THE CAUSE OF RAIN. 



9 1 



hygrometer sometimes remains near saturation without there being 

 precipitation of vapor; and, supposing that the temperature is near 

 3° or 4° C, which is about the mean temperature of the year, it will 

 require a cooling of only one or two degrees centigrade at most 

 for the air to be unable to hold all its vapor and for the excess of it to 

 be transformed into rain. This is confirmed by experiment and ob- 

 servation. 



I will • mention a remarkable example illustrating this point. 

 Not rarely, when the west wind is blowing violently on the top of the 

 Puy-de-D6me, an east wind, blowing opposite to it, prevails at Cler- 

 mont. Then an eddy is formed behind the plateau and the chain 

 of puys that runs from north to south, a little west of Clermont. 

 This eddy gradually becomes a vast whirlwind with a horizontal 

 axis, several leagues long, a few kilometres wide, and seven hundred 

 or eight hundred metres high. It commonly gives rise to an abun- 

 dant and continuous formation of black clouds, which appear in an 

 instant along its length, following its intersection with the upper 

 current. The phenomenon is frequent, and is sometimes produced 

 under very interesting conditions, as on a certain day when the tem- 

 perature at Clermont was five degrees above zero, centigrade, while 

 the hygrometer indicated that the air contained seven tenths of the 

 quantity of vapor required for saturation. Under such conditions the 

 temperature on the Puy-de-D6me would have only had to be a 

 very little above the freezing point for the vapor of the horizontal 

 eddy to be transformed into rain on meeting the upper current 

 coming from the west. Now, on the top of the mountain the ther- 

 mometer marked 4° C. below the freezing point. Hence, every time 

 the lower east wind increased a little, this having the effect of 

 carrying the vapor and the air of the lower regions a little higher, the 

 black clouds could be seen developing with a recrudescence of inten- 

 sity. A few instants afterward a torrential rain fell at Clermont. 



In some cases — and such frequently occur in summer — the min- 

 gling of strata of air of different temperatures is effected by ascend- 

 ing currents. The sky is clear; the moist air in contact with the soil 

 is warmed under the action of the sun, rises, and more or less quickly 

 reaches a much colder stratum. " Light mists are formed; they may 

 frequently be seen rising and spreading out over the warmer or 

 moister spots. On the flanks of the Puy-de-D6me one may often 

 find himself among ascending currents of this sort which succeed 

 one another intermittingly when the air is calm, after a rain; they 

 rise with a velocity of four or five metres at least per second. 



These fogs finally become stationary in a region of the same 

 density with themselves. There they accumulate and form a cloud 

 or a group of clouds that go on developing. When penetrated by the 



