THE ATMOSPHERE. 



experiment, in the year 1654. The section of the hemispheres 

 employed by him measured 113 square inches, and they were held 

 together by a force equal to about three- fourths of a ton. 



Since the atmosphere envelopes the earth, extending, e very wh 

 above its surface to nearly the same height, it presses upon every 

 part of the surface, upon the surfaces of extensive continents and 

 islands, as well as upon those of oceans and seas, with the same 

 force as that which it is shown to exert upon the bladder-glass, 

 and the Magdeburg hemispheres. 



6. Let one end of a glass tube be plunged in a vessel of water, 

 and let the air be partially drawn from the other end by the 

 suction of the mouth applied to it. It will be immediately 

 observed that water will enter the tube, and will rise in it higher 

 and higher the more air is drawn from it by the mouth. This simple 

 experiment, so often made in the sport of children, supplies means 

 of weighing a column of air extending from the surface of the earth 

 to the top of the atmosphere, with as much precision as if that 

 column could be placed in the dish of a balance, and counter- 

 poised by equivalent weights. The water ascends in the tube, 

 because the pressure of the air within the tube being diminished 

 by the suction of the mouth, is less than the pressure of the air 

 upon the surface of the water in the vessel. This latter pressure 

 therefore predominating, forces the water up to a certain height 

 in the tube. The weight of the column of water which thus 

 ascends in the tube, is exactly equal to the excess of the weight of 

 a corresponding column of air, extending from the surface to the 

 top of the atmosphere, over the pressure of the air remaining in 

 the tube ; and it follows, that if the tube were long enough, and if, 

 by the suction of the mouth, all the air could be withdrawn from 

 it, a column of water would rise in the tube whose weight would 

 be exactly equal to that of a corresponding column of the air, 

 extending from the surface to the top of the atmosphere. 



7. Now this experiment was actually made by Pascal, at 

 Houen, in 1646. A tube was procured, measuring 46 feet in 

 length, but as the suction of the air from it was then considered 

 impracticable, the difficulty was surmounted by first closing the 

 tube at one end, and then completely filling it with water. The 

 upper end being then well corked, so as to prevent the escape of 

 the water, the tube was inverted by means of ropes and pulleys 

 properly attached to it, and the corked end being immersed in a 

 reservoir of water, and the tube being erected to the vertical 

 position, the cork was taken out. Immediately the column of 

 water in the tube subsided ; but instead of falling altogether out 

 of it into the reservoir, as many expected would happen, it 

 remained suspended at a height of about 34 feet above the level of 



100 





