CLOUDS. 453 



makes it only possible to determine it visually and approximately. As 

 a general rule, clouds appear more rapid the nearer they are to the sur- 

 face of the earth, and slower the farther they are removed from it. 

 Therefore, the fracto-cumulus, which almost grazes the suiumits of. 

 mountains and tops of trees, is more rapid; while the cirrus, wliich is 

 found in the torrid zone with an altitude of at least 10,000 to ir»,000 

 meters, (six and one-fourth to nine and three-eighths miles,) is the slow- 

 est, seeing that it remains for hours at a time almost immovable. 



We adopt the four following terms: 8low,very slow, rapid, very rapid, 

 which suJBfice to express, with sufficient exactness, all velocities of clouds, 

 because there is no case where we need a more minute nomenclature. 

 The absolute determinations, especially very rapid, being the most diffi- 

 cult to seize, we must guard against makiug use of them before we are 

 perfectly acquainted with the march of the cirrus, which loiters for 

 hours, describing- a little arc, and that of the fracto-cumulits, which has 

 variable velocities. But after a few appearances of clouds with high 

 velocity, the observer will be able to judge them correctly. 



AZIMUTmVL EOTATIOX OF CLOUDS. 



In a note presented to the French Meteorological Society, May lOtli, 

 1864, I showed, fi"om 280,320 observations made at the observatory of 

 Havana, that the law of the rotation of winds formulated in 1827 by M. 

 ])ove is perfectly applicable to clouds, and it is this same rotary direc- 

 tion which determines the rotation of the inferior winds, and modities 

 all the meteorological phenomena ; in a word, that meteorology must be 

 taken /rom above, according to the profound remarks of M. Biot, at the 

 French Academy of Sciences. 



M. Dove's law of the change of winds may be thus recapitulated : 1. 

 AYhen in the northern hemisphere, currents of air coming from the 

 equator, alternate with polar currents, the wind makes the tour of the 

 comjiass oftenest in the order south, west, north, east, and south. 2. In 

 the southern hemisphere it is the reverse, south, east, north, west, and 

 south. 3. The influence of the wind upon meteorological phenomena, 

 combined with this law of its change, shows two parts of the compass, 

 opposed in aU resjiects, the region of the east and that of the west, where 

 the atmospheric variations offer a corresjjon deuce with the instruments, 

 which it is easy to understand. We see, therefore, that this important 

 law of M. Dove can assist us in scientific prevision, if we add M. Buys- 

 Ballot's method of ecarts, which consists in taking the difference between 

 the highest and lowest standing of the barometer, thermometer, &c. 



Now, if the change of clouds, from cirrus to fracto- cumulus — that is 

 to say, from an altitude of at least 10,000 meters to the surface of the 

 earth — really obeys the same law as the change of winds, then our pre- 

 visions acquire a greater degree of certainty. 



In 18G3 the wind at Havana completed twenty- three rotations, con- 

 jointly with cumulus ; these latter, twenty-five rotations ; cirro-cumulus 



