54 Wm. A. Norton on the Variations, fyc. 



the slower rate of cooling after midnight is too common a phe- 

 nomenon to depend upon rain, or prevailing direction or force of 



A m -^ -»_ _& A_^» j_ > A 



wind. 



Wa 



on which the law held good, as a matter of fact only two were 

 rainy. The exceptional days were, with one exception, either 

 rainy or foggy. 



The following table shows the total quantity of rain that fell 



at Philadelphia during each quarter of the years 1842-3-4, and 

 the first two quarters of 1845. 



1842.— Inch. 1843.— Inch. 1844.— Inch. 1845.— In. 



Jan., Feb., March, . . 629 4710 7-305 6-503 



April, May, June, . . 1059 10330 5.244 6604 



July, Aug., Sept., . . 11-39 15-704 10-787 . . 



Oct., Nov., Dec, . . 7-85 8*672 8*017 . . 



Now, if we compare the numbers for 1844 with the mean noc- 

 turnal losses of temperature, in the different quarters of this year, 

 which are 11°, 16°, 16°, 10°. we see that while the fall of tem- 

 perature is the same for the two middle quarters of the year, the 

 quantity of rain fallen is about twice as great for one of them as 

 for the other. The cloudiness of sky for these periods is -92 for 

 the first, and -70 for the second. Again for the middle quarters 

 of the year 1843 the quantities of rain are 10 in *3 and 15 in, 7, 

 while the decrements of temperature are 15° and 12° — the re- 

 verse of what should be the fact, since if rain is the determining 

 cause of the greater loss of temperature in a summer night, it can 

 only have this effect by cooling the earth in summer and warm- 

 ing it in winter. These statements are sufficient to make it evi- 

 dent that the cause we are seeking does not consist in variations 

 in the quantity of rain that falls at different seasons. The truth 

 of this conclusion may be confirmed by the consideration already 

 urged, in considering the influence of the wind, that the laws of 

 the variation of the nocturnal loss of temperature are too general 

 to depend upon a cause which must differ so much in its effects 

 in different places. 



It will have been observed that in considering the question of 

 the influence of variations in the cloudiness of the sky, it was 

 tacitly implied that the numbers representing the proportion of 

 sky covered by clouds represented also the proportion of that 

 part of the sky which is concerned in the nocturnal radiation, 

 that was covered by clouds. This it will be recollected, is the 



part 



Now it is not 



difficult to see that this portion of the sky will be, in the average 

 of months, less cloudy than other parts, and especially than the 

 parts near the horizon. For, the clouds in the horizon are much 

 more distant than those of the zenith, and consequently are seen 

 very obliquely, and as they are generally of considerable thick- 



