450 METEOROLOGY. 



snii, it sometimes presents the seven colors of the spectrum, although 

 usually there is only a single internal tint of orange, terminating at 

 times in a little red. On the contrary, the great halo produced by the 

 moon IS almost always white, and seldom exhibits the tint of orange 

 without the r^d. 



The cirro-cumuli produce the lunar halo of about 2° to 4° radius, 

 which may be triple or formed of sixteen prismatic rings with an inter- 

 nal tint of red. This halo is still more brilliant when it takes place, 

 rarely enough, with cirro-stratus. 



The fracto-cumuU are the only clouds which do not i^roduce halos, 

 but they do produce complete coronas or segments of arcs, according 

 to the extent of the fragments which traverse the lunar disk. These 

 coronas are also prismatic, having a blue internal tint. 



The pallio-cumulus and the cumulus form neither halos nor coronas, 

 but solar and lunar rainbows. In line, aqueous vapor extremely dis- 

 solved, elastic, uniformly distributed in the higher regions of the atmo- 

 sphere, without much altering tbe transparency of the air, gives rise to 

 the formation of a little halo. Their unique coloration, in brown or rus- 

 set, light or dark, as well as their size, is intimately connected either 

 with the density of aqueous or elastic vapors or their altitude •, their 

 dimensions may vary from the borders themselves of the lunar disk up 

 to 2° radius. We find them in every lunation. 



QUANTITY OF CLOUDS. 



"We measure by the eye the blue vault of the sky, or the quantity of • 

 visible clouds, which can then be determined according to a conven- 

 tional scale in decimal fractions from zero (0) to unity, (1.) But it is 

 preferable to take directly the quantity of clouds, and to repeat this 

 calculation for each quadrant, upon each stratum and upon each type, 

 instead of restricting ourselves to the ensemble of the sky, paying no 

 attention to their nature, as has been hitherto everywhere done. 



Here is the mode of proceeding : We explore the first quadrant, 

 and if we find three diiferent types of clouds — for example, the cirrus 

 elevated, cumulus at the horizon, and fracto-cumtihis low and isolated — 

 we judge one after the other, according to their extent in height and 

 breadth, the space which they occupy relatively to the 90^ comprised 

 from north to east, and from the horizon to the zenith of this quadrant. 

 We then write for example in the corresponding columns 0.4 cirrus, 0.3 

 cumulus, and 0.2 fracto-cumulus. If the quadrant examined is completely 

 covered by a single cloud, we mark unity, (1,) and its corresponding 

 type. If, on the contrary, there is no cloud, we place 0. We repeat in 

 turn the same operation in the three other quadrants, from southeast, 

 from southwest, and from northwest. 



At times the quantity of clouds associated with others of a different 

 nature is so small, consisting merely of fragments, that it would be ex- 

 tremely difficult to make a just calculation, and then we mark isolated 



