The vacuum chamber is exhausted during manufacture and sealed. It would naturally a.s.sume the shape of Fig. 157, but the spring S, acting against the atmospheric pressure, pulls it out. As the pressure varies, so does the spring rise or sink; and the slightest movement is transmitted through the multiplying arms C, E, F, to the pointer.
A good aneroid is so delicate that it will register the difference in pressure caused by raising it from the floor to the table, where it has a couple of feet less of air-column resting upon it. An aneroid is therefore a valuable help to mountaineers for determining their alt.i.tude above sea-level.
BAROMETERS AND WEATHER.
We may now return to the consideration of forecasting the weather by movements of the barometer. The first thing to keep in mind is, that the instrument is essentially a _weight_ recorder. How is weather connected with atmospheric weight?
In England the warm south-west wind generally brings wet weather, the north and east winds fine weather; the reason for this being that the first reaches us after pa.s.sing over the Atlantic and picking up a quant.i.ty of moisture, while the second and third have come overland and deposited their moisture before reaching us.
A sinking of the barometer heralds the approach of heated air--that is, moist air--which on meeting colder air sheds its moisture. So when the mercury falls we expect rain. On the other hand, when the "gla.s.s" rises, we know that colder air is coming, and as colder air comes from a dry quarter we antic.i.p.ate fine weather. It does not follow that the same conditions are found in all parts of the world. In regions which have the ocean to the east or the north, the winds blowing thence would be the rainy winds, while south-westerly winds might bring hot and dry weather.
THE DIVING-BELL.
Water is nearly 773 times as heavy as air. If we submerge a barometer a very little way below the surface of a water tank, we shall at once observe a rise of the mercury column. At a depth of 34 feet the pressure on any submerged object is 15 lbs. to the square inch, in addition to the atmospheric pressure of 15 lbs. per square inch--that is, there would be a 30-lb. _absolute_ pressure. As a rule, when speaking of hydraulic pressures, we start with the normal atmospheric pressure as zero, and we will here observe the practice.
[Ill.u.s.tration: FIG. 158.--A diving bell.]
The diving-bell is used to enable people to work under water without having recourse to the diving-dress. A sketch of an ordinary diving-bell is given in Fig. 158. It may be described as a square iron box without a bottom. At the top are links by which it is attached to a lowering chain, and windows, protected by grids; also a nozzle for the air-tube.
[Ill.u.s.tration: FIG. 159.]
A simple model bell (Fig. 159) is easily made out of a gla.s.s tumbler which has had a tap fitted in a hole drilled through the bottom. We turn off the tap and plunge the gla.s.s into a vessel of water. The water rises a certain way up the interior, until the air within has been compressed to a pressure equal to that of the water at the level of the surface inside. The further the tumbler is lowered, the higher does the water rise inside it.
Evidently men could not work in a diving-bell which is invaded thus by water. It is imperative to keep the water at bay. This we can do by attaching a tube to the tap (Fig. 160) and blowing into the tumbler till the air-pressure exceeds that of the water, which is shown by bubbles rising to the surface. The diving-bell therefore has attached to it a hose through which air is forced by pumps from the atmosphere above, at a pressure sufficient to keep the water out of the bell. This pumping of air also maintains a fresh supply of oxygen for the workers.
[Ill.u.s.tration: FIG. 160.]
Inside the bell is tackle for grappling any object that has to be moved, such as a heavy stone block. The diving-bell is used mostly for laying submarine masonry. "The bell, slung either from a crane on the masonry already built above sea-level, or from a specially fitted barge, comes into action. The block is lowered by its own crane on to the bottom. The bell descends upon it, and the crew seize it with tackle suspended inside the bell. Instructions are sent up as to the direction in which the bell should be moved with its burden, and as soon as the exact spot has been reached the signal for lowering is given, and the stone settles on to the cement laid ready for it."[34]
For many purposes it is necessary that the worker should have more freedom of action than is possible when he is cooped up inside an iron box. Hence the invention of the
DIVING-DRESS,
which consists of two main parts, the helmet and the dress proper. The helmet (Fig. 161) is made of copper. A breastplate, B, shaped to fit the shoulders, has at the neck a segmental screw bayonet-joint. The headpiece is fitted with a corresponding screw, which can be attached or removed by one-eighth of a turn. The neck edge of the dress, which is made in one piece, legs, arms, body and all, is attached to the breastplate by means of the plate P^1, screwed down tightly on it by the wing-nuts N N, the bolts of which pa.s.s through the breastplate. Air enters the helmet through a valve situated at the back, and is led through tubes along the inside to the front. This valve closes automatically if any accident cuts off the air supply, and encloses sufficient air in the dress to allow the diver to regain the surface.
The outlet valve O V can be adjusted by the diver to maintain any pressure. At the sides of the headpiece are two hooks, H, over which pa.s.s the cords connecting the heavy lead weights of 40 lbs. each hanging on the diver"s breast and back. These weights are also attached to the k.n.o.bs K K. A pair of boots, having 17 lbs. of lead each in the soles, complete the dress. Three glazed windows are placed in the headpiece, that in the front, R W, being removable, so that the diver may gain free access to the air when he is above water without being obliged to take off the helmet.
[Ill.u.s.tration: FIG. 161.--A diver"s helmet.]
By means of telephone wires built into the life-line (which pa.s.ses 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 b.u.t.ton 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.
[Ill.u.s.tration: FIG. 162.--Diver"s electric lamp.]
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.
[Ill.u.s.tration: FIG. 163.--Divers at work below water with pneumatic tools.]
AIR-PUMPS.
[Ill.u.s.tration: FIG. 164.]
[Ill.u.s.tration: FIG. 165.]
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 pa.s.s by it. On the downstroke (Fig.
165) the edges of the cup expand against the barrel, preventing the pa.s.sage 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 circ.u.mstances cannot act as valves.
PNEUMATIC TYRES.
[Ill.u.s.tration: FIG. 166.]
[Ill.u.s.tration: FIG. 167.]
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.
[Ill.u.s.tration: FIG. 168.--Section of the mechanism of an air-gun.]
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 b.u.t.t. 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 b.u.t.t, 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.
[Ill.u.s.tration: FIG. 169.]
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 a.s.sumes a position in which force C and gravity counterbalance one another.
[Ill.u.s.tration: FIG. 170.]