54 



ELASTICITY OF AIR. 



of the piston with the cylinder, and it will when drawn to the top remain there, 

 without returning to the bottom. In this state let the air be readmitted to the 

 receiver, the piston will then be immediately pressed to the bottom of the 

 cylinder. 



The celebrated experiment of the Magdeburgh hemispheres may be per- 

 formed by means of an air-pump. Two hollow hemispheres constructed of 

 brass, as represented in fig. 9, are so formed that when placed mouth to mouth 



Fig. 9. 



they shall be in air-tight contact. They are furnished with handles, one of 

 which may be screwed off. In the neck to which this handle is screwed is a 

 tube furnished with a stop-cock. The handle being screwed off, let the hem- 

 isphere be screwed on the pump-plate, and the other hemisphere being placed 

 over it, let the stop-cock be opened so as to leave a free communication be- 

 tween the interior of the sphere and the exhausting-tube of the air-pump. 

 The pump being now worked, the interior of the sphere will form the receiver, 

 from which all communication with the external air is cut off, and rarefaction 

 will be produced in it to any degree which may be desired. This being 

 effected, let the stop-cock be closed ; and let the sphere be detached from the 

 pump-plate, and the handle screwed upon it. If then the two handles be 

 drawn in opposite directions, so as to pull the hemispheres from one another, 

 it will be found that they will resist with considerable force. If the diameter 

 of the sphere be six inches, its section through the centre will be about twenty- 

 eight square inches. The hemispheres will be pressed together by a force 

 amounting to fifteen pounds for every square inch in the section. If twenty- 

 eight be multiplied by fifteen, we shall obtain four hundred and twenty-two, 

 which is the amount of the force with which the hemispheres will be held to- 

 gether. If one of the handles be placed on a strong hook, and a weight of 

 four hundred pounds be suspended from the other, the weight will be supported 

 by the pressure of the atmosphere. 



This was one of the earliest experiments in which the effects of atmospheric 

 pressure were exhibited. Otto Guericke, the inventor of the air-pump, con- 

 structed, in 1654, a pair of such hemispheres, one foot in diameter. The sec- 

 tion through the centre of these was about one hundred and thirteen square- 

 inches, which multiplied by fifteen gives a pressure amounting to about seven- 

 teen hundred pounds. If the exhaustion were complete, the hemispheres 

 would be held together by this force ; but, even though incomplete, they were 

 still able to resist a prodigious force tending to draw them asunder. 



It is a consequence of the general theory of gravitation, that under the same 

 circumstances, bodies are attracted in proportion to their mass ; and hence it 



