62 Transactions of the South African PlLilosojiliical Society. 



observations, from July 5 to November 21, 1897, the gauges used 

 were unpainted copper pans, 5 inches in diameter and 5 inches 

 deep, with turned brass rims, by Messrs. Negretti and Zambra. 

 They were fixed in a wooden frame and floated about 3 feet 

 above the ground in a tank of 3 feet cube, so that the gauges dipped 

 about 3|- inches into the water with which the tank was kept nearly 

 iull. The plates of coloured glass covering them were 10 inches 

 square and one-eighth of an inch thick. The five gauges were 

 retained in the same relative position throughout, but the glass 

 plates were permuted after each fortnightly determination of the loss. 

 AVater — rain- water — was added to the gauges as required during 

 each fortnight, so as to keep the evaporating-level as near to the 

 «dge of the rim as safety would allow. There was a clear vertical 

 space of about an inch between the gauge-rims and the glass plates, 

 ±0 allow the exhaled vapour to pass off. Temperatures were taken 

 at intervals, as before, at XIV. and XVII., preferably on clear days. 



The following tests of the absorptive properties of the glass plates 

 will perhaps give the best idea of them : — 



Col. 1 gives the colours. 



Cols. 2 and 3 the relative absorptions of light, as determined by a 

 wedge photometer, received from the sky in the vicinity of, and 

 ■opposite to, the sun respectively. 



Cols. 4 and 5, the temperatures shown by Board of Trade thermo- 

 meters, the thermometers being about 1 inch underneath, and 

 touching the glass underneath respectively. 



Cols. 6 and 7, the colours transmitted, as determined by a direct- 

 vision spectroscope, from the sun and from the sky. 





Light. 



Tempebature. 



■ 



Spectba. 



Col. 



Near 



Opposite 



Under 



Touching 







Sun 









Sky. 









sun. 

 5-76 



sun. 



glass. 



glass. 



















P. 



5-41 



97-2 



104-3 



V 



B' 



G', 



Y' 



K' 



V 



B', G' 



Y' 



R' 



B. 



5-72 



5 09 



98-9 



109-3 





B^ 



G^ 



Y^ 







B°, G° 







G. 



5-58 



4-50 



963 



1110 



v 



B' 



G', 



Y' 



E° 



v°, 



B^G' 



yo 



R" 



Y. 



5-69 



4-82 



98-4 



108-4 



v 



B' 



G^ 



Y', 



E= 





G', 



Y', 



R^ 



K. 



4-34 



3-82 



98-0 



108-7 



v° 



B° 



G°, 



Y3 



K= 







Y^ 



R' 



All the above values are means derived from a number of obser- 

 vations. I was not lucky enough, however, to get calm days for the 

 temperature observations, so that wind has probably somewhat 

 vitiated the numbers. The temperature shown by a thermometer 

 touching the red glass underneath has sometimes been higher and 



