How It Works · Part 11

By means of telephone wires built into the life-line (which passes under the diver's arms and is used for lowering and hoisting) easy communication is established between the diver and his attendants above. The transmitter of the telephone is placed inside the helmet between the front and a side window, the receiver and the button of an electric bell in the crown. This last he can press by raising his head. The life-line sometimes also includes the wires for an electric lamp (Fig. 162) used by the diver at depths to which daylight cannot penetrate.

The pressure on a diver's body increases in the ratio of 4-1/3 lbs. per square inch for every 10 feet that he descends. The ordinary working limit is about 150 feet, though "old hands" are able to stand greater pressures. The record is held by one James Hooper, who, when removing the cargo of the Cape Horn sunk off the South American coast, made seven descents of 201 feet, one of which lasted for forty-two minutes.

A sketch is given (Fig. 163) of divers working below water with pneumatic tools, fed from above with high-pressure air. Owing to his buoyancy a diver has little depressing or pushing power, and he cannot bore a hole in a post with an auger unless he is able to rest his back against some firm object, or is roped to the post. Pneumatic chipping tools merely require holding to their work, their weight offering sufficient resistance to the very rapid blows which they make.

AIR-PUMPS.

Mention having been made of the air-pump, we append diagrams (Figs. 164, 165) of the simplest form of air-pump, the cycle tyre inflator. The piston is composed of two circular plates of smaller diameter than the barrel, holding between them a cup leather. During the upstroke the cup collapses inwards and allows air to pass by it. On the downstroke (Fig. 165) the edges of the cup expand against the barrel, preventing the passage of air round the piston. A double-action air-pump requires a long, well-fitting piston with a cup on each side of it, and the addition of extra valves to the barrel, as the cups under these circumstances cannot act as valves.

PNEUMATIC TYRES.

The action of the pneumatic tyre in reducing vibration and increasing the speed of a vehicle is explained by Figs. 166, 167. When the tyre encounters an obstacle, such as a large stone, it laps over it (Fig. 166), and while supporting the weight on the wheel, reduces the deflection of the direction of movement. When an iron-tyred wheel meets a similar obstacle it has to rise right over it, often jumping a considerable distance into the air. The resultant motions of the wheel are indicated in each case by an arrow. Every change of direction means a loss of forward velocity, the loss increasing with the violence and extent of the change. The pneumatic tyre also scores because, on account of its elasticity, it gives a "kick off" against the obstacle, which compensates for the resistance during compression.

THE AIR-GUN.

This may be described as a valveless air-pump. Fig. 168 is a section of a "Gem" air-gun, with the mechanism set ready for firing. In the stock of the gun is the cylinder, in which an accurately fitting and hollow piston moves. A powerful helical spring, turned out of a solid bar of steel, is compressed between the inside end of the piston and the upper end of the butt. To set the gun, the catch is pressed down so that its hooked end disengages from the stock, and the barrel is bent downwards on pivot P. This slides the lower end of the compressing lever towards the butt, and a projection on the guide B, working in a groove, takes the piston with it. When the spring has been fully compressed, the triangular tip of the rocking cam R engages with a groove in the piston's head, and prevents recoil when the barrel is returned to its original position. On pulling the trigger, the piston is released and flies up the cylinder with great force, and the air in the cylinder is compressed and driven through the bore of the barrel, blocked by the leaden slug, to which the whole energy of the expanding spring is transmitted through the elastic medium of the air.

There are several other good types of air-gun, all of which employ the principles described above.

THE SELF-CLOSING DOOR-STOP

is another interesting pneumatic device. It consists of a cylinder with an air-tight piston, and a piston rod working through a cover at one end. The other end of the cylinder is pivoted to the door frame. When the door is opened the piston compresses a spring in the cylinder, and air is admitted past a cup leather on the piston to the upper part of the cylinder. This air is confined by the cup leather when the door is released, and escapes slowly through a leak, allowing the spring to regain its shape slowly, and by the agency of the piston rod to close the door.

THE ACTION OF WIND ON OBLIQUE SURFACES.

Why does a kite rise? Why does a boat sail across the wind? We can supply an answer almost instinctively in both cases, "Because the wind pushes the kite or sail aside." It will, however, be worth while to look for a more scientific answer. The kite cannot travel in the direction of the wind because it is confined by a string. But the face is so attached to the string that it inclines at an angle to the direction of the wind. Now, when a force meets an inclined surface which it cannot carry along with it, but which is free to travel in another direction, the force may be regarded as resolving itself into two forces, coming from each side of the original line. These are called the component forces.

To explain this we give a simple sketch of a kite in the act of flying (Fig. 169). The wind is blowing in the direction of the solid arrow A. The oblique surface of the kite resolves its force into the two components indicated by the dotted arrows B and C. Of these C only has lifting power to overcome the force of gravity. The kite assumes a position in which force C and gravity counterbalance one another.

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 passing 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 Coxwell 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. Coxwell been able to pull the valve rope with his teeth and cause a descent, both would have died from want of air.

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-cock--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.

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 pass 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.

THE FORCE-PUMP.

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.

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.

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 partition. 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 passage 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 pass 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 passage 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 accumulation 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 passes 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, passes 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 passes 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.

It is a characteristic of fluids and gases that if pressure be brought to bear on any part of a mass of either class of bodies it is transmitted equally and undiminished in all directions, and acts with the same force on all equal surfaces, at right angles to those surfaces. The great natural philosopher Pascal first formulated this remarkable fact, of which a simple illustration is given in Fig. 179. Two cylinders, A and B, having a bore of one and two inches respectively, are connected by a pipe. Water is poured in, and pistons fitting the cylinders accurately and of equal weight are inserted. On piston B is placed a load of 10 lbs. To prevent A rising above the level of B, it must be loaded proportionately. The area of piston A is four times that of B, so that if we lay on it a 40-lb. weight, neither piston will move. The walls of the cylinders and connecting pipe are also pressed outwards in the ratio of 10 lbs. for every part of their interior surface which has an area equal to that of piston B.

The hydraulic press is an application of this law. Cylinder B is represented by a force pump of small bore, capable of delivering water at very high pressures (up to 10 tons per square inch). In the place of A we have a stout cylinder with a solid plunger, P (Fig. 180), carrying the table on which the object to be pressed is placed. Bramah, the inventor of the hydraulic press, experienced great difficulty in preventing the escape of water between the top of the cylinder and the plunger. If a "gland" packing of the type found in steam-cylinders were used, it failed to hold back the water unless it were screwed down so tightly as to jam the plunger. He tried all kinds of expedients without success; and his invention, excellent though it was in principle, seemed doomed to failure, when his foreman, Henry Maudslay,[35] solved the problem in a simple but most masterly manner. He had a recess turned in the neck of the cylinder at the point formerly occupied by the stuffing-box, and into this a leather collar of U-section (marked solid black in Fig. 180) was placed with its open side downwards. When water reached it, it forced the edges apart, one against the plunger, the other against the walls of the recess, with a degree of tightness proportionate to the pressure. On water being released from the cylinder the collar collapsed, allowing the plunger to sink without friction.

The principle of the hydraulic press is employed in lifts; in machines for bending, drilling, and riveting steel plates, or forcing wheels on or off their axles; for advancing the "boring shield" of a tunnel; and for other purposes too numerous to mention.

HOUSEHOLD WATER-SUPPLY FITTINGS.

Among these, the most used is the tap, or cock. When a house is served by the town or district water supply, the fitting of proper taps on all pipes connected with the supply is stipulated for by the water-works authorities. The old-fashioned "plug" tap is unsuitable for controlling high-pressure water on account of the suddenness with which it checks the flow. Lest the reader should have doubts as to the nature of a plug tap, we may add that it has a tapering cone of metal working in a tapering socket. On the cone being turned till a hole through it is brought into line with the channel of the tap, water passes. A quarter turn closes the tap.

Its place has been taken by the screw-down cock. A very common and effective pattern is shown in Fig. 181. The valve V, with a facing of rubber, leather, or some other sufficiently elastic substance, is attached to a pin, C, which projects upwards into the spindle A of the tap. This spindle has a screw thread on it engaging with a collar, B. When the spindle is turned it rises or falls, allowing the valve to leave its seating, V S, or forcing it down on to it. A packing P in the neck of B prevents the passage of water round the spindle. To open or close the tap completely is a matter of several turns, which cannot be made fast enough to produce a "water-hammer" in the pipes by suddenly arresting the flow. The reader will easily understand that if water flowing at the rate of several miles an hour is abruptly checked, the shock to the pipes carrying it must be very severe.

THE BALL-COCK

is used to feed a cistern automatically with water, and prevent the water rising too far in the cistern (Fig. 182). Water enters the cistern through a valve, which is opened and closed by a plug faced with rubber. The lower extremity of the plug is flattened, and has a rectangular hole cut in it. Through this passes a lever, L, attached at one end to a hollow copper sphere, and pivoted at the other on the valve casing. This casing is not quite circular in section, for two slots are cast in the circumference to allow water to pass round the plug freely when the valve is open. The buoyancy of the copper sphere is sufficient to force the plug's face up towards its seating as the valve rises, and to cut off the supply entirely when a certain level has been attained. If water is drawn off, the sphere sinks, the valve opens, and the loss is made good.

THE WATER-METER.

Some consumers pay a sum quarterly for the privilege of a water supply, and the water company allows them to use as much as they require. Others, however, prefer to pay a fixed amount for every thousand gallons used. In such cases, a water-meter is required to record the consumption. We append a sectional diagram of Kennedy's patent water-meter (Fig. 183), very widely used. At the bottom is the measuring cylinder, fitted with a piston, (6), which is made to move perfectly water-tight and free from friction by means of a cylindrical ring of india-rubber, rolling between the body of the piston and the internal surface of the cylinder. The piston rod (25), after passing through a stuffing-box in the cylinder cover, is attached to a rack, (15), which gears with a cog, (13), fixed on a shaft. As the piston moves up and down, this cog is turned first in one direction, then in the other. To this shaft is connected the index mechanism (to the right). The cock-key (24) is so constructed that it can put either end of the measuring cylinder in communication with the supply or delivery pipes, if given a quarter turn (see Fig. 184). The weighted lever (14) moves loosely on the pinion shaft through part of a circle. From the pinion project two arms, one on each side of the lever. When the lever has been lifted by one of these past the vertical position, it falls by its own weight on to a buffer-box rest, (18). In doing so, it strikes a projection on the duplex lever (19), which is joined to the cock-key, and gives the latter a quarter turn.

In order to follow the working of the meter, we must keep an eye on Figs. 183 and 184 simultaneously. Water is entering from A, the supply pipe. It flows through the cock downwards through channel D into the lower half of the cylinder. The piston rises, driving out the water above it through C to the delivery pipe B. Just as the piston completes its stroke the weight, raised by the rack and pinion, topples over, and strikes the key-arm, which it sends down till stopped by the buffer-box. The tap is then at right angles to the position shown in Fig. 184, and water is directed from A down C into the top of the cylinder, forcing the piston down, while the water admitted below during the last stroke is forced up the passage D, and out by the outlet B. Before the piston has arrived at the bottom of the cylinder, the lifter will have lifted the weighted lever from the buffer-box, and raised it to a vertical position; from there it will have fallen on the right-hand key-arm, and have brought the cock-key to its former position, ready to begin another upward stroke.

The index mechanism makes allowance for the fact that the bevel-wheel on the pinion shaft has its direction reversed at the beginning of every stroke of the piston. This bevel engages with two others mounted loosely on the little shaft, on which is turned a screw thread to revolve the index counter wheels. Each of these latter bevels actuates the shaft through a ratchet; but while one turns the shaft when rotating in a clockwise direction only, the other engages it when making an anti-clockwise revolution. The result is that the shaft is always turned in the same direction.

WATER-SUPPLY SYSTEMS.

The water for a town or a district supply is got either from wells or from a river. In the former case it may be assumed to be free from impurities. In the latter, there is need for removing all the objectionable and dangerous matter which river water always contains in a greater or less degree. This purification is accomplished by first leading the water into large settling tanks, where the suspended matter sinks to the bottom. The water is then drawn off into filtration beds, made in the following manner. The bottom is covered with a thick layer of concrete. On this are laid parallel rows of bricks, the rows a small distance apart. Then come a layer of bricks or tiles placed close together; a layer of coarse gravel; a layer of finer gravel; and a thick layer of sand at the top. The sand arrests any solid matter in the water as it percolates to the gravel and drains below. Even the microbes,[36] of microscopic size, are arrested as soon as the film of mud has formed on the top of the sand. Until this film is formed the filter is not in its most efficient condition. Every now and then the bed is drained, the surface mud and sand carefully drained off, and fresh sand put in their place. A good filter bed should not pass more than from two to three gallons per hour for every square foot of surface, and it must therefore have a large area.

It is sometimes necessary to send the water through a succession of beds, arranged in terraces, before it is sufficiently pure for drinking purposes.

THE HOUSEHOLD FILTER.

When there is any doubt as to the wholesomeness of the water supply, a small filter is often used. The microbe-stopper is usually either charcoal, sand, asbestos, or baked clay of some kind. In Fig. 185 we give a section of a Maignen filter. R is the reservoir for the filtered water; A the filter case proper; D a conical perforated frame; B a jacket of asbestos cloth secured top and bottom by asbestos cords to D; C powdered carbon, between which and the asbestos is a layer of special chemical filtering medium. A perforated cap, E, covers in the carbon and prevents it being disturbed when water is poured in. The carbon arrests the coarser forms of matter; the asbestos the finer. The asbestos jacket is easily removed and cleansed by heating over a fire.