112 The Black Bulb Thermometer. 



Art. XVIII. — On the Black Bulb Thermometer as a 

 measure of the Temperature of Solar Radiation. By 

 Mr. R L. J. Ellery, F.RA.S., Government Astronomer, 

 &c., President of the Royal Society of Victoria. 

 [Read 12th October, 1868.] 



On the 24th February I read a short paper giving the 

 results of a series of observations of solar radiation, by 

 which I showed that, so far as my observations then went, 

 the ordinary black glass bulb thermometer did not indicate 

 by many degrees the true temperature of the sun's rays, 

 more especially when the solar radiation was large. It 

 further appeared that a thermometer whose bu]b was covered 

 with a coating of lamp-black and size gave far more reliable 

 indications ; and I pointed out that this was probably owing 

 to the diathermancy of nearly all black glass to the obscure 

 heat rays ; permitting them to reach the surface of the mercury 

 inside the bulb, which reflected them back into space. 



Since then I have obtained a long series of observations 

 with the same two thermometers, as well as with two others 

 from the Adelaide Observatory, sent to me by Mr. Todd, the 

 results of which do not confirm my former conclusions as to 

 the causes of the differences of indications exhibited by the 

 thermometers then used, but rather lead me to the belief that 

 the Indications of Black bulb Thermometers generally, as 

 a measure of the Temperature of Solar Radiation are 

 unreliable. 



The thermometers used were four — 



No. 1. Thermometer by Grimoldi, of Melbourne, with bulb 

 covered by lamp-black. 



2. Thermometer by Casella, of London. Black glass 



bulb. 



3. Thermometer by Negretti and Zambra, of London. 



Black glass bulb. 



Being as follows : — 









No. 1 = — 02 



No. 3 



= 



+ 03 



2 = — 2°-0 



4 



— 



... 00. 



All four were placed on a suitable stand in the open air, 

 with their bulbs exposed to the sun's rays under exactly similar 

 conditions and their indications, which showed the highest 



