WEIGHT OP THE ATMOSPHERE. 47 



exactly counterbalances the weight of the mercurj 

 in the tube, and so keeps it from falling. 



.57. When from any circumstance the weight of the 

 atmosphere is for a time diminished, it counterbalances 

 a less weight of mercury, and the column of mercury 

 in the tube of the barometer falls ; on the other 

 hand, when the weight of the air increases, the mer- 

 cury rises. These fluctuations in the weight of the 

 air are constantly going on with the changes of the 

 weather, and consequently the barometer is a most 

 valuable instrument, because it renders evident to us 

 the changes in the air, which being invisible we should 

 not otherwise know of.. 



58. The higher we ascend in the air, either by 

 going up a hill or by means of a balloon, the less 

 weight of air shall we have above us, and consequently 

 the shorter will be the column in the barometer. 

 Hence the use of that instrument in measuring 

 heights. The mercury falls about one inch for every 

 thousand feet as we ascend from the level of the sea. 



69. We do not feel the weight of the air at all, 

 because it presses equally in all directions, and com- 

 pletely surrounds us. The weight of a column of 

 air resting on a table four feet square, is about four- 

 teen tons ; but this weight does not press upon the 

 table, it is met by a corresponding pressure upwards, 

 from the air below the table, due to the weight of all 

 the surrounding air. 



60. In moving through the air, therefore, we merely 

 displace it, or move it aside ; but as we do not disturb 



