

50 



Win. A. Norton on the Variations 



of the air and of the quantity of vapor suspended in it ; and if it 

 be independent of the density of the air it must be independent 

 of the density of the vapor, so long as this retains the aeriform 

 state. It is also independent of any differences that may subsist 

 between the temperatures and densities of the different strata of 

 the atmosphere, for it is a well established fact that the tempera- 

 ture fails more rapidly, as we ascend in the atmosphere, in a sum- 

 mer than in a winter night. In other words, it holds good what- 

 ever may be the state of all the various particulars upon which 

 the absorptive action of the atmosphere when transparent, can be 

 supposed to depend. 



As for the relative clearness of the sky, I find, on referring to 

 the Report of the Meteorological Observations made at Philadel- 

 phia, that during a period of two years and three months, viz., 

 from March, 1843, to July, 1845, the average clearness of the 

 sky was somewhat greater after midnight than before it. The 

 following numbers show the averages of the 



Mean Sky Covered by Clouds. 



1843 (from March), 

 1844, 



1845 (to July), 



From 6 P. M. to midnight. 



•63 



. -72 . 

 •64 



From midnight to 6 A. M 



. -59 



•62 



. 61 



The numbers for the different quarters of years are as follows : 



Mean Sky Covered by Clouds. 



April, May, June, 

 1843. <( July, Aug., Sept., 



Oct., Nov., Dec, 



Jan., Feb., March, 



1844 1 Apri1 ' May ' June > 



July, Aug., Sept., 



Oct., Nov., Dec, 



Jan., Feb., March 



April, May, June, 



For the entire night, from 6 p. m 



1845. 



- 



From 6 P. M. to midnight. 



53 



66 . 

 72 



71 . 

 90 



65 . 

 61 - . 

 61 . 

 67 



From midnight to 6 A. 



. -51 



•66 



. 59 



•62 



. -83 

 •56 



. -56 



•60 



. 63 



M. 



to 6 a. m., we have 



Jan., Feb., March, 

 April, May, June, 

 July, Aug., Sept., 

 Oct., Nov., Dec, 



1S43. 



•52 



66 



65 



1844. 



1845. 



Average. 



•66 



•60 



•63 



•86 

 60 



•65 



• • 



•67 

 •63 



58 



61 



These numbers show that during these years the clearness of 

 the sky was no greater from March to October than during the 

 first and last quarters of the year, and therefore that the more 

 rapid fall of the temperature at night toward the middle of the 



year, than toward the beginning and end of it, during the inter- 



