354 Professor Forbes's Researches on Heat. 



parately tabulated and projected. It is remarkable enough, how- 

 ever, that there is a striking numerical difference between these 

 results and those obtained when the simple aperture of the pile was 

 employed without any reflector, being merely incased in its square 

 brass tube. The excess of permanent above momentary devia- 

 tion was greater in the latter case than in the former. In other 

 words, the effect of heat (to the same amount) appeared to be 

 more quickly developed in the pile when it was concentrated by 

 the reflector, than when it fell directly on the pile. There is 

 nothing paradoxical in this result, since the distribution of the 

 heat on the sentient extremity of the pile differs in the two cases. 



14. It appears, then, that these effects are developed on the 

 whole in a simple and uniform manner ; and though such an in- 

 vestigation as we have undertaken of the instrument, was neces- 

 sary to give us confidence in the numerical accuracy of the re- 

 sults, all facts of importance might be determined without it : 

 and even quantitative laws ascertained by a judicious conduct of 

 experiments. Many persons, even though not unaccustomed to 

 physical reasoning, have strangely inaccurate conceptions of the 

 limits of possible errors. Nor is there a more important part of 

 the science of experiment than to perceive where physical proof 

 becomes satisfactory, though yet far removed from mathematical 

 certainty. To supply the latter is a humbler and more me- 

 chanical task, which may be undertaken at leisure, as in this 

 paper we shall partly attempt to do. 



§ 2. Polarization of Heat by Tourmaline. 



15. On this subject I have little to add. It does not possess 

 the same theoretical importance as in the case of polarization by 

 other methods. Double refraction is better shewn by the pheno- 

 mena of depolarization by mica, described in my last paper ( art. 46 

 etseq.), and the phenomena attendant on the absorption of one of 

 the doubly refracted pencils, are so capricious and ill understood, 

 even in the case of light, that it might hold or not for heat with- 



