WEIGHT OF WATER IN THE AIR. 183 



were we to descend with the barometer into the 

 earth, so increasing the weight of the column 

 of air pressing upon the mercury, it would 

 rise exactly in proportion as we descended. 

 So also any cause acting in the upper regions 

 of the air to increase the weight of the column, 

 would produce the same effect. Thus the great 

 November wave renders its passage obvious 

 even to the eye, for by reading the height of the 

 barometer, we are able to perceive its gradual 

 rise to its crest, and its gradual recession 

 again. 



It is generally said that it is the pressure 

 merely of the air which causes the rise of the 

 mercury in the tube of the barometer. Recent 

 researches of Professor Dove and Colonel Sabine 

 are tending gradually to develop a very dif- 

 ferent view of this matter. The air constantly 

 contains water in a vaporous and invisible state, 

 which, as well as the gases forming air, properly 

 so called, presses upon the mercury in the cistern 

 of the barometer. When the air is warmer the 

 pressure of this watery vapour is greater than 

 when it is cold and dry. Hence, during the 

 summer, the pressure of the vapour has an im- 

 portant influence in keeping up the level of the 

 mercury; during winter, on the contrary, its 

 pressure is least, but then the pressure of the 

 gases is greater than in summer. When, 



