CHAMBERS'S INFORMATION FOR THE PEOPLE. 



into ABC, the air therein is condensed, for it has 

 no communication with the atmosphere after the 

 water is higher than the bottom of the pipe FG. 

 This condensed air, then, exercises great force on 

 the surface, op, of the water, and raises it in the 

 tube, FG, to a height proportioned to the elasticity 

 of the imprisoned air. 



The principles of the hydraulic-ram are suscep- 

 tible of a very extensive application. In well- 

 constructed rams, the mechanical effect obtained 

 should be from 65 to 75 per cent, of the force 

 supplied. For raising comparatively small quanti- 

 ties of water, such as for single houses, farmyards, 

 &c. the ram is the best mechanism yet introduced. 

 But the concussion, and consequent deterioration 

 of the valves, places a limit to the use of the 

 mechanism when applied to raise large quantities. 



PNEUMATICS. 



Pneumatics, from the Greek word pneuma, 

 ' breath, 1 or ' air,' is a general term for the science 

 of aeriform fluids, embracing both Aerostatics and 

 Aerodynamics. What follows relates chiefly to 

 the weight, pressure, and elasticity of gases ; in 

 other words, to the statical part of the subject. 

 The most interesting motions of air fall under 

 METEOROLOGY. 



Modern science has made us acquainted with 

 a number of distinct airs, besides common air ; 

 these are now usually called gases, and the name 

 tiir is confined to the fluid composing our atmos- 

 phere. Aeriform fluids differ from one another in 

 weight as well as liquids do ; the other properties 

 treated of in pneumatics are common to them all. 

 Whatever, therefore, is established regarding air, 

 is to be understood as applying to gases in 

 general 



WEIGHT OF AIR. 



The idea of air as a material fluid possessing 

 weight, and exerting pressure on everything im- 

 mersed in it, is modern. The ancients thought 

 of it as essentially light, or without weight, and 

 as wanting generally the characteristics of matter. 

 Yet it really possesses all the properties of matter, 

 as surely as water does. A portion of it may be 

 much compressed, indeed, but cannot be squeezed 

 to nothing ; it still occupies space, and is there- 

 fore impenetrable (see MATTER AND MOTION). 

 It also resists sensibly a flat body, such as a fan 

 moving through it ; and when itself in motion, 

 as in a strong wind, it is felt to have a powerful 

 momentum or moving force. 



With regard to the weight of air, nothing in 

 the history of science is more remarkable than 

 that men should have lived so long subject to the 

 great pressure which this weight occasions, with- 

 out discovering it. The fact is, that air acts so 

 little on the senses, that it does not make us 

 aware directly of its existence. The effects pro- 

 duced by its weight were therefore attributed to 

 other causes. The facts, for instance, that go by 

 the name of suction, are all owing to the pressure 

 of the atmosphere. But when water was seen to 

 rush up a pipe from which the air was withdrawn, 

 it was explained by saying that ' nature abhors a 

 vacuum ' that is. an empty space. This explana- 



Fig. 26. 



tion continued to satisfy philosophers till the 

 middle of the seventeenth century. Some mecha- 

 nicians near Florence, having to construct a pump 

 of unusual length, found, to their surprise, that 

 the water refused to rise higher than thirty-four 

 feet. This led to the conjecture, 

 that the weight of the atmosphere 

 was the cause of water rising in the 

 pump ; and Torricelli, the pupil of 

 Galileo, confirmed the conjecture by 

 a happy experiment. 



Mercury weighs nearly 14 times 

 as much as water. If, now, he 

 argued, the atmospheric air can sup- 

 port a column of 34 feet of water, 

 it must also be able to sustain a 

 column of mercury of about i-i4th 

 that height. The experiment is 

 easily made. A glass tube of up- 

 wards of 30 inches in length, and 

 closed at one end, is filled with 

 mercury ; and the finger being firmly 

 pressed on the open upper end, the 

 tube is inverted, and the end closed 

 by the finger is plunged into a vessel 

 containing mercury, CD. The finger 

 l being now withdrawn, the liquid in 

 the tube descends, however long the tube may be, 

 till it stands at the height of about 30 inches above 

 the level of that in the vessel. 



Ah 1 doubt on the subject was shortly after put 

 an end to by the celebrated Pascal, who caused 

 the Torricellian tube, as it was called, to be carried 

 up the Puy-de-D6me, a mountain in France. By 

 ascending a mountain, a part of the atmosphere 

 is left below ; if it is the pressure of the super- 

 incumbent atmosphere, then, that sustains the 

 column, the mercury must sink farther and farther 

 as the elevation is greater. Such was found to be 

 the case, and the question was set at rest for 

 ever. 



That air is ponderable, or has weight, can be 

 put to direct proof. By means of an air-pump, to 

 be afterwards described, vessels of a certain con- 

 struction can be emptied of air, which, in ordinary 

 circumstances, fills every space not occupied by 

 other matter. When a hollow globe of glass or 

 of copper, holding a cubic foot, is thus emptied of 

 air, and weighed, on admitting the air again, it is 

 found to be about an ounce and a quarter heavier 

 than before. As a cubic foot of water weighs 

 looo ounces, water, at the ordinary height of the 

 barometer, is thus about 800 times heavier than 

 air. It is sufficiently accurate for most purposes 

 to consider a cubic foot of air as weighing one 

 ounce. 



Weight of the whole Atmosphere. If the tube 

 (fig. 26) is an inch in area, the column of mercury, 

 PE, 30 inches high, weighs about fifteen pounds. 

 Now, as this weight is sustained by the pres- 

 sure of the atmosphere, that pressure must 

 be equal to a weight of fifteen pounds on 

 eveiy square inch of surface. By multiplying the 

 number of inches on the surface of the globe by 

 fifteen, the product is the weight of the whole 

 atmosphere in pounds. It is the same as the 

 weight of an ocean of mercury spread over the 

 whole globe to the uniform height of 30 inches, 

 or of one of water 34 feet deep, or of one of oil 

 37 feet deep. It has been calculated to amount to 

 77,670,000,000,000,000 tons. 



