106 METEOROLOGY. 



tibles, ill consequence of a partial failure of rain, which then takes place. 

 The recently fallen foliage of Autumn, affords peculiar facilities for the 

 prevalence of extensive fires during the season of the " Indian Sum- 

 mer." 



We must therefore regard, what is ordinarily called smoke and haze^ 

 as very thin cloud. Sometimes this is seen at some elevation in the air, 

 in a great measure disrobing the sun, moon and stars of their splendor, 

 and producing that mild subdued light witnessed in an atmosphere de- 

 nominated hazy. At other times it prevails more abundantly near the 

 surface of the earth, producing that peculiar appearance of objects seen 

 through it denominated smoky. It is generally found to become denser 

 during the night, or early in the morning, in consequence of diminished 

 temperature ; and then, during the day, again to become more rare in 

 consequence of the increased temperature of the air. 



The mechanical condition of the atmosphere accompanying that kind 

 of weather called smoky, is that of comparative rest. During the con- 

 tinuance of this kind of weather, the movements in the atmosphere are 

 feeble, as the most superficial observation will show. The winds, if 

 there be any, are always gentle, except sometimes for a few hours at 

 the commencement. The temperature, also, during the twenty-four 

 hours, is not subject to much variation. But when we leave superficial 

 observation, and have recourse to the indications of proper instruments, 

 we become better satisfied of the truth of the remark just made. The 

 barometer, for example, is slow in its movements, and does not rise 

 much above or fall much below its mean altitude. Any considerable 

 rise or fall is accompanied by a corresponding change in the transpa- 

 rency of the air. The criterion is, that, during the prevalence of any 

 smoky season, the pressure varies within comparatively narrow limits. 



Now this approximate equilibrium in the atmosphere over any sec- 

 tion of country for several days continuously, is, no doubt, the cause 

 of the destruction of its transparency. For it is a matter of observa- 

 tion, and consequently one of the facts of the case, that the murkiness 

 of the atmosphere increases with the continuance of its comparative 

 quiescence, and the density of the former is proportionate to the near- 

 ness of the latter to a perfect equilibrium. 



Agreeably to the principles laid down in No. IV, that form or kind 

 of cloud called smoke can be formed extensively in the atmosphere 

 only during a partial equilibrium of forces. When either the tempera- 

 ture or pressure varies rapidly, the resulting equally quick movements 

 favor the rapid condensation of invisible vapor into dense cloud and 

 rain, by which the moisture is withdrawn from the atmosphere, or bear 



