MATH'S 



great, or the ai-ronaut wishes to doioond, be opens thin, and 



t the gas to enoape. The greatest care 



nuooHiwry in descending, on much of the ga* bad 



escaped, iiii'l uh.Ti tli balloon nears the earth again itoollapM* 

 ruble extent, and a further quantity of *and baa to 

 be let out. 



apnel IH usually suspended from the car, and, catching 



the earth, gives assistance in descending ; but with every oare 



ut shock will tn-qu. ally bo experienced, and the scientific 



instruments taken in th<- cur bo broken. A parachute U *omu- 



tukon up with the balloon. This consists 



of a largo circ-ulur piece of oanvas, with a small 



oar suspended by ropes fixed at intervals round 



:' If tho aeronaut gets into this, the 



oanvas will open out liko an umbrella, and the 



resistance of the air against it will be sufficient 



to break the foroo of tho fall. At present no 



very great results have boon obtained by tho use 



of tho balloon. In war ono has occasionally been 



up fastened to a long rope, and from this 



tin- position of the enemy ban boon noted. Many 



. ible observations as to tho state and t<-ni- 



pcr.it iir.- <>f the air have also boon taken, but no 



important practical results have as yet boon 



red. 



: now seen proofs that tho air has weight, 

 we must see what effects this weight produces. 

 If we lay a piece of iron or any substance on our 

 hand it produces pressure, tho amount of which 

 varies with tho weight of the body, and we 

 should naturally expect the same effect to be 

 produced by tho air. A few simple experiments will show us 

 that this is actually tho case, and that the air does exert a very 

 great pressure on every substance exposed to it. This pressure 

 amounts to nearly 15 pounds on every square inch of surface. 

 If wo have a card measuring 4 inches by 3 inches, the pressure 

 on it from tho air will be 180 pounds. But it will be said this 

 pressure is not felt, nor does the card bend at all ; why is this ? 

 Simply because the pressure is equal in all directions, and 

 therefore that on the lower side balances that on the upper. 

 If we take away the air from tho under side, wo shall soon find 

 that this is tho case. This ma}' easily be done by means of the 

 air-pump. We have only to procure a glass 

 receiver open at each end, and having stretched 

 a piece of bladder or of thin india-rubber over 

 one end, place it on the pump-plate, and exhaust 

 the air (Fig. 3). The pressure above, not being 

 balanced any longer by a corresponding pressure 

 on the under side, will press the bladder down, 

 and, after a few strokes, cause it to burst with 

 a loud report. A thin piece of glass, if it bo 

 flat enough to close the top of the receiver air- 

 tight, may be broken in a similar way, and thus 

 will give a further proof of tho intensity of the 

 pressure. 



Instead of tho bladder or glass the hand may 

 be laid on the receiver, and will be pressed down 

 forcibly as the air is removed. The cupping- 

 glass, formerly so much used, acted on this prin- 

 ciple. The air in it was rarefied either by means 

 of a small syringe, or by heating the glass and 

 allowing it to cool when placed over tho required 

 part of the body ; the flesh was thus drawn up, 

 and then pierced by the lancets. 



If a wooden cup, with a piece of cane let into 

 the middle of it, be mode to fit the top of the 

 receiver and filled with mercury, the pressure of the air, when 

 the receiver is exhausted, will drive the mercury through tho 

 pores of the cane, and it will fall liko a fine shower into a vessel 

 placed to receive it. Care must, however, be taken not to lot 

 any of it run into the pump, as it is almost certain to injure 

 it seriously. 



This pressure is exerted equally in all directions, and honco 

 is almost unnoticed : a simple experiment, which all can try, 

 affords a proof of this. Fill a wine-glass with water, so that 

 it stands a little above the edge, and carefully slide a piece 

 of card over the mouth so as to cover it completely and 

 exclude the air ; the whole may now be inverted without the 



card falling or tba water being piUod. Tho tide* of tho wine- 

 glow nuitain tba downward premium of tb air, and tbo i. . 

 pressure u more than ufficieut to sustain the weight of the 

 water in the gla*. The only advantage of the card hero U 

 that it prevent* the imrface being br./k.-n up, and thus allowing 

 the air to enter at place*. A bottlu with a very small month 

 may bo completely filled and carefully inverted without the 

 water running out, oven though the card be not used. 



Several conjuring trick* are performed upon tkU principle, 

 and many other* are only ingenious application* of scientific 

 principle*, and appear startling mainly became 

 tbe*e principle* are *o little understood by the 

 majority of men. If a small hole be drilled in 

 tho bottom of a decanter, and the stopper pnt in 

 firmly, no liquid will escape. The finger muttt, 

 of coarse, be beld over the opening while tin- 

 liquid is being poured in. The decanter u 

 placed over a stand largo enough to contain the 

 wine, and on the stopper being slightly loosened 

 the liquid will quietly flow out. A cover is placed 

 over it while this is going on. In the same way 

 liquids may bo made to flow from small reservoirs 

 concealed in covers into glasses placed upon them, 

 and thus the wine appear* to have passed from the 

 decanter into the glasses. We Bee, thus, that there 

 is science to be learnt even from some of the tricks 

 of tho conjuror, and a person who understands the 

 principle of these things will be less likely to be 

 deceived. Perhaps the most striking proof of the 

 pressure of the air is afforded by boiling some 

 water in a thin cylinder provided with a stop-cock. 

 After the water has been boiling for a little time, all the air 

 which was within is driven out, and it remains filled with steam. 

 Let tho stop-cock be now closed, and cold water poured over the 

 cylinder ; the steam inside will be instantaneously condensed 

 into water, and tho pressure of the air around will crumple up 

 the cylinder, even though the cylindrical shape is best calculated 

 to resist a pressure of this kind. If the steam inside a boiler 

 bo suddenly condensed by admitting cold water too rapidly, or 

 in any other way, the boiler will sometimes collapse in a similar 

 way. By ascertaining the area it exposes to the air, and mnlti| ly- 

 ing the number of square inches in that by 14J, we shall see how 

 enormous the pressure thus exerted must be. 



Another piece of apparatus, known as the Mag- 

 deburg hemispheres (Fig. 4), furnishes a very 

 good illustration of this pressure. Two hemi- 

 spheres usually made of brass are procured and 

 their edges accurately ground, so that, when 

 pressed together, they fit air-tight. A handle is 

 fixed to each, that on one being so arranged as to 

 screw on the end of a pipe closed by a tap and 

 opening into the hemisphere. This handle is 

 taken off, and the hemispheres pressed together, 

 a little tallow being smeared on their edges ; the 

 pipe is then screwed into the hole in the centre 

 of the pump-plate, and the air removed. When 

 sufficiently exhausted the tap is closed, the appa- 

 ratus unscrewed from tho pump, and the handle 

 replaced. Nearly all the air has now been re- 

 moved from within them, and that aronnd, there- 

 fore, presses them together with great force. If 

 the hemispheres have a diameter of four or five 

 inches, this pressure will bo so great that it will 

 be as much as two men can do to pull them apart. 

 4. If, however, the tap be opened, and the air re- 



admitted, the pressure inside will balance that 

 outside, and they will fall apart by their own weight. It will 

 easily be seen that, if their diameter be 5 inches, the portion of 

 tho pressure of the air which forces them together will bo equal 

 to that on a circular surface of tho same diameter. Now, as the 

 area of a circle is about 3} times the square of its radius, the 

 area of this surface wonld be nearly 20 square inches, and wo 

 may assume that the air inside is so far rarefied that they aro 

 pressed together with a force of 14 pounds per square inch : the 

 force required to separate them is therefore nearly 280 pounds. 

 A pair of hemispheres of this kind were constructed by Otto 

 Guericke of such a size that two teams of horses, pulblng in 

 opposite directions, were unable to separate them. 



