1 40 Prof. Forbes on the Refraction and Polarization of Heat. 



quantities of heat upon this and another instrument. For this 

 purpose I employed two air thermometers of great delicacy ; 

 one was the photometer of Leslie, having one ball covered 

 with lamp black, and exposed to the same source of heat as 

 the pile, whilst the other ball was shaded. The other instru- 

 ment was a vertical differential thermometer, having a hemi- 

 spherical reflector, intercepting a cone of rays 2' 50 square 

 inches in section. I found it impossible to operate with small 

 degrees of heat, which could not be reckoned accurately on 

 the air thermometers, owing to their tardy action ; but, from 

 several experiments, I concluded that the same quantity of 

 heat falling on the photometer ball and on the pile, moved 

 the liquid of the former through ] °, and the needle of the 

 multiplier through 4'^*2. The degrees of the photometer 

 being lOths of 1° cent., one centigrade degree would corre- 

 spond to 42° of the galvanometer (assumed of equal value 

 throughout the scale). The experiment with the differential 

 thermometer, being similarly conducted, gave for the effects 

 of equal quantities of heat, 1° cent, to 62° of the multiplier. 

 If we assume from these experiments that a quantity of heat 

 which raises an air thermometer by one fiftieth of a centigrade 

 degree, affects the galvanometer by 1°, since a quarter of a, 

 degree of the latter is a measurable quantity, and half of that 

 may be estimated as a sensible impression, we may measure 

 an effect of ^^o ^^ ^ centigrade degree, and perceive (by un- 

 assisted vision), an effect of ^^q. 



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 thermometer one three-hundred-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 inconsi- 

 derable, required so long a time to assume their temperature, 



mitie? of the pile, and the spaces round them filled up with copper filings, 

 or some such material. But the experiment could hardly be quite decisive. 



* 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 action ; but any dispersed light (for which we have made 

 allowance), would act on the neighbouring parts. 



