46 WEIGHT OF THE ATMOSPHERE. 



54. Air, like water and solid substances, has weight 

 or gravity ; it presses downwards towards the centre 

 of the earth ; and, as the bulk of the air is very great, 

 the whole weight of it pressing on the surface of the 

 globe must be enormous. It is important to dis- 

 tinguish between the weight of air, and the weight 

 of the atmosphere. By means of a light glass globe, 

 an air-pump, and a good pair of scales, we can 

 readily ascertain the weight of a cubic foot of air, 

 and we find that, under ordinary circumstances, it 

 weighs thirty-one grains. 



55. The weight of the atmosphere is, however, the 

 accumulated weight of many thousand cubic feet of 

 air, for the air surrounds the earth to a height of 

 many miles, floating upon its surface, and pressing 

 downwards in consequence of its weight or gravity. 

 The weight of the column of air which rests upon 

 any given space or surface, may be known by the use 

 of the barometer : and is found to be about two 

 thousand pounds on the square foot. 



56. The barometer consists of a glass tube nearly 

 a yard long, closed at one end, filled with mercury, 

 and inserted in a cup of the same heavy liquid. The 

 height which the mercury stands in the tube, is an 

 exact measure of the weight of a column of air of 

 the same size as the diameter of the tube, but as high 

 as the whole atmosphere. The mercury in the tube 

 being very heavy, presses downwards, and tends to 

 fall in the tube ; but the air without, which also press- 

 es downwards, resting on the mercury in the cup, 



