THE GASEOUS PORTIONS OP THE ATMOSPHERE. «> 



space, every portion of it in that space up to a great height 

 would be found to have an incumbent weight resting upon it 

 proportioned to the dew-point in the part : as, were it not for 

 the pressure of such a column, the vapour would in virtue of 

 its elasticity expand upwards and diffuse itself through the 

 space above, until the quantity and tension became duly pro- 

 portioned to the incumbent pressure and to the temperature 

 of the part. But when the atmospheric space is filled with 

 gases, the vapour that is produced by the evaporation of 

 water, in its ascent to the higher regions, encounters the 

 gases, and is by tliem obstructed and impeded in its expansion 

 into those regions. The elastic force of the vapour, however, 

 in due time enables some of it to force its way through the 

 gases, and if the whole of the vapour while intermixed with 

 , the gases could even slowly diffuse itself through them to as 

 great an extent as it would in free space, and finally attain a 

 state of rest, the upper portion of the vapour would, as if it 

 were free, press on the lower, which by its tension would 

 determine the dew-point in the part. This degree of expan- 

 sion and difiusion, however, it cannot accomplish, because 

 whilst the vapour is impeded in its rising, the cold of the 

 gases in the upper regions of the atmosphere is regularly 

 condensing a part of the vapour as it rises, and this cold 

 efiectually prevents the atmospheric space from being filled 

 with vapour to the extent that it would be if no gases were 

 there to cool it. The degree of tension of vapour at the 

 surface is therefore due, not solely to the incumbent pressure 

 of the upper vapour, but only in part to that cause, and 

 partly to the impediment which the gases present to its 

 expansion, until additional evaporation accumulates vapour 

 sufficient to overcome successively, to a certain extent, the 

 obstruction of the gases, and to produce the dew-point and 

 tension that are actually found. 



In this way evaporation of water from the surface of the 

 sea, near the equator, produces the vapour that has a dew- 

 point of 80*^, and the total weight of the atmosphere there 



