1867.] 



Insolaiion to Atmospheric Humidity. 



357 



confirmation from the fact that the post-solstitial periods of maximum 

 soL^r radiation in autumn, and also the diurnal maxima, coincide with 

 monthly and daily periods of maximum humidity. 



It is proposed to show the extent to which this is the case in England 

 by means of Tables of monthly results of radiation and vapour. 



The first Table exhibits the mean monthly values of solar radiation and 

 vapour-tension for the five months from May to September, at Greenwich, 

 in 1860-64^ 



Table I. 



Monthly Means of Radiation and Vapour at Greenwich (1860-64). 



Month. 



Solar 

 radiation. 



Tension of 

 vapour. 



Weight of 

 vapour. 



May 



97-8 F. 



0-3U in. 



3-5 grs. 





103-2 



0-364 



4-1 



July 



+ 108-2 



+0-393 



+4-4 





: 4-107-5 



+ 0-397 



+4-4 



September .. 



97-5 



0-361 



4-0 



The plus signs ( + ) indicate first and second maxima. 



The maxima, both of radiation and vapour, occur in July and August. 

 The excess of insolation in July is 5°, and the excess in August 4°*3 above 

 the mean in June. The excess of vapour-tension in the two months is "030. 



To obtain results more exactly comparable, the monthly mean tension 

 of vapour for each of the five months was next deduced at the hours of 

 noon, 2 p.m., and 4 p.m., for the years 1842-47, during which two-hourly 

 observations were made at the Royal Observatory. 



In Table II., which contains the monthly means, maximum results again 

 appear principally in July and August. 



Table II. 



Monthly Means of Tension of Vapour at Greenwich at 0^ 2^ and 4^. 



Year. 



May. 



Junet. 



July. 



August. 



September. 



1842. 



0-358 



0-473 



0-436 



0-545 



0-449 



1843. 



0-390(fl) 



0-414 



0-501(i) 



0-509 



0-474 (f) 



1844. 



0-365 



0-429 



0-466 



0-430 



0-450 



1845. 



0-831 



0-493 



0-463 



0-443 



0-402 



1846. 



0-379 



0-483 



0-484 



0-496 



0-471 



1847. 



0-376 



0-391 



0-484 



0-497 



0-403 



(rt) Mean amount of cloud 8-0. {b) Cloud 8-5. (c) Cloud 4-5. 



* The vacuum-thermometer was used at Greenwich fii-st in 18C0. 18G4 is the date 

 of the last published observations. The means of radiation were derived from the 

 daily maxima of the vacuum-thermometer ; the means of vapour from the usual num- 

 ber of diurnal observations. It is believed that the observations for the five years are 

 homogeneous. t See note ^, p. 358. 



2 G 2 



