Art. I. — On the Temperature of Solar Radiation, as 

 Measured by the Black Bulb Thermometer. By Mr. "R. L. 



J. Ellery, F. R A. S., Government Astronomer, &c., 

 President of the Royal Society of Victoria. 



[Read 24th February, 1868.] 



The temperature of solar radiation at the earth's surface, 

 or as it is more commonly known, the " temperature in the 

 sun," is now usually measured by means of a black bulb 

 thermometer fully exposed to the sun's rays, the maximum 

 temperature attained being marked by a self-registering 

 index. These thermometers are now generally enclosed in 

 an outer hermetically sealed tube, from which the air has 

 been exhausted, the bulb of the thermometer itself being 

 made of black glass. Such an instrument has been in use 

 at the Melbourne Observatory for many years past, and the 

 maximum temperature of solar radiation indicated by it has 

 been assumed and published as the greatest heat in the sun. 

 In the "Philosophical Magazine" of March 1866, a letter 

 from Professor Tyndall appeared, discussing the results of 

 some observations of solar radiation made by Mr. Glaisher 

 in one of his balloon ascents. The letter concludes by stating 

 that " If these remarks be correct, or so far as they are 

 correct, the indications of the black bulb thermometer, as at 

 present constructed, are • delusive, and especially so at great 

 elevations." The remarks he refers to relate to the trans- 

 parency of ordinary black glass to the invisible, or the 

 greatest heat rays of the spectrum. From this letter we may 

 conclude that Professor TyndalTs opinion stands thus : — 



" That the black glass, of which black bulb thermometers 

 are usually constructed, is not opaque to all the heat rays of 

 the spectrum, and therefore these rays may pass through the 

 glass of the bulb to the surface of the mercury, and be there 

 reflected back into space without raising the temperature of 

 the mercury, and that the loss from this cause will increase 

 with the height of the thermometer above the earth's 

 surface. 



After reading this I determined to try some experiments 

 in order to ascertain if the maximum heat in the sun's rays 

 was obtained by our black bulb thermometer. I therefore 

 placed another black bulb thermometer alongside, and in aH 



