298 



THE POPULAR EDUCATOR. 



PNEUMATICS. II. 



AIR-PUMP FIRE BALLOON GAS BALLOON PRESSURE OF THE 



AIR MAGDEBURG HEMISPHERES. 



IN the better class of air-pumps two syringes are placed side 

 by side, so as to work alternately ; the construction of the 

 whole arrangement is then similar to that shown in Fig. 2. 

 A A is a pipe which opens through the pump-plate, and also 

 communicates with each of the barrels B B', valves c c, opening 

 upwards, being placed at the bottom of the barrels. The 

 piston-rods are cut into notches, which work in the teeth of 

 the wheel D, so that as the handle E is worked alternately back- 

 wards and forwards they rise and fall, and while the piston in B 

 is rising that in B' is being depressed. The great advantage 

 of this is that the pressure on the handle remains almost 

 constant. When the receiver becomes nearly exhausted, the 

 external air presses very heavily on the top of the piston, which 

 therefore requires a considerable force to raise it ; but when 

 there are two arranged- thus, the pressure on the one nearly 

 balances that on the other, and thus much less labour is re- 

 quired. As the air in the receiver F becomes rarefied, the 

 external air presses on it with considerable force, fixing it 

 firmly on the plate, so firmly, indeed, as to render it almost 

 impossible to stir it. An opening, closed by a screw, is there- 

 fore provided, by which the air can be allowed to enter the 

 receiver when it is desired to remove it from the plate. A 

 gauge to indicate the degree of rarefac- 

 tion produced is affixed to the machine 

 at G. Since at each stroke the air in the 

 receiver expands and fills the cylinder, it 

 is obvious that the larger the cylinder is 

 in comparison with the receiver, the less 

 the number of strokes required to produce' 

 any given degree of exhaustion. 



If the capacity of the cylinder be that 

 of the receiver, J of the air will be removed 

 by the first stroke, and consequently | only 

 will remain ; | of this will be removed by 

 the next stroke, leaving in the receiver 

 5 55, or 1JJ of the original quantity. In this 

 way we see that after five strokes, upwards 

 of | of the air will have been removed, 

 while if the cylinder have only i of the 

 capacity of the receiver twelve strokes 

 will be required to produce the same 

 degree of exhaustion. It is important, 

 then, to have the cylinder as large as 



practicable ; still, since the pressure of the air on it may become 

 thirteen or fourteen pounds to the square inch, there is a limit 

 soon imposed by the power which would be required to work 

 the machine. The vacuum produced by an air-pump constructed 

 in this way is not absolutely perfect, for the pressure of the air 

 in the receiver has to open the valves, and when a certain 

 amount has been removed, that which is left ceases to have 

 sufficient expansive force to do this, and then no further 

 exhaustion can be produced. To obviate this the valves in 

 the best machines, instead of being made of oiled silk, are 

 conical plugs fitting into settings, and are worked by the piston 

 instead of opening by the force of the air. In this way a 

 much more perfect vacuum may be obtained. A still further 

 improvement has been effected by M. Bianchi. His machine 

 has only one barrel, but this is so constructed as to be double- 

 acting. The conical plugs closing the valves at the top and 

 bottom are fixed to opposite ends of a rod which passes through 

 a hole in the piston. As the piston rises it closes that at the 

 top, and forces the air above it out by a separate valve, while 

 at the same time it opens the lower exhaust-valve, and thus 

 draws a further supply of air from the receiver. . Similarly, in 

 descending, it forces out the air below it, and exhausts above. 

 It thus has the advantage of the double-barrel pump without 

 its complications. It is also constructed so that the cylinder 

 oscillates, and the piston is worked by a crank ; thus, instead 

 of the usual alternate motion, which is very awkward, a fly- 

 wheel turned by a winch gives motion to the pump. 



We can now pass on to give other illustrations of the weight 

 of the air. If we take a ball of cork, and suspend it from one 

 end of a scale-beam, placing a piece of metal at the other end 

 ao as just to balance it, and then transfer the whole to the 



receiver of the pump, and remove the air, we shall find that the 

 cork will overbalance the weight, and descend. The reason of 

 this is that the cork displaces a larger bulk of air than the 

 piece of metal, and therefore is supported to that extent by the 

 air ; but as soon as this support is removed it sinks. If, there- 

 fore, we would know the true weight of any substance, we must 

 weigh it in vacuo. 



The ascent of a balloon furnishes another illustration of this. 

 A body floats in water because it has less weight than an equal 

 bulk of water, and in the same way a body will float in air if it 

 has less weight than an equal bulk of air. Now a balloon is so 

 constructed as thus to be lighter, and therefore the weight of 

 the surrounding air buoys it up. If it were possible to con- 

 struct a hollow vessel strong enough to bear the pressure of the 

 air, and yet weighing less than the air it displaces, it would 

 ascend ; this, however, has not been accomplished, nor does it 

 seem at all likely to be done. The simplest balloon is a common 

 soap bubble ; the breath used in blowing it is warm, and suffi- 

 ciently lighter than the air to carry up with it the delicate film 

 of soapy water which envelops it. If. bubbles be blown with 

 hydrogen gas instead of air, they will ascend more rapidly. A 

 peculiar soap solution is now to be obtained, bubbles blown with 

 which may be attached to a small paper disc, and made to take 

 up with them a miniature car. 



The Montgolfier or fire-balloon was the first used, being in- 

 vented towards the end of the last century. It consists of a 

 large hollow vessel, made of varnished 

 silk or thin -canvas, and surrounded by a 

 network of ropes sufficiently strong to 

 support a furnace just under the open 

 mouth of the balloon, and also the car, in 

 which the aeronaut can sit. A large fire 

 being kindled in the furnace, the air 

 inside becomes highly rarefied, and thus 

 weighs so much less than an equal bulk 

 of the external air that it will raise the 

 balloon with its furnace and car. Fresh 

 supplies of the fuel, which is of a highly 

 combustible nature, are laid on when it is 

 desired that the balloon should rise to a 

 greater height, while damping the fire 

 soon causes it to fall. The use of these 

 balloons is, however, attended with great 

 danger, as the flames may catch the car or 

 the balloon itself. 



Hydrogen gas being lighter than heated 

 air, it soon began to be used for inflating 



balloons, which were found to possess great lifting power. Pure 

 hydrogen has less than -j^th the weight of common air, 100 cubic 

 incheo of it weighing only about 2'14 grains. If, then, a balloon 

 having a capacity of 16,000 cubic feet be filled with this, it will 

 possess a lifting power of about half a ton. The hydrogen used 

 was at first commonly made by the action of sulphuric acid and 

 water on pieces of iron or zinc, the gas given off being passed 

 through water to wash it from the acid, which would injure the 

 balloon. It was, however, found to be expensive thus to make 

 it, ri,nd being so light it soon mixed with the air, and thus lost 

 much of its buoyancy ; common coal gas is therefore now 

 generally used, and if it be made at a somewhat higher tem- 

 perature than usual it is sufficiently light for most purposes. 

 Its specific gravity is much less, indeed, than that of pure 

 hydrogen, being about one-half that of the air, but increased 

 size in the balloon will compensate for this, and it has the 

 advantages of being cheap, easily procurable, and moro 

 manageable. 



As the balloon ascends the pressure of the air becomes less 

 and less, and thus the gas in it expands, and would soon burst 

 open the silk, were it not that a large aperture is always left at 

 the neck of the balloon to allow of the escape of any excess. So 

 sensitive is this large body of gas to changes of pressure or 

 temperature, that the fact of passing from a cloud into the rays 

 of the sun makes a very perceptible change in its bulk. 



A number of sand-bags are usually suspended outside the car, 

 and when the aeronaut wishes to ascend he empties some of 

 these, and thus diminishes the weight of the car and causes the 

 balloon to shoot up rapidly. A large valve is also fixed at the 

 top of the balloon, and can be opened by means of a cord which 

 passes down into the car. When the tension becomes very 



