RAIN ON THE ALPS. 237 



vicinity. Hence it might have been imagined, 

 that the great elevation of St. Bernard would 

 reduce the quantity of rain below that at the 

 plain of Geneva. The fact however appears to 

 be far otherwise; and it may demand a little 

 consideration. 



High mountains produce rain, I think un- 

 questionably from their obstructing the horizontal 

 currents of the air, and causing them to ascend 

 into the higher regions of the atmosphere, by 

 which airs of different temperatures are mixed 

 together. Now it is well known, that two por- 

 tions of air, saturated with vapour at their 

 respective temperatures, when mixed together, 

 are incapable of retaining the whole of the vapour: 

 a part of it is precipitated in the form of a 

 cloud or of rain. This is the case too if the 

 portions of air be under saturation, within certain 

 limits. 



The physical principles on which the above 

 statement is supported, are, 1st. — When two 

 portions of air of different temperatures are 

 mixed, the temperature of the mixture is the 

 arithmetical mean of the two temperatures : 

 2nd. — When two portions of air saturated with 

 vapour are mixed together, the quantity of 

 vapour found in the mixture must also be the 



