354 Professor ForsBes’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 
et seq-), 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- 
