and Polarization of Heat. 138 



loni has described, in his paper in the Annates de Chimie for May 

 1 833, a simple and satisfactory method of estimating the relative 

 values of degrees, at different points of the scale. He states, 

 however, that, under 20° of deviation, he found them quite uni- 

 form. In the following experiments, the deviations were gene- 

 rally under 1 5°, and in almost no case exceeded 20°. I have 

 therefore assumed the forces to be as the deviations. Besides, 

 no change of importance would take place from a deviation from 

 this law by a small quantity. 



5. It will be perceived in the experiments which are to be 

 detailed, that the determination of all the more important facts 

 depends generally on whether one effect be greater or less than 

 another, without much regard to their absolute amount. Now, 

 the confidence which we can place in the uniformity of this in- 

 strument, or at least of the small changes capable of affecting it 

 (since it is not liable like thermometers, and especially air-ther- 

 mometers, to advance by starts) is such, as to admit of almost in- 

 definite subdivision, where the relations of small quantities are 

 alone concerned. I conceived, therefore, that, without impairing 

 its sensibility by lengthening the galvanometer needle, we might 

 advantageously magnify the divisions by optical means. This I 

 proposed to do by observing the motions of the index by means 

 of a small telescope, fixing in front of the object glass a lens whose 

 focus is situated at the part of the scale desired to be magnified. 

 It might also be easy, in order to compare larger quantities, to 

 make this micrometric system revolve so as to be always similarly 

 placed as regards the needle, and thus avoid the effects of paral- 

 lax, which at present require constant vigilance. 



6. The method here indicated, I have put in practice with 

 the greatest success in my later researches ; one-tenth of a degree 

 becomes easily visible, and the constancy of the indications fully 

 justify this method of microscopic examination, which has enabled 

 me to verify the most delicate deductions I had drawn from 



