ON BLACK-BULB THERMOMETERS IN VARIOUS MEDIA. 165 



XIX. On Black-bulb Solar -Radiation Thermometers ex- 

 posed in various Media. By G. V. Vernon, F.R.A.S., 

 F.M.S. 



Kead before the Physical and Mathematical Section, February 27th, 1872. 



Being desirous to make some comparisons of the readings 

 of black-bulb thermometers exposed in various media, I got 

 Messrs. Negretti & Zambra to make me a set of three ther- 

 mometers, in addition to the ordinary black-bulb maximum 

 in vacuo. 



The glass tubes containing the thermometers were filled 

 with hydrogen gas, carbonic-acid gas, and atmospheric air, 

 at 32 F., the latter thermometer being described in the 

 tables as filled with compressed air. The instruments were 

 all alike, the glass tubes enclosing them being of equal 

 thickness. The thermometers were all compared with the 

 Greenwich standard, and require no index- error correction. 



The observations were made in the years 1861 to 1865 ; 

 and the period embraced was just four years. Since the 

 latter year the observations have been discontinued ; but 

 the thermometers remain in the same position they were 

 originally placed in. 



In the annexed Tables, Table I. gives the mean monthly 

 readings of the thermometers for each year, with the addi- 

 tional readings of the black-bulb freely exposed and the 

 maximum thermometer in the shade. 



Looking at the yearly means, the black- bulb in vacuo 

 gives the highest mean reading, the one with carbonic-acid 

 gas comes next, followed by the condensed-air one, that 

 filled with hydrogen giving the lowest temperature. 



Examination of the monthly values shows that the 

 maxima for all the thermometers occur in July, and the 



