^ ON THE SEPARATE PRESSURES OF THE AQUEOUS AND 



the equator the dew-point is sometimes at 80*' of temperature 

 when the force of the vapour is considered equal to the 

 weight of a column of mercury an inch high, and this vapour 

 is capable of sustaining such a column by pressing against 

 it; whilst in colder latitudes, where the dew-point is say 

 only 32°, the tension of the vapour is capable of sustaining 

 only one-fifth of an inch of mercury. The total atmospheric 

 pressure being supposed always equal to 30 inches of mercury, 

 it follows that the vapour and the gaseous pressures will be 

 differently proportioned in the two supposed cases : in the 

 former the vapour pressure will be inyth of the whole, whilst 

 in the latter it will be only a 150th part of it. The total 

 atmospheric pressure however is not always equal to 30 inches 

 of mercury, as that pressure varies to the extent of about 

 3 inches, or say from 28 to 31 inches. And on every change 

 of the total atmospheric pressure, it may be necessary to 

 ascertain whether this change is due to an alteration in the 

 vapour or in the gaseous pressure. In the greater fluctua- 

 tions that take place, the alterations amount to much more 

 than the whole of the vapour pressure, and therefore they 

 must, to a large extent, be attributed to an alteration of the 

 gases ; but in other instances the changes are small, and we 

 have to determine whether they are due to an alteration in 

 the pressure of the gases or of the vapour. 



There is no reason to believe that the total quantities of 

 the two gases which constitute the greater portion of the 

 atmosphere are ever altered to an appreciable extent, and 

 the quantity of carbonic acid gas that is in it is so small, 

 as to render notice of it unnecessary, but the quantities of 

 vapour found near the surface of the globe vary considerably 

 in different localities. 



From the tension of vapour at the surface, whatever it 

 may be, the total quantity of it in the incumbent atmosphere 

 at the time has, by modern meteorologists, been inferred; 

 this inference, however, I have in a former paper shewn to be 

 erroneous. Were vapour existing alone in the atmospheric 



