PNEUMATICS. 181 



It sometimes liappens that the united force of the air and gases thus con- 

 fined in the bottle, becomes greater than the cohesive strength of the parti- 

 cles of matter composing the bottle ; the sides of the bottle in such cases give 

 way or burst. 



Those liquors only froth which are viscid, glutinous, or thick, like ale, por- 

 ter, etc., because they retain the little bubbles of air as they rise ; whOe a thin 

 liquor, like champagne, which sufters the bubbles to escape readilj*, sparkles. 

 . 388. The pressure of the atmosphere is connected with tho 



pressure of the action of breathing. The air enters the lungs, not becauso 

 s mosphere ^j^g^ draw it in, but by the weight of the atmosphere forcing 



the act of it into the empty spaces formed by the expansion of the air- 

 reat ng . ^^jjg ^j. ^-^^ lungs. The air in turn escapes from the lungs by 



means of its elasticity ; the lungs, by muscular action, compress the air con- 

 tained in them, and give to it by compression a greater elasticity than the air 

 without. By thia excess of elasticity it is propelled, and escapes by the 

 mouth and nose. 



389. It has been proposed to take advantage of the pressure 



"Wliat is the ^f ^j^g atmosphere for the construction of an atmospheric tele- 

 proposed con- '■ '■ 

 structionofthe graph, or apparatus for conveying tho mails and other matter 



tele-^aph"*^ °'^^^ great distances with great rapidity. The plan proposed 



is as follows ; — a long metal tube is laid down, the interior 

 surface of which is perfectly smooth and even. A piston is fitted to the tube 

 in such a manner as to move freely in it and yet be air-tight. To one sido 

 of this piston the matter to be moved, made up in the form of a cyhndrical 

 bundle, is attached. A partial vacuum is then made in the tube before the 

 piston, by means of large air-pumps, worked by steam-power, located at tho 

 further end of the tube, when the pressure of the atmosphere on the other 

 side of the piston impels it forward through the whole length of the exhausted 

 tube. It has been estimated tliat a piston, drawing after it a considerable 

 weight of matter, could in this way be forced tlirough a tube at the rate of 

 600 miles per hour. 



390. The pressure of the atmosphere is taken advantage of in the con- 

 struction of a great variety of machines for raising water ; the most important 

 and familiar of which is the common, or suction pump. 



Descrihe the The comiiion, or suction pump, consists 



thr'"common ^f a hollow Cylinder, or barrel, open at both 

 pump. ends, in which is worked a movable piston, 



which fits the bore of the cylinder exactly, and is air-tight. 

 The pump is further provided with two valves, one of 

 which is placed in the piston, and moves with it, while 

 the other is fixed in the lower part of the pump-barrel. 

 These valves are termed boxes. 



Fig. 173 represents the construction of tho common pump. The body con- 

 sists of a cyUnder, or barrel, b, the lower part of which, called the suction- 



