A boat sailing across the wind is acted on in a similar manner (Fig.

170). The wind strikes the sail obliquely, and would thrust it to leeward were it not for the opposition of the water. The force A is resolved into forces B and C, of which C propels the boat on the line of its axis. The boat can be made to sail even "up" the wind, her head being brought round until a point is reached at which the force B on the boat, masts, etc., overcomes the force C. The capability of a boat for sailing up wind depends on her "lines" and the amount of surface she offers to the wind.

THE BALLOON

is a pear-shaped bag--usually made of silk--filled with some gas lighter than air. The tendency of a heavier medium to displace a lighter drives the gas upwards, and with it the bag and the wicker-work car attached to a network encasing the bag. The tapering neck at the lower end is open, to permit the free escape of gas as the atmospheric pressure outside diminishes with increasing elevation. At the top of the bag is a wooden valve opening inwards, which can be drawn down by a rope pa.s.sing up to it through the neck whenever the aeronaut wishes to let gas escape for a descent. He is able to cause a very rapid escape by pulling another cord depending from a "ripping piece" near the top of the bag. In case of emergency this is torn away bodily, leaving a large hole. The ballast (usually sand) carried enables him to maintain a state of equilibrium between the upward pull of the gas and the downward pull of gravity. To sink he lets out gas, to rise he throws out ballast; and this process can be repeated until the ballast is exhausted. The greatest height ever attained by aeronauts is the 7-1/4 miles, or 37,000 feet, of Messrs.

Glaisher and c.o.xwell on September 5, 1862. The ascent nearly cost them their lives, for at an elevation of about 30,000 feet they were partly paralyzed by the rarefaction of the air, and had not Mr. c.o.xwell been able to pull the valve rope with his teeth and cause a descent, both would have died from want of air.

[Ill.u.s.tration: FIG. 171.]

The _flying-machine_, which scientific engineers have so long been trying to produce, will probably be quite independent of balloons, and will depend for its ascensive powers on the action of air on oblique surfaces. Sir Hiram Maxim's experimental air-ship embodied the principles shown by Fig. 171. On a deck was mounted an engine, E, extremely powerful for its weight. This drove large propellers, S S.

Large aeroplanes, of canvas stretched over light frameworks, were set up overhead, the forward end somewhat higher than the rear. The machine was run on rails so arranged as to prevent it rising. Unfortunately an accident happened at the first trial and destroyed the machine.

In actual flight it would be necessary to have a vertical rudder for altering the horizontal direction, and a horizontal "tail" for steering up or down. The principle of an aeroplane is that of the kite, with this difference, that, instead of moving air striking a captive body, a moving body is propelled against more or less stationary air. The resolution of forces is shown by the arrows as before.

Up to the present time no practical flying-machine has appeared. But experimenters are hard at work examining the conditions which must be fulfilled to enable man to claim the "dominion of the air."

[34] The "Romance of Modern Mechanism," p. 243

Chapter XVIII.

HYDRAULIC MACHINERY.

The siphon--The bucket pump--The force-pump--The most marvellous pump--The blood channels--The course of the blood--The hydraulic press--Household water-supply fittings--The ball-c.o.c.k--The water-meter--Water-supply systems--The household filter--Gas traps--Water engines--The cream separator--The "hydro."

In the last chapter we saw that the pressure of the atmosphere is 15 lbs. to the square inch. Suppose that to a very long tube having a sectional area of one square inch we fit an air-tight piston (Fig. 172), and place the lower end of the tube in a vessel of water. On raising the piston a vacuum would be created in the tube, did not the pressure of the atmosphere force water up into the tube behind the piston. The water would continue to rise until it reached a point 34 feet perpendicularly above the level of the water in the vessel. The column would then weigh 15 lbs., and exactly counterbalance the atmospheric pressure; so that a further raising of the piston would not raise the water any farther. At sea-level, therefore, the _lifting_ power of a pump by suction is limited to 34 feet. On the top of a lofty mountain, where the air-pressure is less, the height of the column would be diminished--in fact, be proportional to the pressure.

[Ill.u.s.tration: FIG. 172.]

[Ill.u.s.tration: FIG. 173.]

THE SIPHON

is an interesting application of the principle of suction. By its own weight water may be made to lift water through a height not exceeding 34 feet. This is explained by Fig. 173. The siphon pipe, A B C D, is in the first instance filled by suction. The weight of the water between A and B counter-balances that between B and C. But the column C D hangs, as it were, to the heels of B C, and draws it down. Or, to put it otherwise, the column B D, being heavier than the column B A, draws it over the topmost point of the siphon. Any parting between the columns, provided that B A does not exceed 34 feet, is impossible, as the pressure of the atmosphere on the mouth of B A is sufficient to prevent the formation of a vacuum.

THE BUCKET PUMP.

We may now pa.s.s to the commonest form of pump used in houses, stables, gardens, etc. (Fig. 174). The piston has a large hole through it, over the top of which a valve is hinged. At the bottom of the barrel is a second valve, also opening upwards, seated on the top of the supply pipe. In sketch (_a_) the first upstroke is in progress. A vacuum forms under the piston, or plunger, and water rises up the barrel to fill it.

The next diagram (_b_) shows the first downstroke. The plunger valve now opens and allows water to rise above the piston, while the lower closes under the pressure of the water above and the pull of that below.

During the second upstroke (_c_) the water above the piston is raised until it overflows through the spout, while a fresh supply is being sucked in below.

[Ill.u.s.tration: FIG. 174.]

THE FORCE-PUMP.

[Ill.u.s.tration: FIG. 175. Force-pump; suction stroke.]

[Ill.u.s.tration: FIG. 176. Force-pump; delivery stroke.]

For driving water to levels above that of the pump a somewhat different arrangement is required. One type of force-pump is shown in Figs. 175, 176. The piston now is solid, and the upper valve is situated in the delivery pipe. During an upstroke this closes, and the other opens; the reverse happening during a downstroke. An air-chamber is generally fitted to the delivery pipe when water is to be lifted to great heights or under high pressure. At each delivery stroke the air in the chamber is compressed, absorbing some of the shock given to the water in the pipe by the water coming from the pump; and its expansion during the next suction stroke forces the water gradually up the pipe. The air-chamber is a very prominent feature of the fire-engine.

A _double-action_ force-pump is seen in Fig. 177, making an upward stroke. Both sides of the piston are here utilized, and the piston rod works through a water-tight stuffing-box. The action of the pump will be easily understood from the diagram.

[Ill.u.s.tration: FIG. 177.]

THE MOST MARVELLOUS PUMP

known is the _heart_. We give in Fig. 178 a diagrammatic sketch of the system of blood circulation in the human body, showing the heart, the arteries, and the veins, big and little. The body is supposed to be facing the reader, so that the left lung, etc., is to his right.

[Ill.u.s.tration: FIG. 178.--A diagrammatic representation of the circulatory system of the blood.]

The heart, which forces the blood through the body, is a large muscle (of about the size of the clenched fist) with four cavities. These are respectively known as the right and left _auricles_, and the right and left _ventricles_. They are arranged in two pairs, the auricle uppermost, separated by a fleshy part.i.tion. Between each auricle and its ventricle is a valve, which consists of strong membranous flaps, with loose edges turned downwards. The left-side valve is the _mitral_ valve, that between the right auricle and ventricle the _tricuspid_ valve. The edges of the valves fall together when the heart contracts, and prevent the pa.s.sage of blood. Each ventricle has a second valve through which it ejects the blood. (That of the right ventricle has been shown double for the sake of convenience.)

The action of the heart is this:--The auricles and ventricles expand; blood rushes into the auricles from the channels supplying them, and distends them and the ventricles; the auricles contract and fill the ventricles below quite full (there are no valves above the auricles, but the force of contraction is not sufficient to return the blood to the veins); the ventricles contract; the mitral and tricuspid valves close; the valves leading to the arteries open; blood is forced out of the ventricles.

THE BLOOD CHANNELS

are of two kinds--(1) The _arteries_, which lead the blood into the circulatory system; (2) the _veins_, which lead the blood back to the heart. The arteries divide up into branches, and these again divide into smaller and smaller arteries. The smallest, termed _capillaries_ (Latin, _capillus_, a hair), are minute tubes having an average diameter of 1/3000th of an inch. These permeate every part of the body. The capillary arteries lead into the smallest veins, which unite to form larger and larger veins, until what we may call the main streams are reached. Through these the blood flows to the heart.

There are three main points of difference between arteries and veins. In the first place, the larger arteries have thick elastic walls, and maintain their shape even when empty. This elasticity performs the function of the air-chamber of the force-pump. When the ventricles contract, driving blood into the arteries, the walls of the latter expand, and their contraction pushes the blood steadily forward without shock. The capillaries have very thin walls, so that fluids pa.s.s through them to and from the body, feeding it and taking out waste matter. The veins are all thin-walled, and collapse when empty. Secondly, most veins are furnished with valves, which prevent blood flowing the wrong way.

These are similar in principle to those of the heart. Arteries have no valves. Thirdly, arteries are generally deeply set, while many of the veins run near the surface of the body. Those on the front of the arm are specially visible. Place your thumb on them and run it along towards the wrist, and you will notice that the veins distend owing to the closing of the valves just mentioned.

Arterial blood is _red_, and comes out from a cut in gulps, on account of the contraction of the elastic walls. If you cut a vein, _blue_ blood issues in a steady stream. The change of colour is caused by the loss of oxygen during the pa.s.sage of the blood through the capillaries, and the absorption of carbon dioxide from the tissues.

The _lungs_ are two of the great purifiers of the blood. As it circulates through them, it gives up the carbon dioxide which it has absorbed, and receives pure oxygen in exchange. If the air of a room is "foul," the blood does not get the proper amount of oxygen. For this reason it is advisable for us to keep the windows of our rooms open as much as possible both day and night. Fatigue is caused by the acc.u.mulation of carbon dioxide and other impurities in the blood. When we run, the heart pumps blood through the lungs faster than they can purify it, and eventually our muscles become poisoned to such an extent that we have to stop from sheer exhaustion.

THE COURSE OF THE BLOOD.

It takes rather less than a minute for a drop of blood to circulate from the heart through the whole system and back to the heart.

We may briefly summarize the course of the circulation of the blood thus:--It is expelled from the left ventricle into the _aorta_ and the main arteries, whence it pa.s.ses into the smaller arteries, and thence into the capillaries of the brain, stomach, kidneys, etc. It here imparts oxygen to the body, and takes in impurities. It then enters the veins, and through them flows back to the right auricle; is driven into the right ventricle; is expelled into the _pulmonary_ (lung) _arteries_; enters the lungs, and is purified. It returns to the left auricle through the _pulmonary veins_; enters the left auricle, pa.s.ses to left ventricle, and so on.

A healthy heart beats from 120 times per minute in a one-year-old infant to 60 per minute in a very aged person. The normal rate for a middle-aged adult is from 80 to 70 beats.

Heart disease signifies the failure of the heart valves to close properly. Blood pa.s.ses back when the heart contracts, and the circulation is much enfeebled. By listening through a stethoscope the doctor is able to tell whether the valves are in good order. A hissing sound during the beat indicates a leakage past the valves; a thump, or "clack," that they shut completely.

THE HYDRAULIC PRESS.