Chapter IV (2)

215.--=Subtense or Stadia Webs.=--It is very advisable in all levels to have two extra webs, or lines cut on glass, placed one on each side of the central horizontal web or line, fixed at such a distance apart that the image of 10 feet of the staff when placed at 10 chains distance may exactly cut the inner space between the lines. These webs or lines may be used as a means of measuring distances often more exactly than can be performed with the chain if the surface of the land is irregular; or, in any case, they form a good check upon chain measurement. If the webs or lines are separated so as to subtend an arc whose chord is 10 feet at 10 chains, it is easily seen that 1 foot of the staff will represent this chord at 1 chain, and that each ·01 of the foot on the staff will represent 1 link in distance. A diaphragm webbed or lined in the manner described is shown in Fig. 62. There is some difficulty in placing webs in exact position, and allowance should be made for the optical conditions by the addition of a plus factor. This important subject will be fully discussed hereafter in Chapter XII.

216.--=Tripods, or Stands.=--This matter was deferred when describing the Y-level. The same form of tripod is used for both Y-level and dumpy. In this country the tripod is generally made of straight-grained, well-seasoned Honduras mahogany, which stands better than any other wood. When the tripod is folded up for carrying or for putting by it forms a cylindrical pole which is bellied out at about one-third its length from the top, and diminishes downwards and upwards from this point. For a 14-inch Y-level or dumpy the dimensions of the tripod are about 3½ inches at its greatest diameter when closed, tapering off to 2½ inches at both the top and the bottom ends. For a 12-inch level the section is somewhat less. Each leg of the tripod takes an equal section of the cylinder, the inner angle meeting in the axis being at an angle of 120°, as shown in section Fig. 65. _Shovel-pieces_ are shown in Fig. 59 _AA′_ (p. 110), attached to the top of each leg by four screws passing from the brass to the wood. There should be also two screws from a brass plate inside the leg to the shovel-piece, making connection brass to brass: this is important, as fixings from the brass to the wood only become loose and shaky by shrinkage. The shovel-piece is formed into a strong tenon at its upper end, through which a bolt passes connecting the _book-pieces_ together. The book-pieces are plates cut to an angle of 120°, so as to fall true on the tenons of the shovel-pieces. Where hand-work is used for making the tripod head, the book-pieces are attached by three screws; where machinery is used, the head is made in the shaping machine out of a solid casting, which is much better. The tripod head carries a screw about 1½ inches diameter with coarse thread, which fits into a socket on the lower parallel plate of the level, whether Y or dumpy. There should always be a plain piece, technically a _lead_, above the screw. This holds the instrument steady before it is screwed down, and also leads the screw directly to its corresponding thread, thus saving risk of crossing the thread. A common defect in tripod heads is the thinness of the tenon, so that the leg, if twisted, is felt to be rickety. This tenon is better made wide, as shown in the staff head in Fig. 70A, _seq_.

217.--There is a little difference of opinion as to the form of the woodwork of the tripod for 14-inch levels, some preferring an open framed stand in place of the solid form shown in section Fig. 65. These open framed stands are not so compact to carry, and, as the author thinks, unnecessary for levels of 12 inches and under where the tripod head is solidly made. They are well adapted for larger levels and for theodolites, therefore the description of a framed tripod will be deferred to the discussion of these instruments further on.

A few engineers prefer yellow pine for the tripods instead of mahogany: this is much lighter for its relative stiffness, but it is rather soft for the fixing to the shovel-pieces, and therefore scarcely so reliable as mahogany for durability. Where lightness is important the author employs cedar, which is as light as pine but harder.

218.--The lower points of the legs, technically _toes_, are pointed to an angle of about 60°, and are shod on the insides with steel plates to bite the surface upon which the tripod stands when the legs are extended for use. Two brass rings slip over and bind the legs together when the tripod is out of use.

219.--Many years ago the author introduced the plan of having one of the legs to turn up at about 1 foot distance from the toe. This is shown Fig. 63 at _A_, and in detail section Fig. 64. The joint is made perfectly firm by a winged screw at _S_, which screws from a boss cast on the hinge _J_ to a solid metal shoe _P_. When the leg is turned up, the screw fixes it in the female screw _S_. This plan is very convenient for use in mountainous districts, as it enables the level to be set up fairly well without an uncomfortable angle to any of the legs, or risk of the instrument toppling over. This plan is now nearly superseded by a ball joint as a part of the setting-up adjustment.

The tripod head shown under the level of Fig. 59 is by no means the best, but it is the easiest made therefore, it is the general trade form in use, both for the level and theodolite. Some very superior forms will be discussed further on in description of the instruments to which they are attached.

220.--=The adjustments of the Dumpy Level.=--As this instrument does not possess the means of revolving the telescope upon its axis as with the Y-level, the adjustments are somewhat more complicated, and are performed in an entirely different manner when they are to be made by the civil engineer. The differences are not so great in the hands of the optician, as he generally possesses a movable pair of Y's upon which he can adjust the telescope conveniently for collimation within his own works, by supporting the telescope tube in Y's at a position exterior to the bands which surround it. The tools for this adjustment the author has occasionally supplied upon demand with the dumpy level. But what is necessary here will be to give the mode of adjustment which the civil engineer can accomplish at any time without supplementary apparatus.

The bubble is handier to work with when adjusted to reverse in the centre of its run, but it does not really matter, as equally accurate work can be done with it in any other position. Should the bubble not reverse in the centre of its run, adjust the instrument by the levelling screws until it reverses in some position. Say you start with bubble in the centre, and on reversing, it runs towards the eye end of the telescope six divisions, then alter the levelling screws until it is only half this, or three divisions towards the eye end, then, if properly levelled, the telescope will make an entire revolution with the bubble in that position, which will prove that the axis is vertical. The bubble can now be adjusted by the opposing nuts at the one end by means of the tommy pin (provided in the case) until it is in the centre of its run, and it will then reverse in that position instead of three divisions towards the eye end.

221.--_Adjustment to Collimation._--Upon a fairly level piece of ground the staff plate, fully described further on, is trodden well down on the ground, and the level is set up at say 3 chains from this, in which position the staff is read as a back sight. Now in the opposite direction in the same line, at 3 chains distance from the level, a second staff plate, or in defect of this if the surface be not firm, a stake or a boulder, is driven firmly down in the earth, and the staff is placed upon this erect and face to the instrument as a foresight. The instrument is turned half round and the second station is read. These readings of the staves taken will be truly level with each other, if the axis of the instrument has been set up quite vertically, so that the bubble has kept its centre in all positions. This is true although the axis may have been out of collimation. This arrangement is shown in Fig. 66, _L_ the first position of the level taking sights at equal distance from _S_ and _S′_. Let the level be now removed to _L′_: if correct it should cut the staves _SS′_ at equal distances above or below the first readings at _aa′_, which are at equal distances from _bb′_ readings from _L′_, therefore level and parallel with the first reading.

222.--In the dumpy level, as it leaves the hands of any respectable maker, the subsequent adjustments required can never be great, unless the level has suffered a serious fall so as to bend the limb. The rewebbing the stop, if carefully done, would require only a slight readjustment; but it may be convenient to give an exact method for extreme cases, which may be given in detail for clearness, and at the same time we may also consider the influence of the curvature of the earth.

223.--_Original Adjustment of the Dumpy Level to Collimation with consideration of the Curvature of the Earth._--Suppose the readings of the two levelling staves at 10 chains apart, taken with the level placed at intermediate distance as before, read 7·50 and 4·50, and that we now place the level linearly at 1 chain outside the first reading and it reads the near staff 6·50 and the distant staff 5·50, by the inclination of the ground, this would be a + and a - reading; but we require both readings of one sign, and as the distant staff reading is much too high, it is clear we require - readings for correction. The correction will be of the difference of reading in proportion to the distances, calling the lower reading minus--

7·50 - 6·50 = -1, 4·50 + 5·50 = +1, difference = 2.

That is -2′, as our readings are - and as the -2′ is in 10 chains, at 1 chain the distance of - the near staff = -·2, and 11 chains the distant staff = -2·2. The correction will therefore be for the near staff 1 chain distant 6·50 - ·2 = 6·30, and for the distant staff at 11 chains 5·50 - 2·2 = 3·30 = -1·2 below each of the first readings. If the telescope be now collimated to the near staff reading 6·30, by adjusting the screws immediately under it for distance between the limb and the telescope, and the bubble be readjusted to the telescope without moving the instrument or touching the parallel plate screws, the adjustment will be perfect, less the small error due to the earth's curvature in 1 chain. If the telescope be adjusted to the distant staff 3·30, curvature of the earth will be corrected by the level for 11 chains, which is 0·0106 foot or ·01 nearly, the smallest reading we have on the staff.

224.--It was claimed by the late William Gravatt for his method of adjustment,[3] which was equivalent to that given above, but more complicated and with three staves, that the fixed correction for curvature at 10 chains would be uniform in the working of the level _pro ratâ_ for all distances. There is some difference of opinion on this subject: at any rate, a 10 chain correction would only be applicable to very approximately level ground where average 10-chain stations could be taken.

225.--Where space is not at command and curvature correction is not desired, adjustments of the level may be made with care at 1 chain distance on each side of the setting-up of the level with one staff only, which can be moved from one stake to the other, and with the final setting-up of the instrument at 1 chain distance from these stakes as before, art. 221. For this the staff only requires moving twice, if the collimation adjustment is to the last reading only calculated out as above. This close system has a certain amount of merit, that by reading from one staff only for both stations it is more accurate, as any inequality between the divisions of two separate staves is avoided.

226.--=Collimator.=--Optical manufacturers in populous districts, and some observatories, as that of the India Store Department at Lambeth, adjust by means of the collimator by the exact method due to the late eminent German mathematician, Carl F. Gauss, which is hence termed the _method of Gauss_. The collimator consists of any good telescope permanently adjusted to solar focus, with a webbed diaphragm placed in the focus, where it may be illuminated by a lamp or by the reflection of daylight, and provided with means of bringing the telescope to a level position. As the collimator is generally constructed, it consists of an 18-inch telescope, Fig. 67, of the same description as that used for a Y-level, described art. 94, in which the telescope is surrounded by accurately turned collars formed to rest in Y's. The Y's are supported upon a heavy cast-iron stand, of somewhat triangular form, of nearly the length of the telescope, about 6 inches wide at one end and 2 inches at the other. The stand has two feet extended to the full width at the wider end, and one foot at the narrower end under the telescope. Each foot has an adjusting screw. The complete collimator is supported, at about the height of the telescope of the level on its stand, on a very solid pier of stone or brickwork in cement capped with a stout slate slab. The telescope is brought to perfect collimation as with the Y-level, already described art. 200, and the level is fixed true with the axis of the telescope, when the collimation is perfect.

227.--A lamp or gas flame is placed at a short distance from the eye-piece end of the telescope, so as to illuminate the webs that they may be distinctly seen when looking into the objective end of the telescope. In bright daylight, if there is a skylight over, a reflector will answer the same purpose. At the Lambeth Observatory a fine needle-point hole is used instead of webs.

228.--The instrument to be adjusted may be placed at any convenient distance from the collimator. For adjustment of a level, where the collimator is already in adjustment, the level is raised upon its stand until the axis of the telescope sensibly coincides with the axis of the collimator; then if the telescope of the level to be adjusted be focussed into the objective end of the collimator, the illuminated webs will be clearly seen; and if these webs be brought by adjustment of the level exactly to coincide with its own webs, the collimation lines of the two instruments are exactly parallel. In this adjustment it is only necessary to be sure that the vertical axis of the level is truly vertical, so that the bubble reverses without displacement, in which case the whole instrument must then be in perfect adjustment.

229.--It would be very difficult to use this method of adjustment if it were necessary that the axes of the level and collimator should _exactly_ coincide. It is only necessary that they should nearly coincide, on account of the imperfection of object-glasses, which rarely work so well near the edge as towards the centre; otherwise any directly parallel position in front of the object-glass would answer, as the next diagram will show. Let _O_, Fig. 68 be the object-glass of the collimator, whose solar focus is at _F_. Then the rays _PP_, and all other parallel rays falling on the object-glass, will be brought to a focus at _F_; and reciprocally all rays departing from _F_ in passing through this object-glass will leave in parallel lines _PP_. Let _O′_ be the object-glass of a telescope to be collimated, _F′_ its solar focus. Then all rays from _P_ to _P_ departing from _F_ that fall within the parallel space _P′P′_ will be brought to focus at _F′_. When the image at _F_ is illuminated by a lamp _L_, the webs or other index will be clearly seen by the eye-piece at _F′_ when the two telescopes are exactly parallel with each other. In this position the webs of the level are adjusted to make this coincidence. It is easily seen that by this method we eliminate all errors of atmospheric refraction, and are quite independent of the state of the atmosphere for obtaining distinct vision for adjustment.

230.--When two levels are at command, one a Y-level, or even a dumpy in perfect adjustment, the one may be used as a collimator to the other by setting them up at a distance within their focal range on a firm basement floor. A candle or a lamp will give sufficient light to illuminate the webs of the instrument, which is used as a collimator, being certain, of course, that this instrument is first placed in level adjustment and set at _solar focus_, and that the instrument used as a collimator has a good object-glass.

231.--=Improved Dumpy Level.=--The writer has made some improvements in the dumpy level, which have so far met with very general approbation from the profession, Fig. 70. These improvements are directed to ensure much greater sensitiveness in the longer bubble, therefore greater accuracy in the work performed by it; more solidity of construction without increase of weight; and permanence of reading index, with some additional matters. In these improvements the mounting of the longer level tube, instead of being placed in a stiff joint at one end, or between rigid clamping nuts at both ends, has a barrel-fitting at one end which is ground into a parallel hole. This plan admits of circular self-adjustment to the bubble tube, which the clamping of the nuts can never twist or strain during vertical displacement; and the joint can be made perfectly sound with certainty, which saves the risk of accident to the bubble from expansion by heat and some other conditions. A more recent form of cross level, Fig. 69, shown in perspective near the ray-shade in the engraving Fig. 70 has been designed by the author, in which the level casing is bored entirely out of the solid. It is supported upon the side of one telescope strap by three stout pins, the centre one fitting its hole, and the two outer ones are loosely held by cross screws to permit a small amount of adjustment, which is all that is necessary. By this construction the level fixings are made in five pieces only, including screws, instead of thirteen as usual, at the same time making the level more portable and solid for hard wear. The telescope straps are fitted at their stumps solidly down upon the limb, as shown Fig. 60, p. 112. Adjusting screws are placed under this as in the dumpy level described, but the pressure screw is not employed except in case of accident far away from an optician, when it is found to be there ready for use. The limb is framed out into two edge bars: this gives greater vertical sectional strength and resistance to torsion without increase of weight in the instrument. Where a compass is used, this is included in the frame of the limb, as shown in the engraving. The compass is read with a prism, this being much more convenient and exact than looking down upon the divided circle, the instrument being necessarily placed for use at nearly the height of the eye. The compass ring is made of aluminium.

232.--The further improvement, which the author considers of the greatest moment, is that the vertical axis is fixed directly and firmly upon the limb, and not through a loose screw fitting for separation at this point as in the ordinary dumpy. This is shown to be important in that, with the dumpy, where a loose screw is employed, any little difference of screwing down upon the axis when the instrument is set up causes so much derangement of a sensitive bubble in relation to the vertical axis, that the optician is bound to use a rather dull bubble with the ordinary dumpy. Further, a particle of grit or the slightest bruise on the collar in replacing the instrument in its case throws it out of adjustment at this important point. The objection to the author's plan is that it makes the case for the instrument somewhat larger; but the advantage of certainty of permanent adjustment appears to him very far to counterbalance this objection where accuracy is aimed at.

233.--=Tribrach.=--The setting-up adjustment of the instrument is upon tribrach limbs with three screws only. These screws can never strain the vertical axis, which in this instrument is somewhat deeper and more firmly made than that of the dumpy. In the old form of tribrach the points of the screws were held down by a spring plate placed above them. This plate, in carrying the instrument upon its stand over the shoulder, which is the most comfortable way if the stations are not far apart, was very liable to strain sufficiently for the screws to get loose. The author patented a much more solid method, by which the old spring plate is entirely dispensed with. In this plan each screw has a ball at the lower end, which is inserted in a tubular fitting formed in a solid tribrach, made of exact dimensions to take it. The tube is open on the upper side, as shown in longitudinal section _H_, Fig. 71. Many years' experience and the fact that numbers of makers have copied this form since the expiration of the patent, shows this plan to be perfectly successful. The general construction of the lower part of this level may be seen from inspection: _L_ limb, fitted with compass; _C_ axis, in one casting with the limb; _S_ sprang, carrying the socket and supporting the instrument. _PH_ shows the ball head arrangements to the screws. A central screw in this part detaches the tripod. One point is shown at _P_, of which there are three, to support the level upon a wall or rock in cases where the tripod cannot be used--a most important advantage in town levelling. The tripod head is made much more firmly than that of the ordinary construction, by extending two wing fittings from the top of the shovel-plates as wide apart as possible, instead of the narrow tenon fitting before described. The shovel-plates are screwed to the staff by means of a stout nut-plate inside the tripod _F_. Those who have experienced how much defective levelling is due to a shaky tripod head will appreciate this precaution. The general arrangement is also shown in Fig. 70A.

234.--As the tribrach system of adjustment is of somewhat recent adoption to ordinary surveying instruments in this country, it strikes the stranger to it as being more difficult in use. It is really the most simple and expeditious system as is clearly explained by the foregoing diagrams, Figs. 72, 73 of the plan of a level, omitting its lower parts.

235.--The bubble of the level is placed parallel with two of the screws of the tribrach, that is as _B_ and _C_, Fig. 72, and is adjusted to the centre of its run. It is then placed at right angles to the first position, so that the screw _A_ comes directly under the bubble, to be adjusted by this screw only until it again comes in the centre of its run. Fig. 73 shows this second position with the screw _A_ underneath. The level should after this read all round true, but it is well to try it round parallel with the different pairs of screws in all positions to give small adjustment if required. Where there is a cross bubble the level may remain for adjustment in its first position, but it is well to try it all round, as the long bubble is made uniformly the more sensitive.

236.--=The Ray Shade= to the telescope used in the above-described level has two narrow slits opposite each other at 180°. A zero line is carried from one slit to a line on the ray shade fitting when the slits are quite horizontal. Sights through the slits at zero enable an approximate cross-level to be taken. The edge of the tube of the ray shade is divided 30° on each side of the zero line to 2°, so as to take approximate lateral inclines of the surface of the land in levelling. This useful plan of cross-sighting was originally proposed by Gravatt.

237.--The most important variation from the telescope of the dumpy level described is in the diaphragm, where webs or lines of any kind are entirely done away with, and are replaced by a special form of index. This is represented in Fig. 75. The movable part carrying the opening of the diaphragm is placed in a sliding fitting, as previously described, art. 214, for the dumpy level. The index which replaces the web is a finely-pointed needle formed of platino-iridium (platinum ·75, iridium ·25). This alloy has about the hardness of spring-tempered steel, and is, as far as known, perfectly non-corrosive in air or moisture. A pair of vertical points indicate the position for holding the staff. It will be found by experiment that the point reading is much more exact than with the web, as irradiation due to edge reading of the web is entirely avoided, and also the covering of the object as it would be intersected by the web due to the angle its thickness subtends upon the staff, which is very palpable at 10 chains distance. The iridium point is sufficiently strong to be kept perfectly clean by touching it occasionally with the point of a camel-hair brush if it appear dusty. With care this point will last in adjustment for as long a period as the level itself remains in use. Upon first impression the point may not appear so fine as a web, but practically it is more exact, as the previous exaggerated images will show--Fig. 76 is the image of a division of the staff partially covered by a web _WW′_; Fig. 77 that of a magnified image of the point _P_ brought towards a division for reading. It will be readily observed that the fractional part of the 1/100 foot block, which the point _P_ cuts, is much more easily estimated than that in which the web _WW′_ covers a part of a similar block.

Fig. 76. Fig. 77.

_Difference of reading with a web and a point, shown much magnified._]

238.--In early levels of improved construction, as shown Fig. 70, a difficulty was experienced in practice in bringing the index point exactly up to the edge of the line as it is shown in Fig. 77 at _P_. This difficulty has been obviated in recent highest class instruments by making a tangent screw adjustment to the axis as shown under the level in Fig. 78. There was a great objection to the old form of tangent adjustment by clamping on the axis, as this was found to disturb the centre. In the plan shown in the illustration the clamp is left free by jointing to the axis until it grips one of the arms of the tribrach upon a vertical surface; in this way it cannot disturb the axis. The level, Fig. 78, is shown mounted on a framed stand, which is preferred by the Indian Government, and is generally necessary for rigidity for large instruments of over fourteen inches. This will be described further on with theodolites, art. 447, on framed stands.

239.--=Stadia Points.=--The author commonly makes the points, Fig. 75, _VV′_ stadia points, by making the distance of the extreme ends of these subtend an angle, equal to 10 feet of the levelling staff at ten chains distance, or 1 foot of the levelling staff at 100 feet distance (+ a constant to be discussed Chapter XII.), by which measurements of the distance of the staff can by taken or checked by observation through the telescope only.

240.--=Quick setting-up Tribrach.=--One objection has to be made to the tribrach over the four-screw system of adjustment, that the four-screw admits of greater inclination to the tripod, which is important in hilly countries. To remedy this defect the author designed a ball arrangement to the axis, which permits the level to be set 15° to the inclination of the tripod independently of the screw adjustment, so that the level, when the tripod is set at its best angle, may be brought immediately to nearly its final position. The arrangement is shown in the engraving Fig. 79. The axis carries a cup formed in the metal casting, which can be clamped down upon a ball-shaped recess formed upon the tribrach by means of a winged nut placed under it, the wings of which project between the tribrach screws. A very slight pressure is sufficient to firmly clamp the ball. This form of level is now very popular with civil engineers. With a point diaphragm and a tangent screw to the axis, not shown in the engraving, it is, in the author's opinion, the best practical level he has been able to design.

Since the last edition was written the reviser of this work introduced, in conjunction with Mr. Stanley, his new solid bodied engineer's level, which has practically revolutionized the form of dumpy level and has proved such a success that more of this form are now made than all other forms put together. In this level, Fig. 80, the centre, body of telescope, object end and bubble fitting are all combined in one piece of gun-metal, so that although of vastly greater strength and rigidity it does not weigh as much as the old form of tubular body with its collar and stage. This does away with many separate pieces which are usually soldered and screwed together. It thus forms the strongest and most compact level yet made, and with ordinary care it will last in perfect adjustment a lifetime. The pinion for focussing is fitted to the side of the cast body, instead of to a tube, thus greatly increasing its firmness. Its form is equally adapted to the four-screw levelling if desired, as shown on next page, Fig. 81, in which it will be seen the four-screw levelling is of much improved form, giving greater strength and far more wearing and bearing surface to the levelling screws.

The reviser has also patented a new form of spherical joint, which has met with equal favour. This improvement consists of a section of a ball (screwed to fit the stand head) fitted within the lower plate and a simple means of clamping it in any position, which, when released allows of sufficient rocking movement in any direction to compensate for any uneven setting up of the stand. It does not add to the height of the instrument, may be instantly set nearly level, and less than half a turn of the levelling screws will bring the instrument into true position. It is shown fitted to the new engineer's level at Fig. 82 below, but is equally applicable to any other form of instrument.

As ninety per cent. of the orders now for levels are for the form shown at Fig. 82, the reviser ventures to think that this must be favoured by the profession as the best practical instrument yet made.

A further improvement has been made by making the diaphragms interchangeable, so that any form of diaphragm that is preferred may be instantly fitted without disturbing the adjustment, and when lines on glass are used it may be removed for cleaning, and replaced without interfering with the adjustment.

The diaphragms illustrated below, Fig. 83, are usual forms, and it is recommended that when webs are preferred a glass one should be carried as a spare in case of accidents.

E
_Stadia points for
clamp and
tangent levels._

F
_Stadia points
for ordinary
levels._

G
_Stadia glass
diaphragm._

H
_Webs._

J
_Stadia
webs._]

241.--The further discussion of the subject of high-class levels becomes somewhat difficult. Leaving out of consideration the levels sold by the trading optician, who deals in the commercial article but sometimes superadds a little fad, every genuine manufacturer has his pet plans of carrying out details, some of which may be very meritorious, but which could scarcely be described without a fuller discussion than our space permits. There is also, no doubt, a great number of mistakes that have been made in the construction of surveyor's levels. The direction in which the scientific optician generally fixes his attention is to give the advantages of the Y-level in the dumpy form, assuming the civil engineer holds a certain amount of prejudice against the use of the Y, for which, in its old form at least, the writer must admit that he was fully justified. Whether the professional man, nevertheless, will ever depart from the solid construction of the dumpy remains an open question.

242.--=Cushing's Level.=--The level illustrated above, Fig. 84, by the late Mr. Thos. Cushing, F.R.A.S., Inspector of Scientific Instruments for India, would under any circumstances claim attention, from this gentleman's well-known high technical scientific attainments. It has also the merit of being in practical use in India at the present time.[4] The principal improvement in this instrument over the dumpy form, which it otherwise represents, is in the construction of the telescope, which is said to possess all the necessary adjustments of the Y-level. The telescope is firmly fixed in collars soldered to the tube, as in the dumpy. The tube at each end is formed into a stout socket collar. These socket collars are exactly alike, and are ground to fit either the objective or the eye-piece end of the telescope, so that these parts may be reversed, the one for the other. This reversing is nearly equivalent to turning the telescope end for end in the Y-level. The end also rotates in its fitting, which is nearly equivalent to rotating the telescope half a revolution in the Y-level. The reversible ends of the telescope are held in their ground fittings by studs and slides (_bayonet notches_). It is easily seen that by this plan adjustments may be made of collimation and of fixing the line of collimation perpendicular to the vertical axis, as with the Y-level, if the object-glass be originally correctly centred. The stop is of the slide form described for the dumpy, Fig. 61, and a glass diaphragm is used. One important arrangement is also made in this part of the instrument--which is necessary, as glasses become frequently bedewed in the telescope--viz., that the eye-piece end may be removed from its ground fitting and the glass cleaned and replaced without disturbing the adjustment in any injurious degree. The general construction of the instrument can be seen from the illustration. The supports of the telescope have a rocking axis at one end, and are adjusted by capstan-headed nuts at the other. The adjustable support for setting up the instrument is upon Everest's tribrach system for theodolites, to be described further on, in Chapter IX. The tripod head has also wider bearing than is general, which is attained by extending the book-plates into the form of a socket fitting. The illustration given is of a 12-inch level; in the 14-inch an open framed stand is used in place of the solid tripod, as in Fig. 78, which will be described further on, for theodolites. The level is a decidedly good one; but the author has experienced with it some slight defects when compared with his own Y form. The ground collars are a little inclined to bite, particularly if the instrument has been laid by for some time, so that in reversing for adjustment there is great risk of disturbing the instrument. The glass index, although permanent, has the same defect as the web--of covering the image of the staff reading. It also obstructs a little light, and is subject to dew, which the point system avoids. The weight of the instrument is increased by the collar fittings.

243.--=Cooke's Level.=--An instrument somewhat equivalent to the above has been patented by Messrs. T. Cooke & Sons. In this, instead of the objective and eye-piece ends of the telescope only being reversible in the collar fittings, as in Mr. Cushing's level, the entire telescope reverses end for end in an extra outer tube, which is fitted between the collars. This tube also permits the rotation of the whole optical parts about the axis of the telescope for adjustment for collimation, although in a manner more frictional, and therefore more likely to disturb the instrument than in the simple Y adjustment. In this instrument, again, it is easily seen that it is the perfection of the Y-level, without its outward appearance, that is aimed at, and to gain this the weight is increased by extra fittings and double tubes, which are liable to become fixed by a slight dent upon the outer tube. Taken altogether it is not quite so convenient or so simple as the best constructed Y-level; but if it gives the adjustments the optician holds to be most important, in a disguised form it may be acceptable to the civil engineer. We may in this manner, perhaps, from the optician's point of view, count it a certain gain in the same direction as Mr. Cushing's level just described; but if we may accept the late Mr. Wm. Gravatt's ideas, already mentioned, the complication is unnecessary.

244.--A few other structural variations of details may be mentioned, as these are constantly cropping up as new inventions. The bubble tube is sometimes placed upon the stage instead of being upon the telescope. This is thought to protect it. It is not, however, so easy to read it in this position. The compass is sometimes made a loose part--when it is not required on the work its weight is saved. Various forms of locking screws are made to the supports of the telescope; these are only necessary to correct imperfect work. The axis collar is sometimes extended to a limb bearing. This is common in French instruments; it makes the movement stiffer, and is quite unnecessary unless the axis is made too short. A well-known German firm recently brought out a level with internal focussing, by means of an auxiliary lens mounted in a tube inside the telescope, moved by a rack and pinion, but any internal lens is a source of trouble, as it cannot be got at to be cleaned, and in hot, damp climates it becomes bedewed. The device is very old, having been patented in America many years ago and discarded.

245.--=Supplementary Parts to Levels.=--As a rule, supplementary parts fixed to the instrument, beyond the magnetic compass sometimes required, are very objectionable if the object of the level is to be levelling, as these additional parts inevitably increase the weight which has constantly to be borne in carrying the instrument. Supplementary parts have been carried, in various schemes, to the extent of combining the entire level with the theodolite, at the same time nearly combining the united weights of the two instruments. As a rule, professional men rarely care for complex combinations; and even after a limited popularity is granted to extra parts not absolutely required, these are generally finally abandoned. Mention of two such parts, therefore, only will be made, as these owe their introduction to the late William Gravatt, and are found applied to many levels in use, or at least contained in the case with the instrument.

246.--_Bubble Reflector._--This was formerly placed upon all dumpy levels. It consists of a small mirror about 2 inches by 5/8 inch fixed in a frame that is jointed at its lower end to a short piece of tube partly cut away so as to form only a little over a semi-cylinder. This tubular part just clips firmly upon the brass casing tube of the spirit level. The reflector, when placed vertically on the level tube, can be adjusted by its joint, so that the run of the bubble may be observed by reflection in looking above the eye-piece to see that it is in adjustment at the time of taking an observation. Its use was thought to be a precaution in levelling, particularly on marshy ground. The observation of the bubble is less exact than by a side reading, and cannot be relied on.

247.--_Sight Vanes._--Two sight vanes are placed above the telescope, either as loose fittings or to hinge down upon the level tube. One vane has a vertical narrow slit and cross hair; the other has a window with a vertical horse-hair placed in its centre. This arrangement gives sight of distant landmarks in line with the direction of the telescope, upwards or downwards, beyond its field of view. A slider, fixed upon the window sight, reads at its upper edge into divisions cut on the vane, by means of which an approximate rate of forward inclination of the land may be taken. This sighting arrangement adds about half a pound weight to the instrument. It was useful with object-glasses of small field of view, but is useless with good modern glasses of wide angle.

248.--=Lower-class Levels.=--A level is often required by an architect or a contractor for works of limited area, where it is quite unnecessary to go to the expense of a civil engineer's level of refined manufacture. In such cases the level may only be used occasionally and under favourable circumstances, so that extreme solidity is not demanded, neither is distant view in the telescope required. The level generally made for such work is a simple dumpy, without cross bubble, compass, or any extra fittings, and with one eye-piece only.

249.--The instrument Fig. 85 illustrates the author's newest design for a simple level. It has a light form of tripod. The legs clamp directly between angle plates--these are not quite so portable or so neat as cylindrical legs, but they are easily made, very firm, and will bear considerable wear and keep in order. A still cheaper form is made with smaller telescope and turned legs for the tripod.

250.--The illustration Fig. 86 represents the cheapest form of level with a tripod stand that has been constructed, which contains the important factor of a telescope. The telescope has a sliding fitting, which is moved by a knob outside, this being made more quickly than a rack and pinion fitting. The level tube is solidly supported in collars. The adjustment is in one direction only, so that the bubble must be set and examined at the time of reading the staff. The instrument is supported on a _sprang_, jointed at one end and held by a milled-headed screw at the other. Any shakiness of the thread of screw there may be is taken up by a stiff German silver spring between the sprang and the limb. It is sometimes made with a ball and socket joint for first adjustment, but this renders it nearly as costly as a superior level. The tripod head is of simple construction. The legs are oak or ash, and are clamped on the head by bolts. This simple tripod is fairly firm in use. The level is good enough for ordinary building works, laying short drains, etc., within limited areas. It is much more accurate than any form of open sighted level without telescope. Sir George Leach has recently made a modification of this old form of level by placing a pendulum to rock the axis to cross level position, which is a refinement, although rather a costly one.

251.--=Sighted Pocket Level.=--This consists of a tube, which is generally drawn of square section. A pin-hole sight is made in the closed end of the tube, Fig. 87, at _E_. The field end of the tube is left open. The sight is taken by looking through the centre of the pin-hole across the edge of the reflector _R_. A level with a small bubble is placed or inserted in the top of the tube at _B_. The metal casing of this is cut away on the upper and under sides to render the bubble visible from the interior of the tube by means of the reflector _R_, which occupies one half vertical section of the interior of the tube. This is placed at 45° to the axis. The reflector is fixed upon an inner tube so that it may be withdrawn to be cleaned. When the level is set horizontally, a distant object in the direct sight line is seen through half the tube, and simultaneously the reflection of the bubble in the other half appears. A line engraved upon _R_ indicates when the bubble is central, and when these coincide the distant object and the eye are level. The instrument is about 4 inches long, and weighs about 8 oz. in its case.

252.--=Pocket Telescopic Level.=--In the above-described pocket level, where it is made short, the average middle-aged man will not have sufficient accommodation of vision to be able to see the bubble and the screen sharply defined simultaneously with the distant object to which the level is to be taken. In Captain Barrie's[5] level these objections are avoided by making the reflector and bubble form part of a telescope, Fig. 88. An achromatic glass of short focus is used, and the eye-piece is of long focus so as to bring the bubble to focus in the centre of the mirror, which is made of curved form to decrease the apparent size of the bubble. The image of the bubble does not give by bisection a very definite index. The author has found that this level may be much improved by placing a point in the telescope at the mutual foci of the object-glass, eye-piece and the bubble. The appearance of the mirror and point is shown at _B_. The point is shown by a dot at _P_. The curved mirror _R_. The dotted line shows the path of reflection from the bubble. This level will work with very fair accuracy as a hand instrument. Size, about 4½ inches by ¾ inch. Weight in case, about 8 oz.

253.--=Reflecting Level.=--This simple level, Fig. 89, the invention of Colonel Burel, is one of the most portable. When it is used with a fair amount of care it will give good approximate results. It consists of a piece of parallel glass, which has half the surface silvered to form a reflector. It is suspended in such a manner that the glass hangs vertically by gravitation. The position of the mirror to the plain glass may be that shown in the engraving, or horizontally if preferred. The mirror, Fig. 89, is inserted in a solid metal frame suspended from a gimbal, which permits it to hang perfectly free to the action of gravitation. The centres of suspension are made with slightly-rounded knife-edges. A ring at the upper part of the instrument is placed over the thumb or finger to support the instrument when in use. A stout pin passes through a prolongation of the lower part of the frame, screwed or otherwise, which permits adjustment by filing to bring the mirror when it is suspended exactly into a vertical plane. The instrument, fitted into a neat case, weighs from 5 oz. to 9 oz.

254.--_In using the Reflecting Level_, it is held upon the thumb at about arm's length, and adjusted by raising or lowering the arm until the reflection of the pupil of the eye seen in the mirror is exactly bisected by the line cut by the mirror against the clear glass. The distant object seen in front, that cuts this sight line and the image of the pupil of the eye, will then be in true level position with the eye of the observer, provided the air is still, so that the mirror is not deflected from verticality. From the natural unsteadiness of the hand there is some little difficulty of getting this level quite free from oscillation. This may be obviated, or nearly so, by clutching a picket or staff with the hand and suspending the level from the thumb projected out for the purpose, or by resting the hand against a tree or other firm support. Capt. A. H. East, R.A., has suggested to the author a very capital device which he employs for hand instruments. This is to place the handle of a stick (or umbrella) in the waistcoat pocket, to clutch the body of the stick with the hand which holds the instrument, and to steady it with the other hand. In this manner the two arms and the stick form a tripod of surprising steadiness.

255.--=Reflecting Level in Case.=--In windy weather much greater exactness may be secured by placing the pendulous level, just described, in a tubular case, Fig. 90. The case is made of double tubes, so that the aperture cut on one side may by a half turn of the outer tube close and protect the instrument when out of use. The transparent side of the inner case is sometimes closed by thin glass tube of its own internal diameter. It is much better if made with two vertical sides glazed with parallel glass. When this form of instrument is used, it may be, if required, made to fit on the top of a light staff. The eye is then brought with much greater certainty to the point of bisection on the edge of the mirror, and much greater accuracy is thus attained in levelling with it.

256.--=Water Levels.=--The antique form of level, composed of two vials fixed on the ends of a tube and partly filled with water, by which a level is sighted in looking over the surface of the water, is still used to a limited extent in rural districts on the Continent; but the spirit level in some simple form is fast superseding it. The same principle of level, but with long tube, has been found convenient for the surveyor in measuring through close buildings, Fig. 91.

257.--=Browne's Water Level=[6] is found to be a convenient instrument for levelling in close towns. It consists of a pair of glass tubes of about 2 feet in length, placed in a casing tube for protection. The casing tube is divided into inches and parts, or the scale is a detached piece of painted wood, or any rod or rule. A cock at the bottom admits the water to flow to level in the pair of tubes, one of which is shown, Fig. 92. There is a handle at the top which unscrews to fill the level, and a small air cock. It is easily seen that the water finds its level, and the difference of reading of the two standards is the difference of level of the surfaces upon which they are placed. By closing the cocks the level is made portable. In this position it does not matter how high the centre of the pipe is placed--for instance, in crossing over a wall--as the water will still find its level when the cocks are released by syphoning the water from the one side or the other. It is a very convenient and exact level for laying drain pipes in open weather, and for making foundations for heavy machinery, etc., but of course it will not stand frost.

Platelayers' levels and mechanics' levels generally are deferred to consider with useful hand tools and apparatus employed by surveyors in the final chapter.

FOOTNOTES:

[3] See Simms' _Mathematical Instruments_, p. 3.

[4] See pamphlet on _A New Form of Levelling Instrument_, by Thos. Cushing, F.R.A.S., 1879.

[5] Patent No. 69, Wm. Barrie, 1856.

[6] Patent No. 6742, John Browne, June, 1834.