274 RELATION OF PORESTa TO T HE 



produced or yielded a greater rainfall on mountains than on plains ; 

 as in passing over mountain elevations the temperature of the air 

 would experience a great reduction, and a precipitation of the 

 moisture they contained would follow. 



And it may be the case that the presence or the absence of forests 

 on the mountain summits might affect within a limited range the 

 degree to which the temperature of the wave in passing over the 

 ridge would be reduced. So would a fire in a hunter's cabin ! So 

 would the huntsman's breath, and even the huntsman's presence, and 

 every discharge of the huntsman's rifle ! But in the presence of 

 such meteorological phenomena as are under consideration all of 

 these are as nothing. 



And as is the case in such occasional disturbances, so may it be, 

 and most probably it is, with the regular current of the atmosphere 

 passing over a mountain range in its coui'se to the equator from the 

 pole, or striking upon a mountain top in its return to the arctic or 

 antarctic zone. The great change of temperature occurring in the 

 case cited made manifest in such a way as to arrest attention the 

 consequence of such a change in so far as it afi'ected the rainfall ; 

 but any reduction of temperature below the dew-point would, in pro- 

 portion to its extent, produce corresponding effects. Such a reduction 

 of temperature frequently follows the passage of an atmospheric 

 current over a mountain range ; and to the altitude of the mountain, 

 so completely as almost to warrant the use of the term exclusively, ia 

 the greater rainfall on the mountain attributable. 



I have hesitated about using the term exclusively, not on account 

 of the influence which the circumstance of the^ mountain summit 

 beinc clothed or being bare of forests might have in somewhat, but 

 probably to an imperceptible degree, modifying the result ; but I 

 have done so on account of the influence on the rainfall exercised by 

 the contour of the mountain range. 



This subject, the influence on the rainfall exercised by the super- 

 ficial configuration of a country, has engaged the attention of M. 

 Cezanne. From his sequel to Etude sur les torrents des Hautes-Alpes, 

 I translate the following statement : — 



" When a rain-producing wind strikes against an eminence which 

 forces it to rise in the atmosphere, a double efl'ect is produced : the 

 ascent of the air itself tends to a reduction of temperature, and con- 

 sequent condensation of vapour j but besides this, the atmospheric cur- 

 rent turned from its course is subjected to friction against the cooled 



