The Black Bulb Thermometer. 115 



minutes after the source of heat was removed — it would 

 almost seem as if the thick surrounding bulb intercepted the 

 heat rays and stored them, giving them out slowly to the 

 black bulb after the external feed had ceased. No. 4 (the 

 one tested at Greenwich) which has an ordinary black 

 glass bulb with a thin surrounding bulb, was the quickest to 

 absorb the heat from the dark body, and indicated the 

 highest temperature of all four. The one with the blackened 

 bulb (No. 1) was the next in order, and rose almost as rapidly 

 as No. 4, but fell short of its indication by 6° at temperatures 

 above ] 20°. These two read nearly alike at all temperatures 

 when exposed to the sun. 



So far as these observations go, they seem to show that 

 the usual methods of measuring the " heat in the sun" are 

 fallacious ; that among four excellent black bulb thermome- 

 ters from good makers the differences at high temperatures 

 are sometimes as much as 28° Fahrenheit ; and when one is 

 bearing sun's rays at 3 45° or lo0° it is certainly not satisfac- 

 try too find the thermometers indicating only about 120° or 

 125°. We may, I think, safely conclude that some more 

 reliable and accurate instruments are required for the purpose 

 than those usually adopted ; ann although it is probable that 

 a careful selection of the glass used and a strict adherence to 

 one form, size, and as near as possible the same thickness in 

 the construction of the outer bulb, would reduce the discre- 

 pancies to a large extent, there is something more required 

 in the material of the black bulb itself than usually exists 

 in these now used. Tyndal, in his little book on Radiation, 

 note page 29, says : " The black glass chosen for thermo- 

 meters, ahd intended to absorb completely the solar heat, 

 may entirely fail in this object if the glass in which the 

 carbon is incorpored be colourless. To render the bulb of a 

 thermometer a perfect absorbent, the glass with which the 

 carbon is incorporated ought in the first instance to be green." 



This may be accepted as the best way to make the black 

 glass bulbs ; but it is also essential, I believe, to have the 

 surface dead, and not polished, as the ordinary surface of 

 glass always is. With bulbs made of such glass, with the 

 surface deadened, we should probably have a reliable 

 thermometer ; but the necessity for protecting the bulb from 

 currents of air involves other questions which I have pointed 

 out, and which 1 believe, require further experiment for 

 their solution. 



