Chapter VI: Part I: I (4)

The steam turbine, of which so much has been heard lately, is not constructed like an ordinary steam engine with cylinder, slide-valve and other attachments; but more like the Hero engine, with this difference that the steam jet or jets act on a wheel having vanes or blades, the expansion producing a velocity which rotates the wheel containing the vanes. A modern turbine, of the Parsons type, such as are employed on the great Atlantic steamers, is a tremendously high speed engine. It does not derive its power from the static force of steam expanding behind a piston, as in a reciprocating engine. In this case the expanding steam produces kinetic energy of the steam particles, which receive a high velocity by virtue of the expansion, and, acting upon the vanes of a wheel, force it around at a high speed of rotation in the same manner as a stream of water rotates a water-wheel. The expansion produces velocity in a jet of steam, and this is the main difference between the ordinary engine and the modern steam turbine.

Among gas and internal explosion engines there exist some differences, both in construction and in the manner of supplying fuel. The gas-producing engine may be considered the better class, though it has not as yet gained the popularity of the gasolene one. The gas by which this style of engine is operated is produced by a special process, namely, by passing air and steam through a fire of hot coals. After generation the gas passes over a flash-boiler and a portion of its great heat is withdrawn, thus permitting it to enter a scrubber--a cylinder filled with coke and sawdust--while fairly cool. In passing over the flash-boiler the great heat raises all the steam necessary for the production of gas required in the operation of the engine and plant. In passing through the scrubber the gas is not only cooled, but is freed from particles of suspended matter, the coke removing the heavier particles, and the sawdust, the tar, or any other volatile matter that may be left.

One of the most important requirements in a gas-producer is that it shall be adapted to the work it has to do. Its construction should be compact and simple, so as to permit the easy removal of worn out parts. The feeding device should be such as to secure a uniform distribution of fuel.

The blast should be so introduced as to burn out all the carbon in the ash zone, and yet not produce localized combustion along the walls. The construction should permit the easy removal of ashes, and render the machine safe, while the entire process of gasification should be clean. The radiation loss should be low, and the producer must be made efficient to insure satisfaction.

It should be borne in mind that because of the presence of carbon monoxide, producer gas will always be more or less poisonous. The carbon monoxide has a specific toxic effect on the human system, and when inhaled enters into direct combination with the blood, and brings about very dangerous effects.

As water is always required for cooling purposes when running a gasolene engine, it is well to know about how much will be required. One authority says: "The quantity of water required at the ordinary temperature of 60 degrees F. inlet and 150 degrees outlet, to keep the cylinder of gas engines cool is 4.5 to 5 gallons per indicated horse-power-hour. The jacket pipe should be from 1 to 2 inches diameter for engines up to 20 horse-power, while for larger engines the sizes are generally 2 to 3 inches for the inlet and 2.5 to 3.5 inches for the outlet. Tanks for circulating the water are generally made with a capacity for furnishing 20 to 30 gallons per indicated horse-power. This rule may be taken as about correct, but, if anything, it is rather an over-estimation of quantity necessary."

All the foregoing was made as clear as possible to the listeners by Mr. Gregg before the children went to bed.

Next morning Fred called up his artist friend, and got him to come down to gild the name "Caroline" on the boat before the next coat of varnish should be applied. The artist made an outline of the name while George and Jessie stood by and watched the process with considerable interest. They saw him measure off each letter, outline it with a pencil lightly, and then paint inside the lines with a substance known as "gold size," obtained from any store dealing in painters' supplies. While the size was still sticky the artist applied "gold leaf," which he had brought in a little book along with him. Jessie was surprised to see him cut the gold with a thin pallette knife, having a blunt but smooth edge. She watched him pick up the small pieces of gold with a camel's hair brush, which he rubbed in his own hair now and again whenever it would not pick up the gold. The metal was applied bit by bit over and beyond the lines of the letters, and a light puff of breath forced it down to the size. When one side of the boat was finished, so far as laying on the coat of gold was concerned, Jessie was very much disappointed, as the name seemed merely a smudge. She could not make out the letters, but the artist told her to wait until to-morrow and he would show her how well they could be seen. Next day with a flat camel's hair brush he dusted away the surplus gold, and the letters showed up in good style, much to the gratification of Jessie and George. This part of the work being done, the boys took down their varnish pots, and gave the little craft another coat, to make her quite spruce and gay.

Fred, and Nick, who was still in the employ of Mr. Gregg, laid off a space on the ground for tables and seats to accommodate the young folks who were coming to the launch on the following Wednesday. Nick found a number of old cedar posts, and with a saw cut off 18 pieces about two feet long and as many more twice that length. The first were intended to place the seats on; the second lot were to sustain the tables. The spots for the tables were chosen, measured off, and small stakes driven into the ground to show where the posts were to be placed. Five posts were intended for each table--two at each end, two feet apart, and nine feet apart in the length of the table. The single post was placed in the centre of the table both ways. When the stakes were all in place, Nick made holes deep enough to take in the posts so that their tops measured just two feet and two inches above the level of the ground. The tables were to be two feet and six inches high when finished, as that is the regulation height. It was attained, in this case, as follows--First by the height of the posts from the ground, two feet two inches; then by a plank two inches thick laid across the two posts, making the height two feet four inches, and the table top, two inches thick, laid on these cross planks, which brought it up to the required height. A piece of plank the same thickness was nailed on the centre post across, so that it would support the table top. Planks that had been used in the loft of the old barn did service for the table tops, bearing pieces, and the bench seats. The last were constructed in the same manner as the tables, the short posts being let into the ground--three under each seat--and fourteen inches above ground so that when the plank seat was nailed on top of them, the seats were just sixteen inches, the regulation height of stools, benches, and chairs, though it is sometimes varied to suit conditions. The benches were placed about four inches out from the edge of the table and were found to be "just the thing."

When Nick had planted the first post for the tables and got it the right height, he took that one for his guide and by the aid of a long parallel straight edge which he laid on the guide post and the one he was setting, and also a spirit-level on the straight edge, he managed to get all the posts alike in height and this made the tops of the three tables nice and level. It was quite an achievement to have three large tables and six long seats placed in "picnic style" at so small a cost and with so little effort.

In order to have the tables and seats neat and clean, George turned on the garden hose and gave them a good wash off, and when they were dry again the place was as inviting as a country hotel dining-room. When Mrs. Gregg, Jessie, and Grace Scott had the tables set and garnished for the launch, the lay out was charming, none the less so because it was a little rustic.

Another coat of varnish, the third, was given the boat the day before she was to be launched, and Fred had a strong rope attached to the winch, with a heavy iron hook fastened to the end of it. A stout iron ring was bolted to the stern of the boat and made secure. Mr. Gregg had purchased a number of small flags and "burgees" and had one made with the name "Caroline" in large letters wrought on it, ready to be unfurled when the launch was made, and Walter Scott, his mother and sister Grace, and others had been invited to attend.

A number of temporary swings were fixed up by Nick and Fred to the trees, some for the large folks, others for the smaller ones, and everything was at last ready for the great event, which was to take place the next day at two o'clock.

VI

PROPELLER AND OTHER SCREWS

Wednesday morning was light and sunny and the boys were up and dressed somewhat earlier than usual, so, while waiting for breakfast, they took a stroll down to the river, where they found their father looking over the grounds and examining tables, benches, swings, and particularly the foot-bridge; for, as he told Fred, "it was very likely all the guests might be on the bridge at one time and the combined weight would be rather trying if it had not been securely put together." He satisfied himself, however, that the bridge was strong enough to support three times the weight it would be called upon to sustain. Everything else seemed to be sufficiently strong, to apprehend little danger, no matter how much the children romped.

Nick had the grounds nicely raked off; the decayed branches and shrubs he moved, and made everything about the place as clean and as neat as possible. Flags and other decorations were hung or placed about the grounds, on the trees and buildings, but particularly about the tables and the boat house. Newspapers were spread over the tables, linen covers above them, and the whole surroundings took on a most festive appearance.

It was just 11 o'clock when _The Mocking-Bird_ arrived and tied up to the new dock. On board were Mrs. Scott, Grace, and the maid, who came to help, besides several of the invited guests whom Walter had brought down with him. All were welcomed by Fred, Jessie, and George and then the women visitors went to the house to assist Mrs. Gregg.

Mr. Gregg came home from his office earlier than usual and took a half holiday in honour of the occasion. The guests, in little groups, arrived on time, and before the clock struck two Nick had everything prepared for the launch. He and Fred and George had the _Caroline_ nicely placed on the skid, ready to "let go" the winch, and a flag pole was fixed up on the bow of the boat. To this the flag with the name on it was lightly tied, in such a manner that when a string was pulled it would unfurl, and show the name. The string looping up the flag was left long enough to enable Mr. Gregg, standing on the dock, to hold the end in his hand, and by pulling it to loosen the flag as soon as the boat touched the water.

Everything being ready, Walter Scott invited as many of the young people to get into the _Mocking-Bird_ as could crowd on board with comfort, and each was provided with a whistle or a horn, as he ran his boat half way across the river. The children on shore were also given horns and whistles, and all were told to blow as loud as they pleased when the boat touched the water. Mr. Gregg, having Mrs. Gregg and Mrs. Scott standing beside him, gave the word, "Ready!" Nick and Fred answered, "Aye, aye, sir!" and the master of ceremonies called out in a loud voice: "Let her go!"

Nick freed the winch, Fred and George gave a little push, and the _Caroline_ slid down the skids, into the water, without the least hitch. The horns and whistles made a great din, and when the flag was let free to open up and show the name "Caroline" there was another blast of noise by horns and whistles, mingled with voices of the younger people, who cried out with all their might, "Hurrah for the _Caroline_!"

The launch being over, and everything having gone all right, the young people were called to lunch. They all sat at the tables which were nicely garnished and well supplied, and there was plenty of small talk, and much laughter and jollity. After lunch, Fred, Walter and George boarded the _Caroline_, supplied her with gasolene, and tried to run her. They found a little difficulty in starting, but after the engine was warmed up a little, she went off beautifully, and answered her tiller in fine style. The boys ran her up and down the river for a while, then tied her to the dock, and Walter and Fred invited all the girls to "Come and have a sail." The boys were promised one when the two boats returned, which they did in the course of half an hour. The swings were put in use, dancing and romping began, and the afternoon was passed in fun and frolic.

In the evening, Mr. Gregg, Jessie, and the boys took a trip, and Mr. Gregg was well pleased with the boat's performance, particularly with the working of the screw. In mentioning this, he awakened the curiosity of George, who reminded his father that he had not yet explained to them about the screw as a mechanical power.

That evening George was told to bring his blackboard and equipment into the den, and the father at once began explaining the mechanical qualities of the screw. He told of its great usefulness in the industrial arts. As one of the mechanical powers, it may be considered an inclined plane, wrapped spirally round a solid cylinder. The advantage gained by it depends on the slowness of its forward or backward progress, that is, on the number of turns or threads, as they are called, in a given distance. It is always used in combination with a lever of some sort. When employed as a lifting machine it has great power, and is used to produce compression or to raise or move heavy weights. If a screw is formed on the inside surface of a hollow cylinder, it is called a nut, and used to overcome a resistance; either the screw or the nut may be fixed and the other movable. The acting force is generally applied at the end of a lever or wrench or rim of a wheel. Fig. 33 represents a screw and nut operated by a lever or length of radius _r_; _p_ is the pitch of the screw or height of the inclined plane for one revolution of the screw. W is the resistance at the nut and P is the force at the end of the lever _r_. Remembering that, while the resistance W is raised the distance _p_ the force P revolves around a complete circle and moves a distance 2[pi][nu]. Let us now apply the condition [sum] work = 0 and we have

P2[pi][nu] - Wp = 0 or -- = P2[pi][nu]/p (6).

The worm gear (Fig. 34) is a special case of screw and nut, where the latter is replaced by a toothed wheel called a worm wheel. The teeth work in with the thread of the screw or worm, and thus, as the worm revolves, the worm wheel revolves about its axis. P is the force acting on the worm at a radius _r_. _r´_ is the pitch radius of the teeth in the worm wheel and _r´´_ is the radius of the drum on which W acts. Let K, corresponding to W in equation W P (6), be the force at the pitch circle and worm threads due to the force P; then

K = P2W/p (7).

Now apply [sum]m = 0 to the worm wheel and we have

Kr´´ = Wr´´ or K = wr´´ (8).

Substituting the value of K in (7) in equation (8) we have

P2[pi][nu] = Wr´´ or P2[pi][nu] = Wr´´p (9).

Now it is evident that the distance _p´_ moved by W while K moves through the distance _p_ is to _p_ as _r´´_ is to _r´_ or

_p´_ :_p_ :: _r´´_ : _r´_ or _p´_ = _pr´´_/_r´_ (10).

Substituting this value of _pr´´_/_r´_ in equation (9) we have P2[pi][nu] = Wp´ or the condition [sum] work = 0, since 2[pi][nu] is the distance moved by (P) while W moves the distance _p´_.

No provision for friction has been made in any of the examples given, so that allowance must be made for this propensity whenever any of the foregoing rules are applied to practice. The amount of allowance required will vary and must be made to suit conditions.

An endless screw is sometimes used as a component part of graduating machines, counting machines, etc. It is also employed in conjunction with a wheel and axle to raise heavy weights. The distance between the threads of the screw is called the pitch or step. These threads are sometimes square, sometimes acutely pointed or edged, sometimes rounded off on the edges. Power is often applied by means of a lever or other contrivance attached to the end of the screw, or by a long handled wrench (a monkey wrench for instance), which, when turned, moves forward in the direction of its axis, overcoming resistance. In the case of the screw-jack, it may be used to raise a heavy weight. The relation between the force applied and the resistance to be overcome is important to note, for every time the screw performs one revolution it moves forward through a distance equal to the space between one thread and the next.

The Archimedian screw we have read and heard so much about is simply a hollow pipe wound around a cylinder. It was often used in olden times for raising water, but is now only occasionally applied. The lower end of the spiral pipe is, of course, left open and immersed in water, as shown in the illustration (Fig. 35), a device for raising water, the supply stream being the motive power. The oblique shaft of the wheel has extending through it a spiral passage, the lower end of which is immersed in water; and the stream, acting upon the wheel at its lower end, produces its revolution, by which the water is conveyed upward continuously through the spiral passage and discharged at the top. An arrangement like this could easily be constructed at the edge of most rivers to raise water to irrigate the grounds, if so desired, and the little flutter wheel at the bottom of the inclined shaft would be powerful enough to lift all the water required. Fred thought that would be a great scheme, and determined to try his hand at it one of these days, but he was told that a wheel of that kind could only work at intervals, as the river's flow was often running in opposite directions owing to the inflow of the tidal water.

These Archimedian water raisers are often fitted with a crank handle on top, and a man, standing on a platform, turns the crank and thus lifts up all the water the machine will carry. The Archimedian screw is used for many other purposes than raising water. With wide, thin wings, similar to the construction shown at Fig. 36, and enclosed in a case or jacket, it is employed by millers to convey grain and other mill requirements, and it is also good for moving coal, ore, gravel, and like material, but when used for these coarser purposes the propelling blades are made of steel, riveted or bolted to a strong iron shaft. The case or jacket containing the revolving blades, if horizontal, need not be covered on top, as the blades will propel the material without jamming or clogging, if the jacket is smooth inside, and fits fairly close to the blades.

This style of a screw may be used as a sort of turbine water wheel, if cased in a cylindrical penstock or tube, and a body of water allowed to fall into the upper end of the tube. The force of the water will give a rotary motion to the blades and shaft, and, the latter having a geared wheel or pulley attached to its top, motion is imparted to other shafts and wheels.

Another application of the screw is shown at Fig. 37, where one is arranged on a shaft or axle to give a rotary motion. This device is called a "worm and wheel," and is frequently used in the make-up of machine engines and mathematical instruments. The illustration shows how the power or force of a screw may be conceived. For instance, suppose the wheel C has a screw on its axis working in the teeth of the wheel D, having 48 teeth. It is plain that for every time the wheel C and screw are turned round by the handle or crank A, the wheel D will be turned once round. Then, as the circumference of a circle, described by the crank A, is equal to the circumference of a groove round the wheel D, the velocity of the crank will be 48 times as great as the velocity of any given point in the groove. Consequently, if a line C goes round the groove, and has a weight of 48 pounds hung to it, a power equal to one pound at the handle will balance and support the weight. To prove this by experiment, let the circumference on the grooves of the wheels C and D be equal to one another; and then if a weight H, of one pound, is suspended by a line going round the groove of the wheel C, it will balance a weight of 48 pounds hanging by the line G; and a small addition to the weight H will cause it to descend, and to raise the other weight.

If a line C, instead of going round the groove of the wheel D, goes round its axle I, the power of the machine will be as much increased as the circumference of the groove exceeds the circumference of the axle, supposing which to be six times 8, then one pound at H will balance 288 pounds, hung to the line on the axle; thus showing the advantage of this machine as being 288 to 1. A man who can lift by his natural strength alone, 100 pounds, by making use of this combination, will be able to raise 28,800 pounds alone, and if a system of pulleys were applied to the cord H, the power would be further increased to an amazing degree.

When a screw and wheel are attached, as shown, the screw is sometimes called a "worm" and sometimes an "endless screw."

The propeller wheel (Fig. 38) is a screw having a large helical dimension. The example shown has four blades, each of which, when rotated, may be said to make one quarter of a revolution and when at work in the water has the same effect as the working of a nut, producing motion in direction of the axis and so propelling the boat or vessel. The action of the wheel pressing backward against the water tends to push the craft forward.

This figure shows a propeller with four blades, but two and three bladed ones, particularly for small craft, are mostly used. The _Caroline_ carries a two bladed screw and her performances will be entirely satisfactory. The blades, of course, are exactly in line with each other on the shaft, and equally balanced, or of equal weight. A three-bladed propeller should have its extreme points in a horizontal plane, so that they will form an equilateral triangle.

The principal features of a propeller may be described as: diameter, pitch, area, speed of revolution, and slip. The diameter is that of the circle described by the tips of the blades. The pitch, considering the propeller to be a portion of a screw, is the amount which it advances in one turn, supposing it to travel in a solid medium. The blade area is the actual area of all the blades.

The speed of the revolution is customarily reckoned in turns per minute. The slip is the difference between the amount which the propeller actually advances per turn and the amount which it would advance if turning in a solid medium. For example, if the pitch of a screw is 30 in. it would advance 30 in. at each turn if there were no slip. Suppose that it only advances 20 in. per turn, then the slip is 10 in. per turn, or as usually figured, 33-1/3 per cent. As a further example, suppose a propeller of 30 in. pitch, turning 300 turns per minute, drives a boat at the rate of 6 miles per hour. The advance of the propeller in feet per minute is 30/12 × 300 = 750 while the advance of the boat is 6 × 5,280/60 = 528 ft. per minute. The slip is then 750 - 528 = 222, or as a percentage, 222/750 = 29.6 per cent. It might seem at first sight, that a perfect screw propeller should have no slip; but this is a practical and theoretical impossibility.

The most important dimension, from the standpoint of the absorption of power, is the blade area. A certain blade area may be obtained by a relatively wide blade on a small diameter, or by a narrow blade on a relatively large diameter. In the former case the area of the blades bears a greater proportion to the area of the circle through the tips than in the latter case. There are certain limits for this proportion of blade to disc area for well-designed wheels, beyond which it is not well to go. These are as follows:

For two blades .20 to .25.
For three blades .30 to .40.
For four blades .35 to .45.

This means that for a 24 in. diameter propeller, whose disc area is 452 sq. in. the blade area should not, for ordinary use, be made greater than these proportions, as the blades then become so wide as to interfere one with another. Of course where a propeller, for shallow draft, must be unusually small in diameter, the proportion of blade area can be increased, but with some loss in economy. Strictly speaking, for a well balanced propeller, the blade area fixes the amount of power which the propeller can deliver, while the pitch, combined with the turns per minute, governs the speed. As a matter of fact, for the average propeller the two are closely related, each having a certain influence upon the other. To illustrate, a propeller may have a small blade area and so great a pitch that the blades act somewhat like fans and simply churn the water, offering great resistance and absorbing the power of the engine, but doing little effective work toward driving the boat.

To get the measurements for a wheel required to perform a given service, say a three-bladed propeller for a small boat or tug of 20 nominal or 75 indicated horse-power:-- assume that the size determined on is 4 ft. 6 in. in diameter and 7 ft. 6 in. pitch, the diameter of loss may be assumed to be 8 in. swelled to be 11 in. in the middle, and 11 in. long. The tug would be, say, 60 ft. long, 12 ft. beam, and 7 ft. deep. First delineate the path of the point and root of one blade through half a revolution as in Fig. 39. This should be drawn to a scale of not less than 1-1/2 in. to 1 ft. by the ordinary method of projecting a screw thread. The semicircle shows the half plan with twelve equal divisions, and the half elevation is divided into the same number of equal parts. The helix or thread is then obtained by drawing the curves through the intersections of similar divisions. Then _a b_ will be the helix for point of the blade, and _c d_ the helix for the root of the blade. These will be found to be practically straight lines which might have been obtained in a simpler manner if intended for a working drawing only; but it is useful to have demonstrated the proper nature of the full curve.

The practical way of setting off the blade follows: First for dimensions: as 20-in. (pitch) is to 11 in. (length of boss and therefore virtual length of propeller), so is 169.6 in. (circumference due to outer diameter) to the length of circumference occupied by the blade, 169.6 × 11/9 = 20.73, say 20-3/4 in. In Fig. 40 describe a circle equal to the diameter of the propeller, and on each side of the centre line step off 20-3/4 in. to half the scale, making the whole length of arc to scale 20-3/4 in. Draw vertical lines from the ends of the arc, and from the arc on the centre line set up a height of 11 in. and draw horizontal lines. Join _a b_, and this will be the angle of the end of the blade. On the elevation of the propeller circle describe a small circle equal in diameter to the faces of the boss; draw radial lines from the ends of the arc first found, and from the intersection with the boss circle draw vertical lines to cut the horizontal lines of the plan of boss. Join _c d_, and this will be the angle of the blade at the root.

Now describe an arc at every three inches from the circumference within the radical lines; or for large propellers every 6 in. Draw vertical lines from the intersections of the arcs with the radical lines to meet _a c_ and _b d_, as shown, and joining the points thus found, the diagonal lines will represent the plan or angle of the blade to each 3 in. difference of radius--in other words, its real width at the different points, supposing it to be a plain geometrical portion of a screw thread. As a matter of fact, the blades are always more complex than this, the edge being curved to enter the water more easily, to avoid vibration, and also to lessen the risk of fracture in the event of striking any object in the water. Sometimes the blades are curved in the opposite direction, as if the points were being left behind while the blade is advancing.

The next step is to draw a flattened elevation or development of one of the blades, in order to give the actual curves of its outline, and afterward its thickness at various points. Draw a horizontal line from _c_ and _f_ (Fig. 41), and through this a centre line. This will give the length of the blade from the boss, and the centre line of the propeller shaft may be added below. Then take the lengths _a b_ and _c d_ from Fig. 40, and set them off on Fig. 43, as shown, joining all four points. This figure would be the true outline of the blade if there were no curves. The actual outline is found by drawing the curves according to the dimensions.

Lay out the propeller, as shown in Fig. 42, which will give the elevation of the blades, all being alike.

To find the area of a propeller blade, mark it off in parallel lines, say 3 in. apart, and note the width at the centre of each portion. Add the widths together, and divide by the number of widths. This will give the mean width, which must then be multiplied by the length of blade to obtain the area. If the measurements are all in inches, the result should be divided by 144 to give the area in square feet, and then be multiplied by the number of blades to give the total area.

To measure the pitch of a propeller, lay it down on a level surface, hold a straight edge level across centre of blade with a square up from the lower edge, as in Fig. 43. Measure the distance B and H and the radius R from the centre to the part where the measurement is taken; then B : 2wR :: H to pitch, P or P = 2wRH/B. The measurements may be made in more than one place and the average taken, as the blades are sometimes twisted slightly.

Scaling only from the drawing,

P = 2wRH/B

2 × 3.1416 × 1.6 × 1/1.27 = 7.74,

say 7 ft. 9 in. pitch, whereas the intended pitch was 7 ft. 6 in.

A good illustration of the use of the screw may be seen in the carpenter's auger, used for making or boring holes in wood. These tools are provided with a small tapered screw on their points, and this is followed by cutting edges and a larger spiral. The larger spiral is for the purpose of drawing up the chips or shavings. Another tool is made having two blades attached to the bottom of an iron bar formed like the blades of a propeller, which is sometimes employed for boring or digging post holes in clayey or soft soil. The machine is turned by a cross handle on top, and is frequently drawn up to bring out the soil until the hole is deep enough. The ordinary wood screw is one of the most useful of contrivances for fastening wood together, and for attaching to surfaces, hardware, ornaments, or other materials. The adhesive strength of nails is already shown, and the adhesive strength of wood screws, according to Bevan, is set down as follows:

WOOD SCREWS

The following are the thicknesses or diameters corresponding to
the list numbers. Other thicknesses can be interpolated, each size
varying in succession 1/64 in.--

============+====+====+====+====+====+====+====+====+====+====+==== No. | 00 | 0 | 1 | 5 | 10 | 14 | 18 | 22 | 27 | 32 | 40 ------------+----+----+----+----+----+----+----+----+----+----+---- Thicknesses | | | | | | | | | | | in parts | | | | | | | | | | | of inches |1/32|3/64|1/16| 1/8|3/16| 1/4|5/16| 3/8|7/16| 1/2| 5/8 ------------+----+----+----+----+----+----+----+----+----+----+----

An ordinary 2-in. wood screw, driven through a 1/2-in. board into hard wood, was found to be 790 lbs., and a force of about 395 lbs. was required to extract it from soft wood.

When screws are hard to drive or screw in place, a long screw-driver should be used, as screw-drivers with long handles seem to have a much greater leverage than short handled ones in driving screws home. Screws, however, are often split at the head, if care is not taken when using a long driver.

If a screw is rusted, hard to move or withdraw, it can be loosened by applying a hot iron to the head and making it hot. The heat expands the screw and, of course, makes the hole larger, and when the screw cools it contracts a trifle so that it may be withdrawn quite easily.

VII

AEROPLANES

George and Fred were so much interested in the _Caroline_ that they neglected to do some work Mr. Gregg had suggested, but a hint or two from him reminded them that sailing the new boat every day would get so monotonous it would cease to be a pleasure. Fred, therefore, set to work to put the new property in apple-pie order, by cleaning up the grounds, burning the rubbish, and tidying the place generally. Nick, not being needed longer, was allowed to go, with the promise that whenever a man was required about the place, he would be chosen. His departure left all the work to Fred and George, both of whom gladly accepted the duty.

The first thing was to set up three or four long benches on the river bank. These were built exactly in the same manner as the seats alongside the tables. Three short posts were let into the ground for each seat, and a good, sound plank spiked solid to their tops. One of the seats was made four or five inches lower than those at the tables, so as to accommodate the smaller children. The two boys did the work well, though they found it a little hard to dig the holes in the ground and saw off the posts. George's hands became a little blistered and sore, but his mother soon cured them, though she warned him against working too hard or too long at a kind of labour to which he was not accustomed.

After tea was over, it being a fine, warm, spring evening, the whole family went down to the river's edge to sit on the new seats and enjoy the view. Noticing the current of the river, Jessie questioned her father about its going one way sometimes, and then turning in the other direction. Her father explained that it was the movement of the tide that made the water flow against the stream at times, and that when there was no tide, the current took its natural course. This explanation did not seem to satisfy Jessie, and she asked why there were any tides. So Mr. Gregg promised to explain all that was known about tides to her in the near future. "I wish you would," said George, "and tell us about kites, balloons, and flying machines."

"Oh, yes," said the father, "I'll try to do that to-morrow night."

"I'm glad, father," said Fred, "as I want to try and make a model for George before the Fourth if I can, so he can have one to fly across the river that day, instead of fooling with fire-crackers and other dangerous fireworks."

"That's a good idea, Fred," said the father. "A model aeroplane, decorated with silk flags would give a great deal more real pleasure than firing off all the fire crackers in the state. It would be quite easy, now you have a boat, for one of you to be on this side of the river, the other on the opposite side, and to keep a number of little machines going to and fro across the water."

George seemed delighted at the prospect. Walter Scott had also been stricken with the aeroplane fever, and was busy making models, though, as yet, he had not finished any. Both Fred and George were anxious to hear all their father had to say concerning these machines, as they knew he would be thorough, and make it all plain. Mr. Gregg told the boys that to explain fully the theory and practice of building an aeroplane of any kind would take some time, but he would willingly give it for their benefit, and would discuss the subject of aeronautics at length so as to give them some pointers about the design and practical making of flying machines.

THE MONOPLANE MODEL COMPLETE

"A Model Aeroplane, Decorated with Silk Flags Would Give a Great
Deal More Pleasure Than Firing off All the Fire Crackers in the
State"
]

On the following evening, Jessie did not forget to remind her father of his promise to tell them all about "air-ships and things," as she put it.

"All right, my dear," said Mr. Gregg, "I'll take you all into the 'lion's den' shortly after tea. But tell me, why is it you are so anxious to know all about 'air-ships and things'?"

"Oh! that's all right, papa; Fred is going to build a great big ship, as soon as he knows how, and he has promised to take me up to the clouds in it for a ride."

"Well, my dear, it will take some time to tell you all about these things but I will make an attempt. For ages man has wanted to fly, and the Greeks tell us of a mythical personage named Icarius, and another named Dædalus, who flew to the sun. There have been many attempts to fly, both with and without mechanical aid, but history gives us nothing definite on the subject until about the year 1785, when two Frenchmen, named Montgolfiers, built a balloon sixty feet high and forty-three feet in diameter, and filled it with heated air. Attached to the bottom was a light cage made of wicker-work, into which were placed a lamb, a duck, and a rooster. The balloon was cut from its moorings and rose to a height of over 1,400 feet so that these animals were the first that ever went up in a machine made by hands.

"The Montgolfiers attained considerable notoriety, and out of their experiments grew the present dirigible Zeppelin, which measures 446 feet in length, over 42 feet in diameter, and is capable of carrying eight able-bodied men a distance of over 900 miles. This great machine is charged with gas, and driven by four three-bladed propellers, which are run by two gas engines of 110 horse-power. This is simply a monster balloon, suspended in the air by 529 to 700 cubic feet of hydrogen, or coal gas, which is much lighter than ordinary air.

"It may be said there are four distinct kinds of flying machines, each unlike the other in construction and in principle. The first is the old-fashioned balloon which has an envelope or covering of some air-tight fabric, and is inflated with a light gas. To it is attached a framework of some kind called a Nacelle, that carries the aviator, the steering gear, and the necessary engines to operate the propeller or propellers.

"The second kind of flier is the aeroplane, which, as its name indicates, is supplied with 'air planes,' that give it the power of rising and falling at the will of the operator when the machine is in motion. These planes play a very important part in the successful operation of the machine, as I will explain later. The first type of machine is classed as a 'lighter-than-air' machine or a balloon, while the planes of all kinds are classed as 'heavier-than-air' machines. Among other types of 'fliers,' there is the helicoptere, which is raised by screws or propellers on vertical shafts. These revolve rapidly, and drive the machine upward, just as the propeller on the _Caroline_ drives her forward when in rapid motion. Another type, nearly abandoned, is called the ornithoptere, or 'wing flyer.' These machines are built to operate like the wings of a bird, and are provided with the necessary contrivances to work the wings, both vertically and horizontally. This type, like the helicoptere, is not considered practicable, and is virtually abandoned, so that the field is now left altogether to the 'lighter-than-air,' and the aeroplane machines. I do not intend giving you any instruction regarding balloons, or dirigibles, as I think such is unnecessary, but will confine myself altogether to the discussion of aeroplanes.

"It must not be supposed because of the name aeroplane, that the so-called plane is a real plane; it is not. The front edge of an air-ship plane must always be curved, as shown in Fig. 44, so that the air strikes the under surface and is forced under the plane, to buoy up the machine as it moves forward; or, to put it another way, there must be a current of air either natural or artificial on which the machine must float, or it will be drawn by gravitation to the earth. While we cannot see air or wind, we know from experience that it has great power, and for thousands of years ships have been propelled across the seas by this force, acting on sails of some kind. We know how difficult it is to travel against a high wind, and it is this quality in the air that makes it possible to travel through it. The resistance of the atmosphere makes it possible for the aviator to hold his machine suspended in opposition to the laws of gravity, and to drive it forward and upward by means of the revolving propeller acting against this resistance, the motor acting on the same principle and manner as the wheel or propeller of a boat when it is urged forward. If, as I have seen George do, we take a flat stone, a piece of slate, or flat metal, and throw it along the face of the river, in such a manner that its flat surface strikes the surface of the water, it will skim along, striking the water at intervals in its course, until the force given by the hand that threw it is exhausted, when it will drop and sink. The water, though lighter in equal bulk than the stone, is aided by the force given by the hand to buoy up the stone until the force is expended. The curve on the front edge of the planes, when the machine is in motion, really takes in more air than the space allowed for it under normal conditions, and it may be said to be compressed to some extent. If the wind be blowing in the 'teeth' of the machine, the resistance of the air will be greater, and the buoyancy of the machine increased. So, also, if the machine is travelling rapidly, the motion will increase the resistance and the buoyancy at the same time. The moment the propellers stop, gravitation grasps the machine, and if the planes are kept evenly balanced it will quietly and gently descend to the earth. You must particularly bear in mind that wind blowing in the face of a machine tends to hold it up, and that a machine flying rapidly makes its own wind, so that the results are the same.

"The curve on the front of the planes may be called an 'aero-curve,' and much of the success of the machine depends on this curvature of the planes, which gives to the inside of the plane a concave shape of a peculiar character, and to the outside a convex form.

"If you examine the rough drawing I made for you on the blackboard (Fig. 44) you will notice that the upper or convex curve is different from the under or concave one, and it is upon this difference in curvatures that many of the flying qualities of the machine depend. This little section showing the different curves is the one used by many of the successful aviators, though some prefer the form invented by Sir Hiram Maxim, shown in Fig. 44_a_, which does not differ very materially from the previous section shown. In all cases, however, the accepted plane is one of a curved vertical section in which the convex side is uppermost and the upper surface more curved than the lower. Although different authorities disagree as to why this shape of plane is best, all agree that it is so. Sir Hiram Maxim's theory is that the air follows both the upper and lower surfaces of the plane, as shown in Fig. 44_a_, while Phillips holds that the air follows the lower surface of the plane, and, striking the hump, shown at A, Fig. 44, is reflected off the upper surface of the plane, thus forming a partial vacuum on the upper surface, which gives an additional upward pull to the plane. There is, however, little doubt that most of the work is done by the force exerted on the lower surface of the plane.

"Another consideration that enters into the design of the plane is the aspect ratio, or the ratio between the depth of the plane fore and aft, and the width or span. Authorities do not agree about this latter consideration. A practical aspect ratio, one states, is 6 to 1, as, for instance, a plane 39 feet spread by 6 feet 6 inches in depth. In Santos Dumont's _Demoiselle_ the aspect ratio is only 3 to 1. The ideal plane, however, would be a plane of great length and little depth, but this is impossible in the practical machine, as a plane of excessive length would greatly weaken the construction of the machine. Again, the different authorities do not agree as to the shape of the ends of the planes. Lanchester says that an efficient plane must be of rectangular form, and the Voisin and Curtiss planes are rectangular, whereas the wings of the Blériot and the Wright planes are decidedly curved at the tips.

"I will show in other illustrations the method of placing the planes on such machines, as made by Curtiss and some other noted aviators.

"I think I have said sufficient to give you a fair idea of the reason why an aeroplane can be made to navigate the air, but I have not told you how its direction can be controlled. No doubt, if the air were always still and not subject to change, there would be but little difficulty in controlling the direction of the machine, but, unfortunately, this is not the case, so provision has to be made to meet various changes as they occur. A downward current of air causes the plane to change its inclination to the horizontal, so that it will not support the weight, and the machine falls to the ground. To overcome this unsatisfactory state of things, small auxiliary planes are used to counteract the effect of varying air currents. They control the movements of the main planes so that they always bear the same inclination to the horizontal, and they are also used to elevate the machines so as to clear small obstacles. If any great increase in altitude is desired, the speed of the engine must be increased and the planes driven more rapidly through the air, thus giving them more lifting power.

"It may be that in a short time, additional balancing planes will not be necessary, as some other scheme may be invented that will regulate the balance of the aeroplane. Already an Australian inventor, called Roberts, has applied the gyroscope to the aeroplane in order to solve the problem of making it balance automatically. It exerts a balancing force equal to 300 pounds, placed 18 inches on either side of the centre of gravity. The gyroscope is driven by electricity, and controlled by a pendulum which swings right or left, according to the tilt of the aeroplane. Mr. Roberts is also working on a small aeroplane which is to be controlled by wireless telegraphy. His inventions are being tested by the British War Office. There are many other inventors on three continents busily employed in trying to solve the balance problem.

"A very important matter in the construction of the aeroplane is the position of the screw propeller. Sir Hiram Maxim advocates placing it at the rear of the planes, and this construction is carried out in the Wright, Curtiss, Voisin and Baldwin-McCurdy machines, while the tractor screw is used on the Blériot, Antoinette, and Roe fliers. Sir Hiram's theory is that if the screw is placed in front, the backwash strikes the machine, which offers a good deal of resistance to the passage of the air, and retards action; but if the propeller is placed in the rear, the resistance of the machine imparts a forward motion to the air with which it comes in contact, and the screw, running in air that is moving forward, has less slip, and is, therefore, more efficient than the tractor screw.