360 Mr. Powell on Terrestrial Light and Heat. [May, 



^r—r = "uH • These experiments may be contrasted with those 



recently communicated to the Royal Society, in which precisely 

 the same method was applied to terrestrial light and heat, and a 

 remarkable difference in ratio exhibited when the screen was 

 removed. 



Similar remarks apply to the other experiments (19, 20, 42), 

 which appear to me to afford the most satisfactory means, and 

 perhaps the most delicate, we at present possess, of deciding 

 the question as to the existence of any perceptible portion of 

 simple radiant heat in the solar rays. 



The difficulties alluded to 1 have found to occasion much per- 

 plexity in the experiments on terrestrial light and heat in which 

 1 have been for a long time engaged. In those results which 

 form an answer to the prmcipal question existing on the subject, 

 and which are contained in the paper just alluded to, I conceive 

 all fallacy arising from these causes is sufficiently guarded 

 against; and I trust the same may be said of some further inves- 

 tigations on the same topics, to which 1 alluded at the end of 

 my last paper; and which were at first designed to form a 

 second part to the paper communicated to the Royal Society; 

 but which upon further consideration I withdrew.* " 



The principal part of these investigations, and the theory 

 which I have deduced from them, together with some additional 

 remarks, will form the subject of the present paper. 



(3.) Having by the former experiments, as 1 conceive, esta- 

 blished the general fact of two heating radiations emanating 

 from luminous hot bodies, it becomes obvious that we may apply 

 this distinction to explain many results of former experimenters; 

 in particular those of M. de la Roche, before alluded to, will, 

 upon this principle, exhibit an increase in the ratio of the heat- 

 ing power of light to the simple heat in proportion as bodies are 

 more completely luminous. Wishing, however, to examine this 

 and other kindred phenomena upon a uniform principle, I adopted 

 the following application of the differential thermometer, which, 

 though it will not prove the existence of two radiations, enables 

 us, when their distinct existence is assumed, to determine the 

 ratio subsisting between their effect, though not with great 

 accuracy, yet probably sufficiently so for the purpose here 

 wanted. 



The method consists in placing a small screen so as to inter- 

 cept the heat going to the plain bulb. The black bulb is then 

 affected by the sum of the two radiations, or / + //, Then 

 obsemng without the screen in the usual way we have {{), and 



thus obtain (//) and the ratio (-). 1 here suppose the bulbs to 



• I mention this because, owing to an accidental mistake, some account of them was 

 given ill the rejwrt in thvAiiii'ih for March, p. 2'»4. 



