FIG. 98.—A slightly different form of lever.FIG. 98.—A slightly different form of lever.
Suspend a stick with a hole at its center as in Figure 98, and hang a 4-pound weight at a distance of 1 foot from the fulcrum, supporting the load by means of a spring balance 2 feet from the fulcrum. The pointer on the spring balance shows that the force required to balance the 4-pound load is but 2 pounds.
The force is 2 feet from the fulcrum, and the weight (4) is 1 foot from the fulcrum, so that
Force × distance = Weight × distance,or 2 × 2 = 4 × 1.
FIG. 99.—The wheelbarrow lightend labor.FIG. 99.—The wheelbarrow lightend labor.
Move the 4-pound weight so that it is very near the fulcrum, say but 6 inches from it; then the spring balance registers a force only one fourth as great as the weight which it suspends. In other words a force of 1 at a distance of 24 inches (2 feet) is equivalent to a force of 4 at a distance of 6 inches.
FIG. 100.—A modified wheelbarrow.FIG. 100.—A modified wheelbarrow.
One of the most useful levers of this type is the wheelbarrow (Fig. 99). The fulcrum is at the wheel, the force is at the handles, the weight is on the wheelbarrow. If the load is halfway from the fulcrum to the man's hands, the man will have to lift with a force equal to one half the load. If the load is one fourth as far from the fulcrum as the man's hands, he will need to lift with a force only one fourth as great as that of the load.
This shows that in loading a wheelbarrow, it is important to arrangethe load as near to the wheel as possible.
FIG. 101.—The nutcracker is a lever.FIG. 101.—The nutcracker is a lever.
The nutcracker (Fig. 101) is an illustration of a double lever of the wheelbarrow kind; the nearer the nut is to the fulcrum, the easier the cracking.
FIG. 102.—The hand exerts a small force over a long distance and draws out a nail.FIG. 102.—The hand exerts a small force over a long distance and draws out a nail.
Hammers (Fig. 102), tack-lifters, scissors, forceps, are important levers, and if you will notice how many different levers (fig. 103) are used by all classes of men, you will understand how valuable a machine this simple device is.
155. The Inclined Plane.A man wishes to load the 600-pound bowlder on a wagon, and proceeds to do it by means of a plank, as in Figure 93. Such an arrangement is called an inclined plane.
The advantage of an inclined plane can be seen by the following experiment. Select a smooth board 4 feet long and prop it so that the endA(Fig. 104) is 1 foot above the level of the table; the length of the incline is then 4 times as great as its height. Fasten a metal roller to a spring balance and observe its weight. Then pull the roller uniformly upward along the plank and notice what the pull is on the balance, being careful always to hold the balance parallel to the incline.
When the roller is raised along the incline, the balance registers a pull only one fourth as great as the actual weight of the roller. That is, when the roller weighs 12, a force of 3 suffices to raise it to the heightAalong the incline; but the smaller force must be applied throughout the entire length of the incline. In many cases, it is preferable to exert a force of 30 pounds, for example, over the distanceCAthan a force of 120 pounds over the shorter distanceBA.
FIG. 103.—Primitive man tried to lighten his task by balancing his burden.FIG. 103.—Primitive man tried to lighten his task by balancing his burden.
Prop the board so that the endAis 2 feet above the table level; that is, arrange the inclined plane in such a way that its length is twice as great as its height. In that case the steady pull on the balance will be one half the weight of the roller; or a force of 6 pounds will suffice to raise the 12-pound roller.
FIG. 104.—Less force is required to raise the roller along the incline than to raise it to A directly.FIG. 104.—Less force is required to raise the roller along the incline than to raise it to A directly.
The steeper the incline, the more force necessary to raise a weight; whereas if the incline is small, the necessary lifting force is greatly reduced. On an inclined plane whose length is ten times its height, the lifting force is reduced to one tenth the weight of the load. The advantage of an incline depends upon the relative length and height, or is equal to the ratio of the length to the height.
156. Application.By the use of an inclined plank a strong man can load the 600-pound bowlder on a wagon. Supposethe floor of the wagon is 2 feet above the ground, then if a 6-foot plank is used, 200 pounds of force will suffice to raise the bowlder; but the man will have to push with this force against the bowlder while it moves over the entire length of the plank.
Since work is equal to force multiplied by distance, the man has done work represented by 200 × 6, or 1200. This is exactly the amount of work which would have been necessary to raise the bowlder directly. A man of even enormous strength could not lift such a weight (600 lb.) even an inch directly, but a strong man can furnish the smaller force (200) over a distance of 6 feet; hence, while the machine does not lessen the total amount of work required of a man, it creates a new distribution of work and makes possible, and even easy, results which otherwise would be impossible by human agency.
157. Railroads and Highways.The problem of the incline is an important one to engineers who have under their direction the construction of our highways and the laying of our railroad tracks. It requires tremendous force to pull a loadup grade, and most of us are familiar with the struggling horse and the puffing locomotive. For this reason engineers, wherever possible, level down the steep places, and reduce the strain as far as possible.
FIG. 105.—A well-graded railroad bed.FIG. 105.—A well-graded railroad bed.
The slope of the road is called its grade, and the grade itself is simply the number of feet the hill rises per mile. A road a mile long (5280 feet) has a grade of 132 if the crest of the hill is 132 feet above the level at which the road started.
FIG. 106.—A long, gradual ascent is better than a shorter, steeper one.FIG. 106.—A long, gradual ascent is better than a shorter, steeper one.
In such an incline, the ratio of length to height is 5280 ÷ 132, or 40; and hence in order to pull a train of cars to the summit, the engine would need to exert a continuous pull equal to one fortieth of the combined weight of the train.
If, on the other hand, the ascent had been gradual, so that the grade was 66 feet per mile, a pull from the engine of one eightieth of the combined weight would have sufficed to land the train of cars at thecrest of the grade.
Because of these facts, engineers spend great sums in grading down railroad beds, making them as nearly level as possible. In mountainous regions, the topography of the land prevents the elimination of all steep grades, but nevertheless the attempt is always made to follow the easiest grades.
158. The Wedge.If an inclined plane is pushed underneath or within an object, it serves as a wedge. Usually a wedge consists of two inclined planes (Fig. 107).
FIG. 107.—By means of a wedge, the stump is split.FIG. 107.—By means of a wedge, the stump is split.
A chisel and an ax are illustrations of wedges. Perhaps the most universal form of a wedge is our common pin. Can you explain how this is a wedge?
159. The Screw.Another valuable and indispensable form of the inclined plane is the screw. This consists of a metal rod around which passes a ridge, and Figure 108 shows clearly that a screw is simply a rod around which (in effect) an inclined plane has been wrapped.
FIG. 108—A screw as a simple machine.FIG. 108—A screw as a simple machine.
The ridge encircling the screw is called the thread, and the distance between two successive threads is called the pitch. It is easy to see that the closer the threads and the smaller the pitch, the greater the advantage of the screw, and hence the less force needed in overcoming resistance. A corkscrew is a familiar illustration of the screw.
160. Pulleys.The pulley, another of the machines, is merely a grooved wheel around which a cord passes. It is sometimes more convenient to move a load in one direction rather than in another, and the pulley in its simplest form enables us to do this. In order to raise a flag to the top ofa mast, it is not necessary to climb the mast, and so pull up the flag; the same result is accomplished much more easily by attaching the flag to a movable string, somewhat as in Figure 109, and pulling from below. As the string is pulled down, the flag rises and ultimately reaches the desired position.
If we employ a stationary pulley, as in Figure 109, we do not change the force, because the force required to balance the load is as large as the load itself. The only advantage is that a force in one direction may be used to produce motion in another direction. Such a pulley is known as a fixed pulley.
FIG. 109.—By means of a pulley, a force in one direction produces motion in the opposite direction.FIG. 109.—By means of a pulley, a force in one direction produces motion in the opposite direction.
161. Movable Pulleys.By the use of a movable pulley, we are able to support a weight by a force equal to only one half the load. In Figure 109, the downward pull of the weight and the downward pull of the hand are equal; in Figure 110, the spring balance supports only one half the entire load, the remaining half being borne by the hook to which the string is attached. The weight is divided equally between the two parts of the string which passes around the pulley, so that each strand bears only one half of the burden.
We have seen in our study of the lever and the inclined plane that an increase in force is always accompanied by a decrease in distance, and in the case of the pulley we naturally look for a similar result. If you raise the balance (Fig. 110) 12 feet, you will find that the weight rises only 6 feet; if you raise the balance 24 inches, you will find that the weight rises12 inches. You must exercise a force of 100 pounds over 12 feet of space in order to raise a weight of 200 pounds a distance of 6 feet. When we raise 100 pounds through 12 feet or 200 pounds through 6 feet the total work done is the same; but the pulley enables those who cannot furnish a force of 200 pounds for the space of 6 feet to accomplish the task by furnishing 100 pounds for the space of 12 feet.
FIG. 110.—A movable pulley lightens labor.FIG. 110.—A movable pulley lightens labor.
FIG. 111.—An effective arrangement of pulleys known as block and tackle.FIG. 111.—An effective arrangement of pulleys known as block and tackle.
162. Combination of Pulleys.A combination of pulleys called block and tackle is used where very heavy loads are to be moved. In Figure 111 the upper block of pulleys is fixed, the lower block is movable, and one continuous rope passes around the various pulleys. The load is supported by 6 strands, and each strand bears one sixth of the load. If the hand pulls with a force of 1 pound atP, it can raise a load of 6 pounds atW, but the hand will have to pull downward 6 feet atPin order to raise the load atW1 foot. If 8 pulleys were used, a force equivalent to one eighth of the load would suffice to moveW, but this force would have to be exerted over a distance 8 times as great as that through whichWwas raised.
163. Practical Application.In our childhood many of us saw with wonder the appearance and disappearance of flagsflying at the tops of high masts, but observation soon taught us that the flags were raised by pulleys. In tenements, where there is no yard for the family washing, clothes often appear flapping in mid-air. This seems most marvelous until we learn that the lines are pulled back and forth by pulleys at the window and at a distant support. By means of pulleys, awnings are raised and lowered, and the use of pulleys by furniture movers, etc., is familiar to every wide-awake observer on the streets.
164. Wheel and Axle.The wheel and axle consists of a large wheel and a small axle so fastened that they rotate together.
FIG. 112.—The wheel and axle.FIG. 112.—The wheel and axle.
When the large wheel makes one revolution,Pfalls a distance equal to the circumference of the wheel. WhilePmoves downward,Wlikewise moves, but its motion is upward, and the distance it moves is small, being equal only to the circumference of the small axle. But a small force atPwill sustain a larger force atW; if the circumference of the large wheel is 40 inches, and that of the small wheel 10 inches, a load of 100 atWcan be sustained by a force of 25 atP. The increase in force of the wheel and axle depends upon the relative size of the two parts, that is, upon the circumference of wheel as compared with circumference of axle, and since the ratio between circumference and radius is constant, the ratio of the wheel and axle combination is the ratio of the long radius to the short radius.
For example, in a wheel and axle of radii 20 and 4, respectively, agiven weight atPwould balance 5 times as great a load atW.
165. Application.Windlass, Cogwheels.In the old-fashioned windlass used in farming districts, the large wheel is replaced by a handle which, when turned, describes a circle. Such an arrangement is equivalent to wheel and axle (Fig. 112); the capstan used on shipboard for raising the anchor has the same principle. The kitchen coffee grinder and the meat chopper are other familiar illustrations.
Cogwheels are modifications of the wheel and axle. Teeth cut inAfit into similar teeth cut inB, and hence rotation ofAcauses rotation ofB. Several revolutions of the smaller wheel, however, are necessary in order to turn the larger wheel through one complete revolution; if the radius ofAis one half that ofB, two revolutions ofAwill correspond to one ofB; if the radius ofAis one third that ofB, three revolutions ofAwill correspond to one ofB.
FIG. 113.—Cogwheels.FIG. 113.—Cogwheels.
Experiment demonstrates that a weightWattached to a cogwheel of radius 3 can be raised by a forceP, equal to one third ofWapplied to a cogwheel of radius 1. There is thus a great increase in force. But the speed with whichWis raised is only one third the speed with which the small wheel rotates, or increase in power has been at the decrease of speed.
This is a very common method for raising heavy weights by small force.
Cogwheels can be made to give speed at the decrease of force. A heavy weightWattached toBwill in its slow fall cause rapid rotation ofA, and hence rapid rise ofP. It is true thatP, the load raised, will be less thanW, the force exerted, but if speed is ouraim, this machine serves our purpose admirably.
An extremely important form of wheel and axle is that in which the two wheels are connected by belts as in Figure 114. Rotation ofWinduces rotation ofw, and a small force atWis able to overcome a large force atw. An advantage of the belt connection is that power at one place can be transmitted over a considerable distance and utilized in another place.
FIG. 114.—By means of a belt, motion can be transferred from place to place.FIG. 114.—By means of a belt, motion can be transferred from place to place.
166. Compound Machines.Out of the few simple machines mentioned in the preceding Sections has developed the complex machinery of to-day. By a combination of screw and lever, for example, we obtain the advantage due to each device, and some compound machines have been made which combine all the various kinds of simple machines, and in this way multiply their mechanical advantage many fold.
A relatively simple complex machine called the crane (Fig. 116) maybe seen almost any day on the street, or wherever heavy weights are being lifted. It is clear that a force applied to turn wheel 1 causes a slower rotation of wheel 3, and a still slower rotation of wheel 4, but as 4 rotates it winds up a chain and slowly raisesQ. A very complex machine is that seen in Figure 117.
FIG. 115.—A simple derrick for raising weights.FIG. 115.—A simple derrick for raising weights.
FIG. 116.—A traveling crane.FIG. 116.—A traveling crane.
167. Measurement of Work.In Section 150, we learned that the amount of work done depends upon the forceexerted, and the distance covered, or thatW= force × distance. A man who raises 5 pounds a height of 5 feet does far more work than a man who raises 5 ounces a height of 5 inches, but the product of force by distance is 25 in each case. There is difficulty because we have not selected an arbitrary unit of work. The unit of work chosen and in use in practical affairs is the foot pound, and is defined as the work done when a force of 1 pound acts through a distance of 1 foot. A man who moves 8 pounds through 6 feet does 48 foot pounds of work, while a man who moves 8 ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth of a foot pound of work.
FIG. 117.—A farm engine putting in a crop.FIG. 117.—A farm engine putting in a crop.
168. The Power or the Speed with which Work is Done.A man can load a wagon more quickly than a growing boy. The work done by the one is equal to the work done by the other, but the man is more powerful, because the time required for a given task is very important. An engine which hoists a 50-pound weight in 1 second is much more powerful than a man who requires 50 seconds for the same task; hence in estimating the value of a working agent, whether animal or mechanical, we must consider not only the work done, but the speed with which it is done.
The rate at which a machine is able to accomplish a unit of work is calledpower, and the unit of power customarily used is the horse power. Any power which can do 550 foot pounds of work per second is said to be one horse power (H.P.). This unit was chosen by James Watt, the inventor of a steam engine, when he was in need of a unit with which to compare the new source of power, the engine, with his old source of power, the horse. Although called a horse power it is greater than the power of an average horse.
An ordinary man can do one sixth of a horse power. The average locomotive of a railroad has more than 500 H.P., while the engines of an ocean liner may have as high as 70,000 H.P.
169. Waste Work and Efficient Work.In our study of machines we omitted a factor which in practical cases cannot be ignored, namely, friction. No surface can be made perfectly smooth, and when a barrel rolls over an incline, or a rope passes over a pulley, or a cogwheel turns its neighbor, there is rubbing and slipping and sliding. Motion is thus hindered, and the effective value of the acting force is lessened. In order to secure the desired result it is necessary to apply a force in excess of that calculated. This extra force, which must be supplied if friction is to be counteracted, is in realitywaste work.
If the force required by a machine is 150 pounds, while that calculated as necessary is 100 pounds, the loss due to friction is 50 pounds, and the machine, instead of being thoroughly efficient, is only two thirds efficient.
Machinists make every effort to eliminate from a machine the waste due to friction, leveling and grinding to the most perfect smoothness and adjustment every part of the machine. When the machine is in use, friction may be further reduced by the use of lubricating oil. Friction can never be totally eliminated, however, and machines of even the finest construction lose by friction some of their efficiency, while poorly constructed ones lose by friction as much as one half of their efficiency.
FIG. 118.—Man's strength is not sufficient for heavy work.FIG. 118.—Man's strength is not sufficient for heavy work.
170. Man's Strength not Sufficient for Machines.A machine, an inert mass of metal and wood, cannot of itself do any work, but can only distribute the energy which is brought to it. Fortunately it is not necessary that this energy should be contributed by man alone, because the store of energy possessed by him is very small in comparison with the energy required to run locomotives, automobiles, sawmills, etc. Perhaps the greatest value of machines lies in the fact that they enable man to perform work by the use of energy other than his own.
Figure 118 shows one way in which a horse's energy can be utilized in lifting heavy loads. Even the fleeting wind has been harnessed by man, and, as in the windmill, made to workfor him (Fig. 119). One sees dotted over the country windmills large and small, and in Holland, the country of windmills, the landowner who does not possess a windmill is poor indeed.
For generations running water from rivers, streams, and falls has served man by carrying his logs downstream, by turning the wheels of his mill, etc.; and in our own day running water is used as an indirect source of electric lights for street and house, the energy of the falling water serving to rotate the armature of a dynamo (Section 310).
A more constant source of energy is that available from the burning of fuel, such as coal and oil. The former is the source of energy in locomotives, the latter in most automobiles.
FIG. 119.—The windmill pumps water into the troughs where cattle drink.FIG. 119.—The windmill pumps water into the troughs where cattle drink.
In the following Chapter will be given an account of water, wind, and fuel as machine feeders.
171.Small boys soon learn the power of running water; swimming or rowing downstream is easy, while swimming or rowing against the current is difficult, and the swifter the water, the easier the one and the more difficult the other; the river assists or opposes us as we go with it or against it. The water of a quiet pool or of a gentle stream cannot do work, but water which is plunging over a precipice or dam, or is flowing down steep slopes, may be made to saw wood, grind our corn, light our streets, run our electric cars, etc. A waterfall, or a rapid stream, is a great asset to any community, and for this reason should be carefully guarded. Water power is as great a source of wealth as a coal bed or a gold mine.
The most tremendous waterfall in our country is Niagara Falls, which every minute hurls millions of gallons of water down a 163-foot precipice. The energy possessed by such an enormous quantity of water flowing at such a tremendous speed is almost beyond everyday comprehension, and would suffice to run the engines of many cities far and near. Numerous attempts to buy from the United States the right to utilize some of this apparently wasted energy have been made by various commercial companies. It is fortunate that these negotiations have been largely fruitless, because much deviation of the water for commercial uses and the installation of machinery in the vicinity of the famous falls would greatlydetract from the beauty of this world-known scene, and would rob our country of a natural beauty unequaled elsewhere.
FIG. 120.—A mountain stream turns the wheels of the mill.FIG. 120.—A mountain stream turns the wheels of the mill.
172. Water Wheels.In Figure 120 the water of a small but rapid mountain stream is made to rotate a large wheel, which in turn communicates its motion through belts to a distant sawmill or grinder. In more level regions huge dams are built which hold back the water and keep it at a higher level than the wheel; from the dam the water is conveyed in pipes (flumes) to the paddle wheel which it turns. Cogwheels or belts connect the paddle wheel with the factory machinery, so that motion of the paddle wheel insures the running of the machinery.
FIG. 121.—The Pelton water wheel.FIG. 121.—The Pelton water wheel.
One of the most efficient forms of water wheels is that shown in Figure 121, and called the Pelton wheel. Water issues in a narrow jet similar to that of the ordinary garden hose and strikes with great force against the lower part of the wheel, thereby causing rotation of the wheel.Belts transfer this motion to the machinery of factory or mill.
173. Turbines. The most efficient form of water motor is the turbine, a strong metal wheel shaped somewhat like a pin wheel, inclosed in a heavy metal case.
FIG. 122—A turbine at Niagara Falls.FIG. 122—A turbine at Niagara Falls.
Water is conveyed from a reservoir or dam through a pipe (penstock) to the turbine case, in which is placed the heavy metal turbine wheel (Fig. 122). The force of the water causes rotation of the turbine and of the shaft which is rigidly fastened to it. The water which flows into the turbine case causes rotation of the wheel, escapes from the case through openings, and flows into the tail water.
The power which a turbine can furnish depends upon the quantity of water and the height of the fall, and also upon the turbine wheel itself. One of the largest turbines known has a horse power of about 20,000; that is, it is equivalent, approximately, to 20,000 horses.
174. How much is a Stream Worth?The work which a stream can perform may be easily calculated. Suppose, for example, that 50,000 pounds of water fall over a 22-foot dam every second; the power of such a stream would be 1,100,000 foot pounds per second or 2000 H.P. Naturally, a part of this power would be lost to use by friction within the machinery and by leakage, so that the power of a turbine run by a 2000 H.P. stream would be less than that value.
Of course, the horse power to be obtained from a stream determines the size of the paddle wheel or turbine which canbe run by it. It would be possible to construct a turbine so large that the stream would not suffice to turn the wheel; for this reason, the power of a stream is carefully determined before machine construction is begun, and the size of the machinery depends upon the estimates of the water power furnished by expert engineers.
A rough estimate of the volume of a stream may be made by the method described below:—
Suppose we allow a stream of water to flow through a rectangular trough; the speed with which the water flows through the trough can be determined by noting the time required for a chip to float the length of the trough; if the trough is 10 feet long and the time required is 5 seconds, the water has a velocity of 2 feet per second.
The quantity of water which flows through the trough each second depends upon the dimensions of the trough and the velocity of the water. Suppose the trough is 5 feet wide and 3 feet high, or has a cross section of 15 square feet. If the velocity of the water were 1 foot per second, then 15 cubic feetof water would pass any given point each second, but since the velocity of the water is 2 feet per second, 30 cubic feet will represent the amount of water which will flow by a given point in one second.
FIG. 123.—Estimating the quantity of water which flows through the trough each second.FIG. 123.—Estimating the quantity of water which flows through the trough each second.
175. Quantity of Water Furnished by a River.Drive stakes in the river at various places and note the time required for a chip to float from one stake to another. If we know the distance between the stakes and the time required for the chip to float from one stake to another, the velocity of the water can be readily determined.
The width of the stream from bank to bank is easily measured, and the depth is obtained in the ordinary way by sounding; it is necessary to take a number of soundings because the bed of the river is by no means level, and soundings taken at only one level would not give an accurate estimate. If the soundings show the following depths: 30, 25, 20, 32, 28, the average depth could be taken as 30 + 25 + 20 + 32 + 28 ÷ 5, or 27 feet. If, as a result of measuring, the river at a given point in its course is found to be 27 feet deep and 60 feet wide, the area of a cross section at that spot would be 1620 square feet, and if the velocity proved to be 6 feet per second, then the quantity of water passing in any one second would be 1620 × 6, or 9720 cubic feet. By experiment it has been found that 1 cu. ft. of water weighs about 62.5 lb. The weight of the water passing each second would therefore be 62.5 × 9720, or 607,500 lb. If this quantity of water plunges over a 10-ft. dam, it does 607,500 × 10, or 6,075,000 foot pounds of work per second, or 11,045 H.P. Such a stream would be very valuable for the running of machinery.
176. Windmills.Those of us who have spent our vacation days in the country know that there is no ready-made water supply there as in the cities, but that as a rule the farmhouses obtain their drinking water from springs and wells. In poorerhouses, water is laboriously carried in buckets from the spring or is lifted from the well by the windlass. In more prosperous houses, pumps are installed; this is an improvement over the original methods, but the quantity of water consumed by the average family is so great as to make the task of pumping an arduous one.
The average amount of water used per day by one person is 25 gallons. This includes water for drinking, cooking, dish washing, bathing, laundry. For a family of five, therefore, the daily consumption would be 125 gallons; if to this be added the water for a single horse, cow, and pig, the total amount needed will be approximately 150 gallons per day. A strong man can pump that amount from an ordinary well in about one hour, but if the well is deep, more time and strength are required.
FIG. 124.—The toy pin wheel is a miniature windmill.FIG. 124.—The toy pin wheel is a miniature windmill.
The invention of the windmill was a great boon to country folks because it eliminated from their always busy life one task in which labor and time were consumed.
177. The Principle of the Windmill.The toy pin wheel is a windmill in miniature. The wind strikes the sails, and causes rotation; and the stronger the wind blows, the faster will the wheel rotate. In windmills, the sails are of wood or steel, instead of paper, but the principle is identical.
As the wheel rotates, its motion is communicated to a mechanical device which makes use of it to raise and lower a plunger, and hence as long as the wind turns the windmill, water is raised. The water thus raised empties into a large tank, built either in the windmill tower or in the garret of the house, and fromthe tank the water flows through pipes to the different parts of the house. On very windy days the wheel rotates rapidly, and the tank fills quickly; in order to guard against an overflow from the tank a mechanical device is installed which stops rotation of the wheel when the tank is nearly full. The supply tank is usually large enough to hold a supply of water sufficient for several days, and hence a continuous calm of a day or two does not materially affect the house flow. When once built, a windmill practically takes care of itself, except for oiling, and is an efficient and cheap domestic possession.
FIG. 125.—The windmill pumps water into the tank.FIG. 125.—The windmill pumps water into the tank.
178. Steam as a Working Power.If a delicate vane is held at an opening from which steam issues, the pressure of the steam will cause rotation of the vane (Fig. 126), and if the vane is connected with amachine, work can be obtained from the steam.
FIG. 126.—Steam as a source of power.FIG. 126.—Steam as a source of power.
When water is heated in an open vessel, the pressure of its steam is too low to be of practical value, but if on the contrary water is heated in an almost closed vessel, its steam pressure is considerable. If steam at high pressure is directed by nozzles against the blades of a wheel, rapid rotation of the wheel ensues just as it did in Figure 121, although in this case steam pressure replaces water pressure. After the steam has spent itself in turning the turbine, it condenses into water and makes its escape through openings in an inclosing case. In Figure 127 the protecting case is removed, in order that the form of the turbine and the positions of the nozzles may be visible.
FIG. 127.—Steam turbine with many blades and 4 nozzles.FIG. 127.—Steam turbine with many blades and 4 nozzles.
A single large turbine wheel may have as many as 800,000 sails or blades, and steam may pour out upon these from many nozzles.
FIG. 128.—The principle of the steam engine.FIG. 128.—The principle of the steam engine.
The steam turbine is very much more efficient than its forerunner, the steam engine. The installation of turbineson ocean liners has been accompanied by great increase in speed, and by an almost corresponding decrease in the cost of maintenance.
179. Steam Engines.A very simple illustration of the working of a steam engine is given in Figure 128. Steam under pressure enters through the openingF, passes throughN, and presses upon the pistonM. As a resultMmoves downward, and thereby induces rotation in the large wheelL.
AsMfalls it drives the air inDout throughOandP(the openingPis not visible in the diagram).
As soon as this is accomplished, a mechanical device draws up the rodE, which in turn closes the openingN, and thus prevents the steam from passing into the part ofDaboveM.
But when the rodEis in such a position thatNis closed,Oon the other hand is open, and steam rushes through it intoDand forces up the piston. This up-and-down motion of the piston causes continuous rotation of the wheelL. If the fire is hot, steam is formed quickly, and the piston moves rapidly; if the fire is low,steam is formed slowly, and the piston moves less rapidly.
The steam engine as seen on our railroad trains is very complex, and cannot be discussed here; in principle, however, it is identical with that just described. Figure 129 shows a steam harvester at work on a modern farm.
FIG. 129.—Steam harvester at work.FIG. 129.—Steam harvester at work.
In both engine and turbine the real source of power is not the steam but the fuel, such as coal or oil, which converts the water into steam.
180. Gas Engines.Automobiles have been largely responsible for the gas engine. To carry coal for fuel and water for steam would be impracticable for most motor cars. Electricity is used in some cars, but the batteries are heavy, expensive, and short-lived, and are not always easily replaceable. For this reason gasoline is extensively used, and in the average automobile the source of power is the force generated by exploding gases.
It was discovered some years ago that if the vapor of gasoline or naphtha was mixed with a definite quantity of air, and a light was applied to the mixture, an explosion would result. Modern science uses the force of such exploding gases for the accomplishment of work, suchas running of automobiles and launches.
FIG. 130.—The gas engine.FIG. 130.—The gas engine.
In connection with the gasoline supply is a carburetor or sprayer, from which the cylinderC(Fig. 130) receives a fine mist of gasoline vapor and air. This mixture is ignited by an automatic, electric sparking device, and the explosion of the gases drives the pistonPto the right. In the 4-cycle type of gas engines (Fig. 130)—the kind used in automobiles—the four strokes are as follows: 1. The mixture of gasoline and air enters the cylinder as the piston moves to the right. 2. The valves being closed, the mixture is compressed as the piston moves to the left. 3. The electric spark ignites the compressed mixture and drives the piston to the right. 4. The waste gas is expelled as the piston moves to the left. The exhaust valve is then closed, the inlet valve opened, and another cycle of four strokes begins.
The use of gasoline in launches and automobiles is familiar to many. Not only are launches and automobiles making use of gas power, but the gasoline engine has made it possible to propel aëroplanes through the air.
181."As difficult as for water to run up a hill!" Is there any one who has not heard this saying? And yet most of us accept as a matter of course the stream which gushes from our faucet, or give no thought to the ingenuity which devised a means of forcing water upward through pipes. Despite the fact that water flows naturally down hill, and not up, we find it available in our homes and office buildings, in some of which it ascends to the fiftieth floor; and we see great streams of it directed upon the tops of burning buildings by firemen in the streets below.
In the country, where there are no great central pumping stations, water for the daily need must be raised from wells, and the supply of each household is dependent upon the labor and foresight of its members. The water may be brought to the surface either by laboriously raising it, bucket by bucket, or by the less arduous method of pumping. These are the only means possible; even the windmill does not eliminate the necessity for the pump, but merely replaces the energy used by man in working it.
In some parts of our country we have oil beds or wells. But if this underground oil is to be of service to man, it must be brought to the surface, and this is accomplished, as in the case of water, by the use of pumps.
An old tin can or a sponge may serve to bale out water from a leaking rowboat, but such a crude device would beabsurd if employed on our huge vessels of war and commerce. Here a rent in the ship's side would mean inevitable loss were it not possible to rid the ship of the inflowing water by the action of strong pumps.
Another and very different use to which pumps are put is seen in the compression of gases. Air is forced into the tires of bicycles and automobiles until they become sufficiently inflated to insure comfort in riding. Some present-day systems of artificial refrigeration (Section 93) could not exist without the aid of compressed gases.
Compressed air has played a very important role in mining, being sent into poorly ventilated mines to improve the condition of the air, and to supply to the miners the oxygen necessary for respiration. Divers and men who work under water carry on their backs a tank of compressed air, and take from it, at will, the amount required.
There are many forms of pumps, and they serve widely different purposes, being essential to the operation of many industrial undertakings. In the following Sections some of these forms will be studied.