TRANSACTIONS OF SECTION A. 505 



Fifth Beport of the Committee for considering the best methods of record- 

 ing the direct Intensity of Solar Radiation. — See Reports, p. 40. 



4. On the Blach Bulb Thermometer in Vacuo. 

 Bij Professor Herbert M'Leod, F.B.S. 



This instrument, which is generally employed for measurinp: solar radiation, does 

 not appear to give universal satisfaction, for it is said that no two instruments give 

 the same result when placed side by side. No doubt the imperlection of the 

 vacuum may account for this in a great measure, but Ijesides this there appear to 

 be other causes, 



"When such an instrument is exposed to the rays of the sun a large proportion 

 of the radiation passes through the enclosing case, and also traverses the opposite 

 side of the globe. Some, however, is absorbed by the blackened thermometer bulb, 

 and this then begins to radiate heat of low refrangibility which is incapable of 

 passing through the enclosing case ; as a consequence the latter becomes heated, so 

 that the thermometer bulb is in a warmer enclosure than at first. The quantity of 

 heat thus radiated will be diminished the smaller the bulb of the thermometer, and 

 some years ago I suggested to Mr. Hicks to make a thermometer with a very small 

 bulb. Such a one was made, and I am informed that it gave readings ten degrees 

 hif/her than any other instrument. As this was exactly opposite to my expectation, 

 perhaps I may be excused for not attempting any explanation. Some months ago I 

 ordered two instruments with very small bulbs, one to be in a thick case and the 

 other in a thin one. When the instruments came I foimd that one of the cases, 

 which appeared the thicker, was devitritied and rough, and produced a very marked 

 sliadow when held before a screen exposed to sunshine, so I was not surprised 

 to find that this thermometer always indicated a slightly lower temperature than 

 the one with the clear glass. It was returned to the maker, and I was then 

 informed that the bulb was a thin one, and the devitrification was caused by 

 blowing the bulb before the lamp. This, therefore, supported the theory that I 

 had formed on the subject. These two thermometers were used from May UO to 

 June 0. The mean of the readings of the instrument in the thick case was 119'ii° F., 

 and the mean obtained by the instrument in tlie thin case was 116-8°. I then had 

 one of the tliermometers enclosed in a case of very thick glass. The thermometers 

 were then tested with a thermopile, to determine the quantity of radiant heat 

 that would pass through the enclosing cases. The source of heat was an albo-carbon 

 flame, and the cases of the tliermometers were interposed in succession between the 

 flame and the cone of the thermopile. 



Case of thermometer with large bulb transmitted about 26 per cent, of the 

 Tadiant heat. 



Thin case of thermometer with small bulb transmitted about 23 per cent, of the 

 radiant heat. 



Thick case of thermometer with small bulb transmitted 11^ per cent, of the 

 radiant heat. 



These thermometers were exposed to the sun's rays for the first 27 days of 

 August, and tlie means of the readings are as follow : — 



liarge bulb instrument .... 126"7° F. 

 Small bulb with thin case .... 119"9 „ 

 Small bulb with thick case . . . . 118"3 „ 



Although the case of the instrument with the large bulb allowed a larger 

 percentage of the rays from a low temperature source to pass through, yet the 

 amount of heat radiated from the large bulb was so great that the case was warmed 

 sulHciently to cause the instrument to read, on the average, nearly G° F. higher than 

 the small bulb instrument. It should lie noted, however, tiiatthe small bulbs are 

 not so dead black a.sthe large bulb, which is coated with lampblack, the small bulbs 

 being made of black glass. 



According to the theory enunciated above, the thermometer with the thick 



