204 



veral heights, 0, 3281, 6562, 9843, 13,124, and 16,405 feet, 

 and assuming, as the most unfavourable supposition, that 

 throughout the entire column, between two points of observa- 

 tion, the dew-point is the same as at the base, he finds the total 

 amount of vapour in a column 19,686 feet high. To this he 

 adds one-fourth, as the amount of vapour existing above the 

 highest point, and thus obtains the total amount of vapour ex- 

 isting in the entire atmospheric column. 



The pressure of such an amount of vapour would only pro- 

 duce, at the base of the column, a tension corresponding to a 

 dew-point of 47°, the actual dew-point being 81°. Professor 

 Patton infers, therefore, that the actual tension of the vapour 

 at the surface of the earth cannot depend solely upon] the 

 amount of vapour by which it is pressed. The same conclu- 

 sion is deduced from the observations of M. Guy Lussac. 



Professor Patton confirms this conclusion by arguments 

 drawn from the meteorological phenomena of the tropics. He 

 asserts that the hygrometer gives no indication of the mois- 

 ture which produces the tremendous tropical rains, amounting 

 sometimes to ten inches within twelve hours. 



In further confirmation of his views, as to the mutual ac- 

 tion of gases, the author adduces the following argument : — 

 If vapour exerts no pressure upon dry air ; and if we can by 

 any means cut off the lowest stratum of vapour, we should 

 thus diminish the total pressure by the weight of the vapour 

 existing in the atmospheric column. Acting on this prin- 

 ciple, Professor Patton takes a bent glass tube, both ends of 

 which are open and turned upwards. Into this tube he pours 

 a small quantity of mercury, having previously filled one arm 

 nearly to the bottom with powdered chloride of calcium. The 

 effect of this substance being to render the air which is in 

 contact with it perfectly dry, Professor Patton argues that, ac- 

 cording to the ordinary theory, the atmospheric vapour can 

 exert no pressure upon the mercury in that side. Hence, the 

 mercury in the two arms of the tube ought to show a diffe- 



