Few el — Measures of Intensity of Solar Radiation. 385 



these very small temperature differences and the correspond- 

 ing intensities of radiation which produce them, as in the case 

 of the exposed plate in the registering actinometer ; so that the 

 higher the temperature, the greater the amount of solar or 

 other radiation required to raise the temperature of the strips 

 a given very small amount, which we have denoted by d. By 

 the law of Dulong and Petit the relation between i and d for 

 any given temperature r is given by (4), but by Stefan's law it 

 is given by (9). Proceeding as before, except neglecting the 

 terms containing c and r as factors, we get, with the relation 

 of Stefan's law, instead of (11), 



j =ix(jy i =ix(i+ e ^^ (14) 



the latter form being only approximate and not very accurate 

 where A is large. It is seen, therefore, that the observed inten- 

 sities I, in order to be comparable, must be multiplied into a 

 factor which is a function of the temperature in order to reduce 

 them to what they would have been with an absolute tempera- 

 ture equal to T . Or, if we wish to reduce the observed value 

 of I at temperature T to that of I at temperature T, we must 

 divide by this factor. 



From Langley's Researches on Solar Heat, Tables 108 and 

 110, p. 136, we get the following average values of the sums 

 of all the deflections of the several wave-lengths, namely : for 

 Lone Pine 710, and for Mountain Camp 12S4, using only the 

 observations of September 2 and 3 in the latter. These are 

 the uncorrected deflections, to which large corrections have to 

 be applied, especially to those of the shorter wave-lengths, 

 but the corrected results must give nearly the same ratio 

 between Lone Pine and Mountain Camp. Langley found from 

 actinometer observations that the intensities of solar radiation 

 at Mountain Camp were about one-ninth greater than at Lone 

 Pine. Increasing, therefore, 710 by one-ninth, we get 789 for 

 what the sum of the deflections at Mountain Camp should have 

 been, if they are proportional to the intensities of the radiation 

 below and above, instead of 1284. Now this is much too great 

 a difference to be all due to difference of temperature, accord- 

 ing to the formula of (14). Temperature observations at Lone 

 Pine and Mountain Camp, which are required for an applica- 

 tion of the formula, are not given, except in one case it is 

 stated that the temperature of the galvanometer at Lone Pine 

 was 97° F. But from observations of wet- and dry-bulb ther- 

 mometers about the same time for hygrometric purposes, it 

 may be inferred that the differences of temperature between 

 Lone Pine and Mountain Camp could scarcely have been as 

 much as 20° C. Using this for the value of J in (14), and 



