and at the Pass of Great St Bernard. 103 



The most striking circumstance with regard to the rain is the 

 great excess of it, compared with that at Geneva. Though the 

 average rain at Geneva for the thirty-two years was SO inches 

 annually, yet the average quantity for the same ten years as 

 those which were observed at St Bernard was only 26 inches 

 annually ; so that the rain at St Bernard is nearly two and a 

 half times as much as that at Geneva. From the observations 

 made in great Britain, it appears to be an established fact that 

 more rain falls in the hilly parts of the country than the plains ; 

 but it also appears, that the quantity of rain in a low situation, 

 is greater than that in an elevated situation in the vicinity. 

 Hence it might have been imagined, that the great elevation of 

 St Bernard would reduce the quantity of rain below that of the 

 plain of Geneva. The fact, however, appears to be far other- 

 wise ; and it may demand a little consideration. High mountains 

 produce rain, I think, unquestionably 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 dif- 

 ferent temperatures are mixed together. Now, it is well known, 

 that two portions of air, saturated with vapour at their respec- 

 tive temperatures, when mixed together, are incapable of retain- 

 ing the whole of the vapour ; a part of it is precipitated in the 

 form of a cloud or 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 sup- 

 ported, are, 1*^, When two portions of air of different tempera- 

 tures are mixed, the temperature of the mixture is the arithme- 

 tical mean of the two temperatures,, ^d, When two portions of 

 air saturated with vapour are mixed together, the quantity of 

 the vapour found in the mixture must also be the arithmetical 

 mean of the quantities found in each ; but it is only a quantity 

 proportional to the geometrical mean that can be supported in 

 the state of vapour, by the mean temperature; and as the geo- 

 metrical mean is always less than the arithmetical mean, the ex- 

 cess must needs be precipitated. 



This accounts for more rain falling in mountainous countries 

 than in plains ; but the question at present is. How does it hap- 

 pen that more falls on great elevations amongst the Alps tlwin 

 on the plains below ? 



