174 M. MELLONI ON THE POLARIZATION OF HEAT. 



tion perpendicular to one of the two sides of which this double 

 basis was formed, and the plane of primitive refraction remain- 

 ing always vertical, by resting the lamina first upon one, then 

 upon the other basis, and disposing the second plane of refrac- 

 tion, first vertically, then horizontally, the actions indicated in 

 the following table were obtained. 



By comparing each positive variation with the corresponding 

 negative variation, we see that the first is constantly superior to 

 the second, for the two sources of heat possessing hght, and in- 

 candescent platina ; and the contrary for the radiation of the 

 lamp; the difference, which is five or six hundredths for the 

 three latter cases, amounts to nearly twelve-hundredths for the 

 first. But, from the nature of the experiments, says Mr. Forbes, 

 the table tends generally to show the coincidence of the two va- 

 riations*. I know not whether this tendency will appear suffi- 

 cient to physicists in general. 



It is really alarming when we see that the effects produced 

 scarcely amount to a few^ fractions of a degree for obscure heat, 

 the subject in which we are chiefly interested; for it is very 

 difficult to estimate smaller quantities than a quarter of a degree 

 upon the circle of the thermomultiplier, which is scarcely more 

 than five centimetres in diameter : on the other hand, circum- 

 stances, in appearance the most unimportant, may produce varia- 

 tions equal at least in extent to the deviations observed in the 

 first two cases. It is true Mr. Forbes has adapted a mici'ome- 

 trical system to his galvanometer, by means of which he can, 

 with greater ease, appreciate, as he thinks, even tenths of a de- 

 gree; he also endeavours to secure himself from perturbing causes 



* The following are the author's own expressions : " The table generally 

 points to a coincidence, and that as close as by the natiu-e of the experiments 

 we should perhaps be warranted in expecting." — Trans, of the R. S. of E. 

 vol. xiii. p. 162., [ov Lend, and Ediub. Phil. Mag. vol. vi. p. 366. Edit.] 



