oZO RADIANT HEAT. 



blackened, was exposed to the direct rays of the sun; but, as Dr. 

 Ritchie observes, no correction seems to have been made for the 

 variable causes which abstract caloric from the blackened ball of the 

 exposed thermometer. — (Edinb. Journ. of Science, v, 107.) 



In the same paper is described the method proposed by Sir J. F. 

 W. Herschel; his object was to ascertain, by direct experiment, the 

 relative heating power of the sun's rays; this he did by exposing in 

 a glass vessel, or large thermometer, at different times and places, 

 a deep blue liquid, for a given time, to the direct rays of the sun, 

 noting the increase of temperature, which was purposely rendered 

 very small by properly a'djusting the capacity of the instrument, then 

 shading the sun's direct rays, and leaving it exposed for an equal 

 time to the free influence of all the other heating and cooling causes, 

 radiation, conduction, wind, &c. , and again noting the effect of these. 

 The same difference of these, according to their signs, was the effect 

 of the mere solar radiation. Dividing this by the time of exposure, 

 he had the momentary effect or differential co-efficient, which is the 

 true measure of the intensity of radiation. 



Professor Cumming has been engaged in researches, the object of 

 which was to obtain a measure of the total heating effect of the sun's 

 rays. He has communicated for this report an account of his inves- 

 tigations, of which the following is the substance. 



His instrument consists of a bent tube in the form jq, one side ter- 

 minating in a black bulb containing ether, or sulphuretof carbon; the 

 other a graduated tube closed at the bottom; into this, on exposure 

 to the sun, some of the liquid is distilled over from the bulb; and the 

 quantity measured on the scale is^roportional to the amount of radia- 

 tion, when all interfering causes are allowed for; and these are esti- 

 mated by comparative observations. 



The experiments have been varied by exposing the bulb and 

 screening the other part, or by exposing the whole instrument equally 

 to the sun; and by making contemporaneous observations with the in- 

 strument wholly uncovered, or covered totally or partially by a glass 

 to protect it from currents of air. 



The Professor has endeavored to make a standard scale by regis- 

 tering the sun's radiation on clear days every half hour, or hour, in 

 the usual manner, and comparing them with the contemporary distil- 

 lation; or by placing the two sides of the instrument in two vessels of 

 water at unequal temperatures, and noting the distillations in given 

 times by ascertained differences of temperature. 



The instrument is filled with ether in the same manner as Wollas- 

 ton's Cryophorus (from which the suggestion was taken;) but there 

 is an inconvenience, arising from the circumstance of the difference 

 of pressure under which the instrument is hermetically sealed, which 

 renders two instruments not strictly comparable; this he proposes to 

 remedy by sealing a standard instrument when exhausted to a known 

 pressure by the air pump. 



The ether or sulphuret of carbon employed must be perfectly pure, 

 or there is a re-absorption. The circumstance of being exposed to 

 the air, or covered, makes great differences in the indications j 



