METEOROLOGY. 



405 



the city has, therefore, a better chance to escape the injuries of late 

 spring and early fall frosts than the adjacent country by thirty-six 

 days. While the former enjoys a period of 204 days without frost, 

 the latter must be satisfied with one of but 168 days, or a period by 

 one sixth less. 



On the morning of the 23d of April, the day on which the latest 

 spring frost occurred here, the thermometer marked 32° at 5.15 a. m., 

 and 39° at 7 a. m. On the morning of the 9th of October, the day 

 of the earliest fall frost, the thermometer marked 28° at 6 a. m., and 

 32 1 , at 7 a. m. ; while Dr. Engelmann's gave 42^° at 7 a. m. 



7. The monthly means of clearness and calmness combined are found 

 to be in proportion to those of the differences of temperature between 

 the two places of observation, with only some very slight modifications, 

 as seen by the results of the following table, where the numbers of 

 calms are multiplied by the means of clearness : 



In trying to account for the facts mentioned above, several causes 

 could be named as contributing more or less towards the differences of 

 temperature. But the preceding table seems, more than anything 

 else, to demonstrate that the principal cause of the depression of tem- 

 perature in these cases is radiation of heat, a most universal and pow- 

 erful cause, which may, by the interposition of certain mediums, be 

 either retarded in its action or altogether suspended. Such a medium 

 may be found in masses of clouds, as well as in those of smoke, the 

 presence or absence of which may account for most of the differences 

 between the two stations of observation. 



We can, therefore, with good reason, assign chiefly to the absence 

 of smoke the lower temperature near the city limits, although it is 

 more than probable that the process of cooling may have been assisted 

 to some extent by the gently sloping sides of the creek valley ; for it 

 it is well known that the air cooled upon adjacent swells or eminences 

 of the ground, and then gliding down into lower places, does accelerate 

 the cooling action of radiation of the slopes and bottoms. 



The inhabitant of a large city, who in his daily walks of business 

 pays little or no attention to the great phenomena of the atmosphere, 

 but who, having heard or read something about radiation of heat to- 

 wards an unclouded sky, may begin to entertain doubts about these 

 truths of science when distressed with a close and sultry atmosphere 



