4t ON THE SEPARATE PKESSUBES OF THE AQUEOUS AND 



being supposed equal to 30 inches of mercury, the vapour has 

 been presumed to be equal in weight to an inch of mercury. 

 The tension of the vapour being found equal to the inch of 

 mercury, it has been assumed that its weight was also equal. 

 Tension and weight of vapour are not however identical, and the 

 tension in this case is due to partial confinement under a parti- 

 cular temperature, and confined vapour we know may have its 

 tension increased by heat to a great extent without any increase 

 of its weight. It should be remembered too, that it is not near 

 to the surface of the earth alone that the gases obstruct the 

 expansion of the vapour, but in their whole mass, through which 

 the vapour is regularly trying to force its way by the exertion 

 of its power of expansion. The vapour passing from the 

 surface upwards has to force its way successively through the 

 whole mass of the obstructing medium, and each portion of the 

 vapour below becomes, to that which is expanding above it, a 

 base, or fulcrum, from which the elastic force is exerted, — so 

 that the whole resistance of the gases through which the 

 rising vapour is penetrating is transmitted downward through 

 the lower vapour, and that aggregate resistance contributes to 

 the formation of the degree of vapour tension that is found 

 near the surface. The tension of vapour shewing a dew-point 

 of 80*, does not therefore indicate that vapour equal in weight 

 to an inch of mercury exists in the atmospheric column. The 

 quantity of vapour, for the reasons just given, must be con- 

 sidered much less, and it may for the present be supposed to 

 be equal to only half an inch of mercury. 



But it does not follow that, when there is tropical vapour 

 equal in weight to only half an inch of mercury, there must 

 be a weight of 29i inches of the gases, in order to make up 

 the total 30 inches, the assumed weight of the atmosphere. 

 For the cold of the gases may be, and is, regularly condens- 

 ing some of the vapour as it ascends, and converting it into 

 small globules of water, which remain in the atmosphere as 

 water and not as vapour. This is commonly seen in the mist 



