that's it ; 



blending, produce cirro-stratus; 

 c and E cirro-cumulus; E and 

 D cumulo-stratus ; F is generally 

 of short duration, and retains its 

 specific f:>rm. There are thus 

 four primary, and three second- 

 ary, or compound forms of clouds. 

 Clouds, although formed of light 

 vapours, preserve their forms for 

 a. considerable time, because the 

 depths of the atmosphere, like 

 the depths of the sea, are still, 

 or move only in great volumes. 

 The winds that rattle against 

 our windows, and hiss among 

 the trees, are the atmospheric 

 waves converted into breakers, 

 like the waves on the sea-shore, 

 by objects that resist their pro- 

 gress. When two opposing cur- 

 rents of wind meet, then the 

 clouds are seen to break up 

 rapidly, and assume new combi- 

 nations in the currents by which 

 they may be borne away. Moun- 

 tains cause rains by driving the 

 winds which strike against their 

 sides, 2, upwards into the cold 



171. 



regions, 3, where the vapours, be- 

 coming condensed by the lower 

 temperature, descend as rain, 4. 

 The atmosphere presses upon 



172. 



the earth's surface with a force 

 equal to fifteen pounds to the 

 square inch at the level of the sea. 

 This may be proved by taking a 

 piece of glass tube, 5, closed at 

 one end, and having a be re 

 equal to one square inch, in 

 length some- 

 thing more 

 than thirty in- 

 ches. Till this 

 with mercury 

 ( quicksilver), 

 and then, plac- 

 ing the thumb 

 over the ori- 

 fice, 6, immerse 

 it in a basin of 

 mercury, 7-, the 

 mercury will 

 sink in the 

 tube until at 

 the height of about thirty inches 

 from the level of the mercury in 

 the basin, when it will rest.* It 

 is the pressure of the air on the 

 mercury in the basin which ba- 

 lances the weight of the column 

 of mercury in the tube, and that 

 is equal to about fifteen pounds. 

 If the pressure upon the mercury 

 in the basin were not upon every 

 part of its surface equal to the 

 weight of the column in the 

 glass tube, it is obvious that 

 the merqury within the tube 

 would descend into the basin. 

 The depth of the atmospheric 

 body which envelopes the earth 

 is greater at the equator, 8, 8, 

 than elsewhere, because the diur- 

 nal revolution of the globe upon 

 its own axis, which has imparted 



* The experiment is, of course, usually performed 

 with a smaller tube, the proportions of which afford 

 data for calculation. But for the simplcity, we sup- 

 pose the tube to be exactly equal to a square inch ia 

 bore. 



