General Science Part 15

175. Quant.i.ty 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 quant.i.ty of water pa.s.sing 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 pa.s.sing each second would therefore be 62.5 9720, or 607,500 lb. If this quant.i.ty 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 poorer houses, water is laboriously carried in buckets from the spring or is lifted from the well by the windla.s.s. In more prosperous houses, pumps are installed; this is an improvement over the original methods, but the quant.i.ty 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.

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.

[Ill.u.s.tration: FIG. 124.--The toy pin wheel is a miniature windmill.]

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 from the 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.

[Ill.u.s.tration: 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 a machine, work can be obtained from the steam.

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.

[Ill.u.s.tration: FIG. 126.--Steam as a source of power.]

[Ill.u.s.tration: 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.

The steam turbine is very much more efficient than its forerunner, the steam engine. The installation of turbines on 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 ill.u.s.tration of the working of a steam engine is given in Figure 128. Steam under pressure enters through the opening _F_, pa.s.ses through _N_, and presses upon the piston _M_. As a result _M_ moves downward, and thereby induces rotation in the large wheel _L_.

[Ill.u.s.tration: FIG. 128.--The principle of the steam engine.]

As _M_ falls it drives the air in _D_ out through _O_ and _P_ (the opening _P_ is not visible in the diagram).

As soon as this is accomplished, a mechanical device draws up the rod _E_, which in turn closes the opening _N_, and thus prevents the steam from pa.s.sing into the part of _D_ above _M_.

But when the rod _E_ is in such a position that _N_ is closed, _O_ on the other hand is open, and steam rushes through it into _D_ and forces up the piston. This up-and-down motion of the piston causes continuous rotation of the wheel _L_. 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.

[Ill.u.s.tration: 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 quant.i.ty 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, such as running of automobiles and launches.

In connection with the gasoline supply is a carburetor or sprayer, from which the cylinder _C_ (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 piston _P_ to 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.

[Ill.u.s.tration: FIG. 130.--The gas engine.]

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 aeroplanes through the air.

CHAPTER XVIII

PUMPS AND THEIR VALUE TO MAN

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 be absurd 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.

[Ill.u.s.tration: FIG. 131.--Carrying water home from the spring.]

182. The Air as Man's Servant. Long before man harnessed water for turbines, or steam for engines, he made the air serve his purpose, and by means of it raised water from hidden underground depths to the surface of the earth; likewise, by means of it, he raised to his dwelling on the hillside water from the stream in the valley below.

Those who live in cities where running water is always present in the home cannot realize the hardship of the days when this "ready-made"

supply did not exist, but when man laboriously carried to his dwelling, from distant spring and stream, the water necessary for the daily need.

What are the characteristics of the air which have enabled man to accomplish these feats? They are well known to us and may be briefly stated as follows:--

(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure, weighs 1-1/4 ounces.

(2) The air around us presses with a force of about 15 pounds upon every square inch of surface that it touches.

(3) Air is elastic; it can be compressed, as in the balloon or bicycle tire, but it expands immediately when pressure is reduced. As it expands and occupies more s.p.a.ce, its pressure falls and it exerts less force against the matter with which it comes in contact. If, for example, 1 cubic foot of air is allowed to expand and occupy 2 cubic feet of s.p.a.ce, the pressure which it exerts is reduced one half. When air is compressed, its pressure increases, and it exerts a greater force against the matter with which it comes in contact. If 2 cubic feet of air are compressed to 1 cubic foot, the pressure of the compressed air is doubled. (See Section 89.)