158 EFFECTS OF FORESTS ON RAINFALS. 



opposite side ; the highest summits aloue emerge above the inunda- 

 tion ; but down the stream the clouds seem to be falling into an un- 

 fathomable void ; they are seen to be disappearing, and the slope on 

 this side, lit up with sunshine, offers a smiling contrast to the other, 

 which, immersed in the storm, is shrouded in a thick veil. The rain 

 in such circumstances is invisible to the observer, but he makes its 

 acquaintance a little later, when in his descent he passes through the 

 clouds, or, if the storm have then passed away, when he finds all the 

 streamlets on his road overflowing their banks." 



When one is enabled thus to look down upon the clouds he may 

 see the atmospheric current in obedience to the laws of hydraulics. 

 It has its rapids, and its eddies, it strikes with fury against jutting 

 obstacles, and when it is contained between regular sloping hills it 

 abandons the convex mound, and presses itself out against the 

 concave bank. Thus is the aerial current made manifest. 



Thus far we have had to do with the production of clouds ; the 

 transition to the consideration of the production of rain is easy. 

 This is thus detailed by Sir John Herschel* : " When the sun 

 shines on a cloud which absorbs its beat, the cloud itself is neces- 

 sarily evaporated, and the vapour by its levity tends to produce an 

 upward current, and thus to counteract the effects of gravity on 

 the globules of which it consists. A globule of water jg^jypths of 

 an inch in diameter, in air of 5-6 ths the density on the surface, or 

 at the height of about 5000 feet, would have its gravity counter- 

 acted by resistance, with a velocity of descent of one foot per second 

 (supposing no friction and no drag) ; and even if the terminal 

 velocity were reduced to half that quantity by these causes, would 

 still require some upward action to enable it to maintain its level, — 

 a circumstance which sufl&ciently accounts for the lower level gene- 

 rally observed of cloud during the night. It is more probable, 

 however, that when not actually raining, a cloud is always in process 

 of generation from below and dissolution from above, and that the 

 moment this process ceases, rain in the form of ' mizzle ' commences. 

 In a word, a cloud would seem to be marely the visible form of an 

 aerial space in which certain processes are at the moment in 

 equilibrio, and all the particles in a state of upward movement. 



In whatever part of a cloud the original ascensional movement of 

 the vapour ceases, the elementary globules of which it consists, 



* " Meteorology : " a volume reprinted from the Encyclopaedia Britannica . 



