10 Obscurations of the Atmosphere. 



There is in Siberia, M. Erman has informed us, an entire 

 district, in which, during tlie winter, the sky is constantly 

 clear, and where a single particle of snow never falls. 



I am willing to overlook the perturbations of the terres- 

 trial temperatures which may be connected with a greater or 

 less abundant emission of light or solar heat, whether these 

 variations of emission depend on the number of spots which 

 are found accidentally scattered over the sun's surface, or 

 whether they originate in some other unknown cause ; but it 

 is impossible for me not to draw the reader's attention to the 

 obscurations to which our atmosphere is from time to time 

 subject, without any assignable rule. These obscurations, 

 by preventing the light and solar heat from reaching the 

 earth, must disturb considerably the course of the seasons. 



Our atmosphere is often occupied, over spaces of consider- 

 able extent, by substances which materially interfere with 

 its transparency. These matters sometimes proceed from 

 volcanoes in a state of eruption. Witness the immense 

 column of ashes which, in the year 1812, after having been 

 projected from the crater of the island St Vincent to a great 

 height, caused at mid-day a darkness like that of night in 

 the island of Barbadoes. 



These clouds of dust appear, from time to time, in regions 

 where no volcano exists. Canada, in particular, is subject 

 to such phenomena. In that country recourse has been had, 

 for an explanation, to the burning of forests. The facts do 

 not always appear to agree exactly with this supposition. 

 Thus, (m 16th October 1785, at Quebec, clouds of such ob- 

 scurity covered the sky, that it was impossible, even at noon, 

 to see in what direction one was going. These clouds covered 

 a space of 120 leagues in length by 80 broad. They seemed 

 to come from Labrador, a country very thinly wooded ; and 

 they presented none of the characters of smoke. 



On the 2d July 1814, clouds similar to the above sur- 

 rounded some vessels in the open sea on their way to the 

 River St Laurence. The great obscurity lasted from the 

 evening of the 2d till the afternoon of the 3d. 



With regard to the object we have here in view, it is of little 



