick become fouled the flame cannot freely burn. For this
reason it is better to use gas, whenever obtainable.

The maintenance of an approximately uniform temperature is obtained by
allowing the heated air of the egg-drawer to escape through the two
ventilating shafts (V). The swing-valves of these are opened or closed
by means of the regulator (R). This latter consists of a glass bowl
prolonged into a tube, about 8 in. long and three-eighths of an inch
in diameter. The glass tube swings upon an axis (A) which is situated
as near as possible to the bowl of the regulator. The axis is
connected with a crank (C´) which is disposed so as to act as a lever
upon the vertical shaft (S), which in its turn is connected with the
upper crank (C); this works the axis (A´) of the swing-valves, and so
can open or close the apertures of the ventilating pipes. The bowl of
the regulator is filled with mercury to such an extent that at the
temperature of 100° F., and when the tube is slightly inclined upwards
from the horizontal it just flows slightly into the tube from the
bowl. On the lever-crank (C´) a weight is slung by a sliding
adjustment, and is so placed that when the temperature of the
egg-drawer is 103° it just balances the tube of the regulator when it
is slightly inclined upwards. Should the temperature of the drawer now
rise higher the mercury flows towards the distant end of the tube and,
causing it to fall down, brings about a rotation of the regulator axis
and as a consequence the opening of the ventilating valves. A
transverse stay prevents the limb of the regulator from quite reaching
the horizontal when it falls. As the temperature cools down the
mercury contracts and retraces to the nearer end of the tube and to
the bowl, and consequently results in the upward inclination of the
limb; the valves are thus closed again.

The egg-drawer (E) is specially constructed so as to imitate as nearly
as possible the natural conditions that exist under a sitting hen. The
drawer is of wood and contains a zinc tray (Z) into which cold water
is placed. Fitting into the zinc tray is another zinc compartment, the
floor of which is made of a number of zinc strips (X) transversely
arranged and placed in relation to each other like the limbs of an
inverted V. The limbs are so disposed that those of one series do not
touch the adjacent ones, and in fact a space is left between them.
Thus a number of parallel troughs are formed, each of which opens
below into the moist air chamber of the cold water tray beneath. In
practice these troughs are covered with flannel which is allowed to
dip into the water of the tray. Thus the eggs lie in a series of damp
troughs and their lower surfaces are therefore damper and colder than
their upper ones. This incubator, if carefully worked and the
necessary practical details observed, has the reputation of being an
efficient machine.

Somewhat similar to the Halsted incubator, but differing from it in
the nature of the boiler and in the temperature regulator, is the
Graves incubator, made in Boston, U.S.A. The incubator itself (fig. 3)
consists of an incubating or egg-drawer (E) heated from above by a
warm-water tank (T). Below the egg-drawer is a tank containing cold
water, the vapour of which passes through the perforated floor of the
former and keeps the air of the egg-chamber slightly humid. Above the
warm-water tank is an air chamber (AC) to serve as a non-conducting
medium and to prevent therefore undue loss of heat. Above this is a
nursery or drying chamber (N), closed in, with a movable lid.

The warm-water tank is heated by means of a simple boiler (B) from
which an inlet tube (I) carries heated water to the tank; the tube
traverses the length of the tank and discharges at its farther end
(not shown in the diagram). From the nearer end of the tank an outlet
tube (O) passes out and opens into the boiler at a slightly higher
level than the inlet one. The boiler is heated by an evenly burning
lamp below, of special construction. The rectangular tube through
which the wick passes is bevelled at its outer end, and upon this
bevelled edge a metal flap (F) is allowed to rest more or less
closely, according as the flame is to be smaller or larger
respectively. The wick is, of course, bevelled to correspond to the
form of its tube. The metal flap is raised or depressed by means of
levers connected with the heat-regulator. When it is depressed upon
the wick the flame is lessened; and it becomes proportionately bigger
as the flap is raised more and more.

The heat-regulator consists of a glass tube (T) which runs the whole
width of the incubation chamber and lies in contact with the floor of
the warm-water tank; it is filled with alcohol. Externally to the
incubator this tube is connected with a U-shaped one containing
mercury. The free limb of the U-tube contains a piston (P) which rests
upon the surface of the mercury in that limb. From the piston a piston
rod (PR) passes vertically upwards and is connected with a lever (L)
which operates, through the agency of a second lever (L´) the
movements of the ventilating valve (V) inserted over the orifice of
the ventilating shaft (A) which opens from the roof of the incubator
drawer. The lever (L) is further connected with a spiral spring (S)
which works the metal flap of the lamp already described. The height
of the piston in the U tube can be so adjusted, by varying the
quantity of mercury in the tube, that when the temperature of the
incubation drawer is 103° F., the ventilating valves are closed and
the wick is burning to its full extent. Should the temperature rise,
the alcohol in the glass tube (T) expands and causes the mercury in
the free limb of the U tube to rise. This carries with it the piston,
and this movement brings about the opening of the ventilating valves,
and at the same time, through the agency of the lever (L) and the
spiral spring (S) the metal flap is brought down upon the wick,
cutting off more or less of the flame. Should the temperature then
fall to 103° or lower, the contraction of the alcohol reverses these
movements, the valve closes, and the wick once more burns to its full
extent.

In practice, the boiler and the temperature regulator are duplicated,
there being a set on both sides of the incubator. Any slight
irregularity on the one side may be thus compensated for by the other
side.

Graves's incubator has the reputation of being a good machine.

Among the most recent type of incubators made in England is that of
Charles Hearson. This differs from any of those described in the
simplicity and ingenuity of the heat regulator, and in that the tubes
which traverse the water tank are hot-air flues, carrying the air
heated by the flame and not warm water. Consequently a further
simplification is introduced inasmuch as no boiler is required.

The essential features of this incubator are shown in fig. 4. The
internal parts of the incubator are insulated by a double wall, the
interspace being packed by a non-conducting material, which is not
shown in the figure. The incubation or egg-drawer (E) is heated by the
warm-water tank (T). Beneath the egg-drawer is a zinc tray (Z), so
constructed that in the central part the floor is raised up into a
short cylinder. Around the raised cylinder is a wide trough containing
water and into this dips a canvas cloth which is stretched out over a
perforated zinc support (F). By this means an extended moistened
surface is produced which allows of a rapid evaporation. The floor of
the incubator, which is raised by short feet from the table on which
it stands, is perforated in the central portion by a number of holes,
and which are so situated that they lie beneath the raised cylinder of
the cold-water tray (Z). The incubation-drawer is thus supplied
continuously by a slow current of moistened air because the air in the
upper part of the drawer, i.e. in contact with the floor of the
warm-water tank, is the warmest and lightest. It therefore tends to
diffuse or pass through the narrow slits between the drawer and the
walls of the incubator, and also through the aperture in the front
wall of the egg-drawer, through which a thermometer is laced. To
replace the air thus lost, fresh air passes in through the holes in
the bottom of the incubator, and on its way must pass through the
pores of the damp canvas which dips into the water in the zinc tray
(Z).

The warm-water tank is heated by an inlet (I) and outlet (O) flue
which are, however, continuous. The inlet flue opens out from a
vertical chimney (C), the air in which is heated either by a gas flame
or that of an oil lamp. The outlet or return flue passes back through
the width of the tank and opens independently to the exterior. The
vertical chimney (C) is capped by a lid (L) capable of being raised or
lowered upon its orifice by the lever (L´). When the cap is resting
upon the chimney all the heated air from within the latter passes
through the flues and heats the water in the tank. If the cap is
widely raised, practically all the heated air passes directly upwards
through the chimney and none goes through the flues. If the cap be but
slightly raised, part of the heated air goes through the flues and
part directly escapes through the aperture of the chimney. The
movement of the lever (L´) which raises the cap (L) is determined by
the thermostatic capsule (S), situated within the egg-drawer.

The principle upon which this capsule is designed is that the boiling
point of a liquid depends not only upon temperature but also upon
pressure. A given liquid at ordinary atmospheric pressure will boil at
a certain degree of temperature, which varies for different
substances. But if the pressure be increased the boiling point of the
liquid is raised to a higher degree of temperature. A liquid when it
boils passes into a gaseous condition and in this state will occupy a
very much larger volume--some two or three hundred times--than in the
liquid condition. If, therefore, a hermetically sealed capsule with
flexible sides be filled with some liquid which boils at a given
temperature, the sides of the capsule will distend when the
temperature of the air round the capsule has been raised to the
boiling point of the liquid within it. The distension of this capsule
can be used to raise the lever (L´). The thermostatic capsule is
placed on a fixed cradle (F) and is filled with a mixture of ether and
alcohol, the proportions being such that the boiling point of the
mixed liquid is 100° F. Between the capsule and the lever (L´) is a
vertical rod (V), articulating with the lever as close as possible to
its fulcrum (M). The articulation with the lever is by means of a
screw, so that the necessary nice adjustment between the height of the
rod (V), the thickness of the capsule and the position of rest of the
damper (L) upon the chimney, can be accurately made. The temperature
at which it is desired that the liquid in the capsule shall boil can
be determined by sliding the weight (W) nearer or farther to the
fulcrum of the lever (L´). The farther it is moved outwards, the
greater is the pressure upon the thermostatic capsule and consequently
the higher will be the boiling point of its contained liquid. By means
of the milled-head screw (A), the height of the lever at its outer end
can be so adjusted that when the liquid of the capsule is not boiling
the damper (L) closes the chimney, but that when it does boil the
damper will be raised sufficiently high from it. If the weight is
pushed as far as it will go towards the fulcrum end of the lever, the
temperature of the egg-drawer will never rise more than 100° F.
because at this temperature and under the pressure to which it is then
subjected, the liquid in the capsule boils, and consequently brings
about the raising of the damper. It matters not, therefore, how high
the flame of the gas or lamp be turned, the temperature of the
egg-drawer will not increase, because the extra heat of the enlarged
flame is passing directly outwards through the chimney, and is not
going through the flues in the tank. In order to raise the temperature
within the incubation chamber to 102° or 103°, or any other desired
degree, the weight (W) must be moved outwards along the lever (L´),
about 1 in. for every degree of temperature increase desired. This
thermostatic capsule works admirably, and the incubator will work for
months at a time and requires no adjustment, however much, within the
limits of our climate, the external temperature may vary. The capsule,
like all other thermostats in which the expansible substance is a
liquid, is, however, dependent upon external pressure for the point at
which its contained liquid boils and therefore, for the degree of
temperature prevailing within the incubator drawer. It is therefore
responsive to variations in atmospheric pressure, and as the barometer
may fall 1 or 2 in., this may possibly make a difference of two or
three degrees in the fluctuation of temperature within the egg-drawer.
It is not, of course, often that such large oscillations of the
barometer occur, and as a matter of practical experience, under
ordinary conditions, this incubator will work for months together
without attention with only half a degree variation round the point at
which it was set.

Greenwood's incubator (fig. 5), named the Bedford, resembles Pearson's
in that hot-air flues (F and F´) and not hot-water pipes, traverse the
water tank (T). And the method of regulation of the temperature is
much the same, i.e. a thermostat (V) operating upon a lever which
raises a cap (C) from off the aperture of the main flue (F) and thus
allows all the heat of the flame to pass directly outwards, without
passing through the series of flues (F) which horizontally traverse
the water-tank. Fresh air enters through a wide circular aperture (A)
which surrounds the main flue, and it thus becomes partially warmed
before entering the egg-chamber. The eggs are placed upon a perforated
floor (E) lying over water baths (B). The water tank (T) lies in the
centre of the incubation chamber and is traversed through its central
axis by the main hot-air flue (F). From this, four horizontal flues
pass outwards through the water and open into small vertical flues,
which in their turn communicate with the exterior.

The thermostat (V) consists of a glass tube of peculiar form. This is
closed at the end of its short limb and open at its other extremity on
the long limb. The bent portion of the tube is filled with mercury and
between the mercury column and the closed end is a small quantity of
ether. The thermostat is lodged in a box (G), which forms part of the
lever (L). At one end this lever is pivoted to a fixed arm, and at the
other to the vertical rod which operates the ventilating cap (C). If
the temperature should rise, the ether in the thermostat expands and
pushes the mercury column up along the inclined long limb. This
disturbs the equilibrium of the lever (L), and it descends downwards,
pulling with it the vertical rod, and thus raising the cap over the
main flue. If the temperature falls the reverse series of changes
occur. The temperature at which the cap will be raised can be adjusted
within limits by the position of the weight (W) and by the adjustment
of the degree of inclination of the thermostat.

The Proctor incubator, made at Otley, is apparently, in its main
features, similar to the Greenwood.

Somewhat similar, in certain features, to the Greenwood is the
Winchcombe. Its improved form, in which metal replaces the wood
casing, is named the Gladstone. In it there is a combination of the
hot-air and the water-tank systems of warming the incubation chamber.
The wall of the incubator is double, and the space between the outer
and inner wall is packed with a non-conducting material. The
incubation chamber is heated above by a water-tank (fig. 6 T) which is
traversed by a main vertical flue (F) and four subsidiary horizontal
ones which discharge externally. The main flue, however, in passing up
to enter the water tank traverses the egg-chamber, and therefore
serves to warm it, as in the hot-air type of incubator, by the heat of
the flue itself. Around the lower half of the flue is a water vessel
consisting of two concentric containers (C), holding water. In the
space between these concentric containers, fresh air passes in through
the aperture (A), and before it reaches the egg-chamber it passes
through coarse canvas which dips into the water in the containers, and
is therefore kept permanently moist. The containers are filled from a
water tank (S) outside the incubator. Air passes out from the
egg-chamber through the aperture (O). The temperature is regulated by
a bimetallic thermostat (see below), which operates two levers, that
by their arrangement can raise or depress the cap (D) over the main
flue (F). The temperature at which this occurs will be determined,
within limits, by the position of the adjustable weight (W).

Tomlinson's incubator, designed in 1880, is novel in principle. It
possesses a very large water tank, holding 15 gallons for every
hundred eggs. Through this tank there pass two hot-air horizontal
flues, lying in the same plane. The novelty of the construction lies
in the great volume of water used and in the disposition of the flues
towards the top of the tank. It is said that very little circulation
of water takes place beneath the flues, because warmed water rises
instead of falling. The great body of water below the flues will
therefore only take up heat relatively slowly, and will, on account of
its bulk and its physical properties, but slowly lose it. Should the
flame fall in power, or even go out for ten or twelve hours, it is
claimed that no serious loss of efficiency of the apparatus will
result.

Regulation of the temperature is by means of an air tube, the air in
which expanding bulges out an india-rubber diaphragm and this moves a
lever. The lever operates a valve which allows more or less of the
heated air to escape from the egg-drawer.

(b) _Hot-air Incubators._--W. H. Hillier's Incubator (fig. 7) is
circular in form and is constructed of a double-walled metal case. The
space between the two walls is packed with a non-conducting material.
The incubation or egg-chamber (C) is warmed by a circular heating box
(H), and the air in this is heated by a lamp. The roof of this box
forms part of the floor of the incubation chamber and from it a main
flue (F) and four smaller ones (F´) pass upwards through the roof of
the incubator and discharge to the exterior. Fresh air passes in to
the incubator through two tubular channels (A and A´) on either side
of the heating box and escapes through a hole in the roof, which
serves at the same time as a passage for one of the rods (D) in
connexion with the temperature regulating apparatus.

This apparatus (T) consists of a glass tube of ½ in. bore, and which
is bent into the form of a circle of 5 in. diameter. The tube is
fastened to a wooden disk, which rotates upon a pivot and in so doing
operates a vertical rod (D), which in its turn works the cap (V) which
covers the orifice of the main flue. The tube is partly filled with
mercury and is closed at one end. At this end there is contained some
spirit. As the temperature rises, this expands and pushes the mercury
column farther along the tube. The equilibrium of the position of rest
is thus disturbed, and the wooden disk consequently rotates, carrying
with it the vertical arm, the downward movement of which raises the
cap (V) of the flue. The temperature at which it is desired that this
valve shall uncover the flue, can be adjusted within the necessary
limits by sliding the weight (W) along the horizontal arm and by the
amount of mercury present in the bent tube. The air of the incubation
chamber is rendered sufficiently moist by the evaporation of water in
the vessel (G).

In the Cornell incubator (New York) more personal attention is
required than in other forms, since the ventilation of the egg-chamber
is not wholly automatic but is regulated according to the results of
observation. The great difficulty in ventilation is the proper
combination of fresh air and moisture. The Cornell Incubator Company
has endeavoured to obviate this difficulty by carrying out a series of
observations on the rate at which evaporation occurs in incubating
eggs under natural conditions. The rate of evaporation is measured by
the size of the air-space within the egg-shell at successive days.
This they have ascertained, and with their incubators they furnish a
book of instructions in which diagrams showing the size of the air
space on the 1st, 5th, 10th, 14th and 18th days are given. Examination
of the eggs should therefore be made every two or three days, and the
result compared with the diagrams. The incubator is provided with an
adjustable ventilator and this should be so arranged that evaporation
is neither too great nor too little. The ventilator should never be
wholly closed, and if when closed to its minimum evaporation is still
too great, then water should be placed in the moisture pans. In all
cases lukewarm water should be placed in these on the 18th day and the
ventilating slide opened wide.

It will thus be seen that in this machine there is an attempt to do
away with the addition of water to the incubator drawer during the
greater part of the period of incubation, and to rely upon the aqueous
vapour naturally present in the atmosphere. This attempt is based upon
the fact that water vapour is lighter than air, and will therefore
rise to the top in any enclosed volume of air. If the direction of the
ventilating current is downwards in the incubation chamber, and if it
is slow enough, it is thought that the water vapour will be sifted out
and tend to accumulate to a sufficient extent in the chamber. In the
Cornell incubator consequently the ventilating current passes first
upward through an external heater in order to warm it, whence it is
then deflected downwards into the egg-chamber and diffuses through its
perforated bottom. Then it passes along a space beneath the chamber
into a space in the left-hand wall of the incubator and out to the
exterior through an adjustable and graduated ventilating slide.

These incubators are hot-air machines, and the hot-air chamber is
situated above the egg-drawer and is traversed by several flues
opening out from a main one. The temperature regulating apparatus
appears to be similar to that of Hearson's machine and operates by a
thermostat, which through the agency of levers opens or closes a valve
over the main flue.

The Westmeria incubators (Leighton Buzzard) are of two patterns. One
type is built on the hot-air principle and the other on the hot-water
system. In both forms the heated air from the heating surfaces is
deflected down on the eggs and escapes through the perforated bottom
of the egg-drawer. The inlet air is first warmed by contact with the
main flue. The thermostat is similar to that in the Hillier machine
(fig. 7) and consists of a coil mounted on an axis, round which it can
rotate. The coil is filled with mercury and is closed at one end.
Between this end and the mercury column is a short column of air. By
expansion of the air under a rising temperature, the mercury column is
displaced and brings about a rotation of the disk to which the coiled
tube containing it is attached. This rotation raises the cap over the
main flue.

All the incubators so far described have been constructed with the
idea of obtaining as nearly as possible a uniform temperature. But in
E. S. Renwick's incubator (America) no attempt is made to obtain
uniformity in temperature. On the other hand, it is designed to give a
periodical oscillation from one extreme to the other of a limited
range, about 3°, of temperature. This is accomplished by means of a
thermostatic bar made of plates of brass and vulcanite fastened
together. This is connected with a clockwork and detent arrangement,
which simultaneously opens a valve and actuates the lamp flame. The
temperature falls to the lower limit of its range before the
thermostatic bar is sufficiently bent to set the clockwork arrangement
operating in the reverse direction, by which the valve is closed and
the lamp flame increased. The temperature then rises to the higher
limit, when the bending of the thermostatic bar again releases the
detent and the clockwork opens the valve and reduces the flame.

The incubator is said to succeed well. It also possesses a mechanical
arrangement by which all the eggs can be periodically turned on
rollers at once.

_Size._--The incubators which have been described are of relatively
small size, and the numbers of eggs which they can incubate are
strictly limited. For commercial purposes, however, operations of a
much larger magnitude are desirable and necessary. And there can be no
doubt that for these purposes the incubators of the future will be of
great size and will contain from 15,000 to 30,000 eggs or more at a
time. Already, at Aratoma Farm, Stamford, New York State, there is
established a large incubation room, containing several thousands of
eggs, and in which the heat regulation is controlled in part by the
personal efforts of attendants. It constitutes almost a complete
return, with added accessories, to the methods of the Egyptians, and
to those of John Champion.

_Bacteriological Incubators._

These differ from bird Incubators in that the heating surface of the incubation chamber generally surrounds all sides of it and there is, as a rule, no special arrangement for bringing about a more or less humid condition of the contained air. In some forms there is an arrangement to ensure a continuous supply of fresh and moist air, but in the majority the incubation chamber obtains its supply of fresh air vicariously. In some forms the chamber of the incubator is heated by a warm water tank of a simple kind, which extends round all its sides. But in other forms a series of tubes or flues passes through the water in this tank and thus simulates in principle the tube boiler. This latter form utilizes the heat of the flame to a greater degree than the former kind. In yet other forms the incubation chamber is heated by warm air chambers which surround it or flues which traverse it. Most bacteriological incubators are square or rectangular in form, but some bacteriologists prefer cylindrical forms, presumably on account of the ratio of volume to surface in connexion with the water tank.

One of the best known and most generally used of the cylindrical and
water-tank kind is that of Dr d'Arsonval. It consists of two copper
cylinders (fig. 8 C and C´), each terminating in a cone below. Between
the cylinders is a wide interspace, in order that a large volume of
water may be contained. This interspace therefore constitutes the
water-tank of the incubator. The upper orifice of the inner cylinder
is closed by a movable double lid, which contains an interspace filled
with water. The outer cylinder has an oblique form at its upper end
and is permanently closed. The result attained by this slope of the
lid of the outer cylinder is that the water tank, which is fed from
the highest point, becomes completely filled. The aperture at the
highest point of the outer cylinder is plugged with a caoutchouc plug
and through a perforation in this a glass tube (T) is placed. In the
side of the outer cylinder below this, there is a wide and rimmed
aperture, to which a gas regulator of special construction is fixed.

This regulator was designed by Théophile Schloesing, and consists of a
brass box, supplied with a rim (L) which fits on to the corresponding
rim (L´) on the aperture of the incubator. Stretching across the
orifice thus connecting the brass box of the regulator with the
water-tank of the incubator is a thin india-rubber diaphragm (D). At
its outer end a perforated cap (R) screws on to the brass box. Through
the perforation the inlet gas tube passes (I); the outlet gas tube (O)
leaves the brass box below and passes direct to the gas burners. The
inlet gas tube is fitted at its inner end with a sliding flanged
collar (F), which is kept pressed against the rubber diaphragm by a
spiral spring. Just behind the collar the inlet tube is perforated by
a small hole, so that the gas supply is never wholly cut off, even
though the rubber diaphragm completely occludes the inner aperture of
the pipe.

The mode of working of the regulator is as follows: when the water
tank of the incubator is filled with distilled or rain water at the
temperature required, it presses upon the india-rubber diaphragm with
a certain degree of pressure. By screwing the inlet pipe in or out, as
required, it can be so adjusted that the diaphragm does not occlude
its inner aperture, and consequently the full volume of gas can pass
through to the burners below. The temperature of the water in the
water-tank therefore begins to rise, and in consequence the volume of
the water to increase. This results in the water rising up into the
tube (T), and therefore the dynamical pressure which is exercised by
the water upon every part of the two cylinders of the incubator and
consequently also upon the india-rubber diaphragm of the regulator is
increased. As this pressure increases, the diaphragm becomes bulged
outwardly and reduces the volume of gas passing through the aperture
of the inlet pipe. At a certain point, of course, the diaphragm
completely occludes the aperture, and the gas supply is wholly cut
off, except for the very small hole, forming a by-pass, in the pipe,
behind the collar. This hole is just sufficiently big to allow the
minimum amount of gas requisite to keep the flames burning to pass
through. The temperature will, therefore, begin to fall, the volume of
water to decrease with its resulting descent from the glass tube (T)
and consequent decrease in the dynamical pressure of the water upon
the diaphragm. The latter therefore retracts away from the aperture of
the inlet tube, and more gas consequently passes through; the flames
again increase in size and the temperature rises once more. And as
soon as the volume of water, owing to the rising temperature, has
increased to the extent correlated with the temperature at which the
apparatus has been set to work, it will have risen once more in the
tube (T), and the gas will be again cut off. The three burners are
placed upon a support that can be moved vertically up or down along
one of the legs of the incubator. The flames are protected from
draughts by mica chimneys. Ventilation is provided by an adjustable
valve (V´) in the cylindrical termination of the incubator at its
lower end, and by tubular orifices, also fitted with valves (V) in the
lid above.

The incubator is very reliable and may be worked within very narrow
limits of variation, provided that the gas-supply be regulated by a
gas-pressure regulator, that the height of the water in the tube (T)
is maintained by daily additions of a few drops of distilled water,
and that the incubator itself be protected from draughts.

Another form of d'Arsonval incubator has a glass door in the side of
it and a slightly modified form of the heat regulator.

Other cylindrical forms of incubators are made by Lequeux of Paris. In
one of these the heat regulator is a bimetallic thermostat, the
movements of which are enlarged by a simple series of levers, so that
a valve can be automatically adjusted to allow more or less heat from
the flame to pass through the heating flue.

In another form there is a movable interior, and an arrangement for
keeping the air in the incubation chamber saturated. It is governed by
a bimetallic thermostat of the Roux type.

In Dr Hüppe's improved form of his incubator, which is approximately
square in form, the double-walled water tank is completely surrounded
externally by an air chamber, which is heated by the passage through
it of the products of combustion of the two flames. The heated gases
escape through an adjustable aperture at the top. In the earlier form
the water tank was traversed by a number of hot-air flues, and there
was consequently no external hot-air chamber. There is an arrangement
of tubes for ventilation, which allow fresh air to enter the lower
part of the incubation chamber and to leave it at the top. The
incoming air is warmed before it enters. The walls are made of
lead-coated steel, and externally the incubator is covered with
linoleum. In the more expensive forms the inner chamber is of copper.
The temperature may be controlled by any of the simpler mercury
thermostats described below.

Dr Babes' incubator is somewhat similar, but the water tank is not
surrounded by a hot-air chamber. Instead it is traversed by a number
of vertical flues through which the heated gases from the flames pass.
Ventilation is provided for and there is an apparatus for controlling
the humidity of the air in the incubation chamber. As in Hüppe's
incubator, the bottom is conical in form. The walls of the incubator
are of lead-coated steel, and externally they are covered with
linoleum; there are two doors, an inner one of glass and an outer one
of metal. The temperature may be controlled as in Hüppe's incubator.

Hearson has designed several forms of bacteriological (biological)
incubators, made by Chas. Hearson & Co., Ltd. Some are heated by a
petroleum lamp and others by a gas flame. In the form heated by a
lamp, for which, however, gas can be substituted, the incubation
chamber is surrounded by a water tank (fig. 9, A) and the lowest part
of this is traversed by an in-going (L) and an out-going flue. The
mode of regulation of the temperature is by means of a thermostat
which operates the movements of a cap (F) over the main flue (V), and
it is identical in its chief features with the method employed in the
chicken incubator. The thermostat (S) is situated in the upper part of
the incubation chamber.

In the other form (fig. 10) for which gas is used exclusively, there
are no flues traversing the water tank. This latter is heated from its
conical floor by a burner beneath the incubator. The heat regulation
is controlled by a thermostat of the same nature as in the form of
incubator just described, but instead of operating by lowering or
raising a cap over a main flue, so as to direct the heated gases
either through the water tank if the temperature is falling, or
through the main flue directly to the exterior if it is rising, it
actuates a gas-governor, so that the flame itself is increased or
diminished in size according to the needs of the incubator. The
gas-governor (fig. 11) is fixed to the roof of the incubator. The
horizontal arm (D) is the same that raises the cap (fig. 9, F) over
the flue in the other form of incubator, but in this case it simply
acts as the bearer of the sliding weight. Beyond its fulcrum (fig. 11,
G) it is continued into a detent-like spur (B) which pushes down upon
a button attached to a rubber diaphragm, when the thermostat within
the incubator is expanded by a rise in temperature. The button thus
forced down, more or less completely closes the inlet gas aperture,
and so reduces or cuts off the gas supply to the flame. There is a
by-pass to prevent the flame from going out completely, and the size
of this can be adjusted by the screw (S). Hearson's incubators have
the reputation of very accurate performance and practically need no
attention for months, or even years.

Schribaux's incubator is a hot-air form. Its walls are of metal, but
it is cased externally with wood, which serves as the insulating
material. Against the inner metal wall of the incubator, and upon its
internal surface, there are disposed a number of vertical tubes, which
open through the roof above into a common discharging funnel. Below,
at the bottom of the incubator they receive the heated gases of
several burners, which as they pass through them radiate their heat
evenly throughout the incubation chamber. In each side wall, at the
bottom of the chamber, is an adjustable ventilating valve.

Inside the incubation chamber, and situated against its left-hand
wall, is a U-shaped bimetallic thermostat of the Roux design,
described below. This very accurately controls the temperature of the
incubator.

(c) _Cool Incubators._--In bacteriological laboratories there are two
standards of temperature, one chiefly for the culture of
non-pathogenic organisms and the other for the pathogenic forms. The
first standard of temperature lies between 18° and 20° C., and the
second between 35° and 38° C. But in hot countries, and even in
temperate regions during the summer, the external temperature is much
higher than the former of these two standards, with the result that
many cultures, especially the gelatine ones, are spoiled. The
difficulty is often partially overcome by running cold water through
the incubator.

Hearson, however, has constructed a "cool biological incubator," in
which by an ingenious device the expansion or contraction of the
thermostatic capsule deflects a horizontal pipe (C) (fig. 12), through
which cold water from an ordinary tap is kept running, in one of two
directions. If it is deflected so as to open into the tube (D), the
cold water passes into the tank (F), where it is warmed by a gas
flame, and thence it passes into the water-jacket of the incubator. If
it is deflected so as to open into the pipe (E), it then runs through
the ice tank (B), containing broken ice, before passing through the
water-jacket of the incubator. If it poured into neither of these
pipes it then simply passes out through the pipe (H) to the waste pipe
(N). By this device the temperature of the incubator can be kept
constant at any desired point, even though it may be some 30° to 40°
C. below that of the external air.

Dr Roux has also designed an incubator which can be maintained at a
constant temperature below that of the surrounding air. This also
depends upon the principle of carrying water through an ice-safe,
which then traverses a pipe within the incubator chamber before
passing into the water-jacket of the machine. The heat-regulating
apparatus is a bimetallic thermostat. The incubator is made by Lequeux
of Paris.

The most recent forms of all kinds of incubators, made by Hearson of
London, Lequeux of Paris and Lautenschläger of Berlin are both heated
and regulated by electricity. The heating is accomplished by electric
radiators.

In Hearson's machines the regulation of the temperature is brought
about by the breaking or making of the electric current, through the
lifting or depression of a platinum contact, actuated by the expansion
or contraction of the thermostatic capsule.

In Roux's apparatus, made by Lequeux, the make and break is attained
by the movement of one limb of a bimetallic thermostat, and in some
forms a resistance coil and rheostat are placed in the circuit.

At the Pasteur Institute in Paris, and at other large laboratories in
France, the bacteriological incubator is raised to the dimensions of a
room. In the centre of this room is a large boiler heated by
gas-burners, the fumes from which pass through a large flue to the
outside. The flame of the burners is regulated by a bimetallic
thermostat. The gas by-pass can be regulated by an attendant. The
cultures are contained in vessels placed on shelves, which are ranged
round the side of the room.

_Human Incubators._

The first incubator designed for rearing children who are too weak to survive under normal conditions, or who are prematurely born, is that of Dr Tarnier. It was constructed in 1880 and was first used at the Paris Maternity Hospital. Its form is that of a rectangular box measuring 65 × 30 × 50 centimetres (fig. 13). It is divided into an upper and lower chamber; the former contains the infant, while the latter serves as a heating chamber, and in reality is simply a modified water-tank. The partition (P) which divides the incubator into two chambers does not extend the whole length of it, so that the upper and lower chambers are at one end of the apparatus in communication with each other. It is through this passage that the heated air from the lower chamber passes into the upper one containing the infant. The narrow bottom chamber C serves to prevent loss of heat from the base of the water-bottles. The outside air is admitted into the lower chamber at the opposite end, through an aperture (A), and passing over a series of bottles (B) containing warm water, becomes heated. The air is rendered adequately moist by means of a wetted sponge (S) which is placed at the entrance of the lower chamber into the upper. The warmed and moistened air is determined in its direction by the position of the outlet aperture (O), which is situated above and just behind the head of the infant. It contains a helix valve (H) and the rotation of this is an indication that the air is circulating within the incubator.

The child is kept under observation by means of a sliding glass door (G) situated in the upper or roof wall of the incubator. Immediately beneath this, and attached to one of the side walls, is a thermometer (T) which records the temperature of the air in the infant-chamber. The temperature should be maintained at 31° to 32° C. The precise limit of temperature must of course be determined by the condition of the child; the smaller and weaker it is, the higher the temperature must be.

The warm water vessels contain three-quarters of a pint of water and four of them are sufficient to maintain the required temperature, provided that the external air does not fall below 16° C. The vessels are withdrawn and replaced through an entrance to the lower chamber, and which can be opened or closed by a sliding door (D).

The walls of the incubator, with the exception of the glass sliding door, are made of wood 25 millimetres thick.

The apparatus appears to have been successful, if by success is understood the indiscriminate saving of life apart from all other considerations, since the mortality of infants under 2000 grammes has been reduced by about 30%, and about 45% of children who are prematurely born are saved.

Dr Tarnier's apparatus requires constant attention, and the water in the warm water vessels needs renewing sufficiently often. It is not provided with a temperature regulator and consequently fluctuations of internal temperature, due to external thermal variations, are liable to occur.

In Hearson's Thermostatic Nurse these drawbacks are to a large extent obviated. This "Nurse" consists fundamentally of an application of the arrangements for heating and moistening the air and for regulating the temperature of Hearson's chick incubator to Dr Tarnier's human incubator. As in this latter form, there are two chambers (fig. 14), an upper (A) and a lower (B), connected with each other in the same way as in Tarnier's apparatus. The upper chamber contains the infant, but the lower is not a heating but a moistening chamber. Through apertures (M) in the bottom of the lower chamber, the external air passes through, and as in the chick incubator it then passes through perforations in the inner cylinder of a water tray (O) and thence over the surface of the water in the tray, through a sheet of wet canvas, to the chamber itself. Hence it passes to the infant chamber and ultimately leaves this through a series of perforations round the top. The air in both chambers is heated by a warm-water tank. This tank forms the partition which divides the incubator into upper and lower chambers and is made of metal. Through the water contained in it, an incoming (R) and an outgoing (R) to the left flue, continuous with each other, pass. These two flues are related to each other as in the chick incubator (see above) and the inlet flue is heated in the same way and the outlet flue discharges similarly. The heat-regulating apparatus is identical with that in the chick incubator, and the thermostatic capsule (S) is placed in the upper chamber, near the head of the infant.

The child is placed in a basket which has perforated walls, and is open above. The basket rests upon two shallow supports (D) situated on the upper surface of the water-tank partition. The child is kept under observation through a glass door in the upper or roof-wall of the incubator.

In Great Britain this apparatus is in use at various hospitals and workhouses throughout the country, and provided there is no great fluctuation of barometric pressure, it maintains a uniform temperature.

_Thermo-Regulators or Thermostats._

Certain special forms of thermo-regulators, adapted to the requirements of the particular incubators to which they are attached, have already been described. It remains now to describe other forms which are of more general application. Only those kinds will be described which are applicable to incubators. The special forms used for investigations in physical-chemistry are not described. There are various types of thermo-regulators, all of which fall into one of two classes. Either they act through the expansion of a solid, or through that of a liquid. They are so adjusted, that, at a certain temperature, the expansion of the material chosen causes the gas supply to be nearly completely cut off. The gas flame is prevented from being wholly extinguished by means of a small by-pass.

We will first describe those which act through the expansion of a
liquid. A very efficient and cheap form is that described by F. J. M.
Page in the _Journal of the Chemical Society_ for 1876. The regulator
consists of a glass bulb (fig. 15 B), continuous above with a tubular
limb (L). At the upper part of the limb is a lateral tubular arm (A)
which bends downwards and constitutes the outlet pipe. At the upper
extremity of the limb there is a short and much wider tube (T), the
lower end of which slides upwards or downwards along it. The upper end
of this wider tube is closed by a cork and through a perforation in
this a very small glass tube (G) passes downwards into the limb of the
regulator to a point a short distance below the exit of the outlet
tube. The exact height of the lower aperture of the small tube can be
varied by sliding the wider tube up or down along the limb. The
by-pass (P) consists of a transverse connexion between the inlet and
outlet gas pipes, and the amount of gas which travels through the
short circuit thus formed is regulated by means of a stopcock. The
by-pass, however, can be formed, as suggested by Schäfer (_Practical
Histology_, 1877, p. 80), by making an extremely small hole in the
small inlet tube, a little way above its lower extremity. But unless
this hole be small enough, too much gas will be allowed to pass, and a
sufficiently low temperature therefore unattainable. The regulator is
filled with mercury until the top of the column reaches within ½ in.
of the exit of the outlet tube, the bulb is placed in the incubator
chamber, and gas is allowed to pass through it. By pushing down the
inner inlet tube (G) until its aperture is immersed beneath the
mercury, the gas supply is cut off, with the exception of that passing
through the by-pass. The stopcock is now turned until only the
smallest flame exists. The inlet pipe is then raised again above the
mercury, and the flame consequently increases in size. The temperature
of the incubator gradually rises, and when the desired degree is
reached, the inlet tube is pushed down until the end is just beneath
the surface of the mercury. The gas supply is thus cut off at the
desired temperature. If the temperature of the incubator falls, the
mercury contracts, the aperture of the inlet tube is uncovered, the
gas supply is renewed and the flame increased. The temperature will
then rise until the required point is reached, when the gas supply
will again be cut off. A uniform temperature which oscillates within a
range of half a degree is thus attained.