114 The Black Bulb Thermometer. 



other surface of the sheet of copper. As the copper got 

 heated and radiated, the thermometers commenced slowly to 

 rise until they reached an indicated temperature of 150°, 

 when the burner was removed and the temperature fell to 

 about 80°. Readings were obtained at every 10°, both 

 whilst they were rising and falling. Nos. 3 and 4 were 

 compared with No. 1 in the same way. The results were as 

 follows : No. 2 behaved almost exactly as it did under solar 

 heat ; the difference between its indications and those of 

 No. 1 increasing with the temperature until when No. 1 read 

 145°, No. 2 indicated only 130°. 



No. 3 was exceedingly slow in rising. At 90° with No. 1 it 

 was 12° behind ; at 100°, 70° behind ; and at 120° it was 23° ; 

 at 150°, however, it was only 14°d. The highest temperature 

 indicated by No. 1 was 150°, while No. 3 reached 140° a 

 considerable period after No. 1 had ceased to rise. A second 

 experiment gave almost identical results. It was found, 

 however, that No. 3 continued to increase its indications after 

 the source of heat was removed, but never reached the tem- 

 perature indicated by No. 1. In cooling, too, it fell very 

 slowly, reading 125° when No. 1 read 109°, and so on. The 

 surrounding bulb is about two and a half inches diameter, 

 and of very thick glass. It remained hot a long time after 

 the bulb of No. 1 was quite cool. No. 4 was now tried 

 against Nos. i and 3. The relations between 1 and 3 were 

 the same as before, while No. 4 (the one tested at Green- 

 wich) read 3° more than No. 1 at 80°, 6° more at 120°, 5° at 

 138°, and 3° at 145° ; and they increased and decreased in 

 their indications simultaneously. No. 4 has a surrounding 

 bulb of about one inch and three-quarters and the glass is 

 thin, while that of No. 1 is about one inch diameter and of 

 somewhat thick glass. 



These results are not quite confirmatory of the belief 

 which I expressed in my former paper, that coating the black 

 glass bulb with some dead opaque pigment, so as to destroy 

 every trace of diathermancy, would render the indications of 

 solar radiation thermometers more reliable and inter compar- 

 able. The anomalous results obtained from No. 3 are somewhat 

 inexplicable. Exposed to the sun's rays this thermometer 

 always indicates the highest temperature of all four ; exposed 

 to radiant heat from a dark surface it is quite the reverse ; 

 it is found to be exceedingly slow in absorbing the heat 

 rays, and equally slow in parting with them ; and it was 

 observed that the mercury continued to rise for several 



