266 WRITINGS OF JOSEPH HENRY. [1855- 



because the precipitation begins at a definite temperature 

 due to a definite height ; its form will be that of a mushroom, 

 bulging out and gradually increasing in altitude ; in short, 

 will be precisely that form of cloud which is denominated 

 cumulus, and which may be seen during a moist warm day 

 forming in a still atmosphere, gradually extending upward 

 until the precipitation of vapor begins to be so copious that 

 the particles of water coalesce and form drops of rain, which 

 falling down directly through the base of the cloud, leave 

 but a remnant of very attenuated vapor, which is blown 

 away and forms, according to Mr. Espy, the cirrus or hair 

 clouds. 



We can also readily infer from the same principle that so 

 long as a current of air moves horizontally over a plain of 

 uniform temperature, no precipitation will take place; but 

 if in its course it meets with a mountain, up the acclivity of 

 which it will be obliged to ascend and thus come under 

 a less pressure and lower temperature, a precipitation must 

 ensue. We have in this way a natural explanation of the 

 effect of a mountain in causing a cloud and a fall of rain, 

 and need not refer the phenomena to the unscientific expla- 

 nation of attraction so frequently given ; we say unscientific, 

 because the attraction of gravitation at a distance on an atom 

 of vapor, is almost infinitely small, and could have no ap- 

 preciable efiect in drawing the clouds. If we suppose, in ad- 

 dition to the preceding case, that the air, after ascending to 

 the top of the mountain and forming a cloud by the precip- 

 itation of its moisture, descends on the other side to the same 

 level, it will arrive at the earth much dryer than it went up. 

 If the height of the mountain is not sufficient to reduce the 

 temperature enough to produce a rain, but merely a cloud, 

 and if we suppose the current of air to continue its course, 

 and to descend to the same level on the other side, it will, 

 as it descends, become condensed as it comes under greater 

 pressure; the temperature will increase for a like reason to 

 that which caused its diminution in the ascent. 



We have in this way an explanation of the paradoxical 

 appearance of a strong wind blowing across the top of a 



