and Polarization of Heat. 139 



quantity, and half of that may be estimated as a sensible impres- 

 sion, we may measure an effect of ^— of a centigrade degree, 

 and perceive (by unassisted vision), an effect of ¥ ^. 



1 4. In the case of the moon's rays, concentrated 3000 times, 

 we have seen that it is improbable that even the last effect was 

 produced. The whole sensitive extremity of the pile being larger 

 than the moon's image, was not brought into action ; but if we 

 compare their relative dimensions,* we shall still find that it is 

 improbable that the direct light of the moon would raise a ther- 

 mometer one three-hund red-thousandth part of a centigrade degree, 

 at least in this climate. 



15. The value of the thermo-multiplier consists not so much in 

 the minuteness of its indications, which may easily be equalled 

 by employing large enough thermometers, but in the certainty 

 and rapidity of its action. Air thermometers, such as I compared 

 it with, though the size of the balls was inconsiderable, required 

 so long a time to assume their temperature, that, when exposed 

 simultaneously with the thermal pile to the source of heat, the 

 latter had almost assumed its maximum effect before the others 

 had sensibly moved ; and it is obvious that, in delicate experi- 

 ments, where constancy in the producing cause is presumed, ra- 

 pidity of execution is essential. In short, with an air thermo- 

 meter (which requires from 10 to 15 minutes to give a single re- 

 sult), the greater part of the experiments to be described would 

 have been impossible from this cause alone, and the remainder 

 would have been tedious beyond measure. It will therefore be 

 conceived that were thermometers enlarged so as to give as mi- 

 nute indications as the multiplier, they would be utterly unma- 

 nageable. 



16. Of all the researches of M. Melloni on radiant heat that 



* The moon's image contained 0.114 square inches, whilst the area of the pile is 

 about 0.40. Hence little more than a fourth of the pile was brought fully into ac- 

 tion ; but any dispersed light (for which we have made allowance), would act on the 

 neighbouring parts. 



