1883.] S. A. Hill— T^d Measurement of Solar Uadiation. 3 



above quoted figure in its much darker colouration throughout on the 

 upperside, more especially on the outer margins \ but on the underside it 

 is much paler. 



II. — On the Measurement of Solar Radiation hj means of tlie hlach-bulh 

 Thermometer in vacuo. — By S. A Hill, Esq. B. Sg. Metl. Bep. to 

 N. W. P. and Oudh, Communicated hy H. F. Blanford, Esq. F. R. S. 



[Received March 29th :— Read April 4th, 1883.] 



The interesting results of sun thermometer observations, published by 

 Mr. Blanford at page 72, Vol. LI, Part II, of the Journal of the Asiatic 

 Society of Bengal, suggest the possibility of making use of the instrument 

 to measure the heat received from the sun. Even with all the precautions 

 adopted by Mr. Blanford, the excess of the maximum temperature in the 

 sun, above the maximum in the shade, is affected by variations in the follow- 

 ing and perhaps other conditions, as well as by variations in the heat emit- 

 ted from the sun. 



I. The thickness of the atmosphere traversed by the sun's rays, 

 which, for moderate degrees of obliquity, may be taken to be proportional 

 to the secant of the sun's zenith distance. 



II. The absorptive power of the clear transparent atmosphere, which 

 probably varies with the proportion of water vapour in it. 



III. The quantity of haze and dust in the air. 



IV. The radiating and reflecting powers of the ground surface in the 

 vicinity of the thermometer. 



V. The excess of the maximum air temperature above the tempera- 

 ture at the hour, when insolation is most intense. 



The last mentioned condition is subject to a very distinct annual 

 variation. At Allahabad, where hourly observations have been made on 

 four days in each month since 1875, the insolation is most powerful on 

 clear days within a few minutes of noon, while the average excess of the 

 maximum above the noon temperature of the air is the following : 



Jan. 



Feb. 



Mar 



Apl. 



May 



June 



October 



Novr. 



Deer. 



35*» 



3-5° 



3-9° 



4-1° 



3-9° 



4-2° 



4-3« 



4-4° 



2'9° 



In order to obtain comparable values for the several months we should 

 therefore add these corrections to the figures for Allahadad given by Mr. 

 Blanford. In table I the figures in Mr. Blanford's second table are thus 

 corrected, and the table is extended down to the ead of 1882. 



