Caloric by Fricticni. 371 



tween the two faces of the pile, causes the needle to traverse so 

 much that the angle of difference is quite appreciable. 



To reduce the question to the most simple expression pos- 

 sible, two bodies of the same nature were selected, which were 

 bad conductors of caloric, which were equal in all their dimen- 

 sions, and which presented no other difference than that which 

 arose from the different conditions of their surfaces. These 

 bodies were conveniently fixed on glass stalks. The rubbed sur- 

 faces were each placed in contact with one of the faces of the pile. 

 When the two surfaces have the same temperature, the needle 

 remains in repose, owing to the circumstance that the two ther- 

 mo-electrical currents are equal, and flowing in opposite direc- 

 tions, they destroy each other. But when the temperature is 

 not the same, the needle speedily traverses, and the angle of 

 movement enables us to measure the difference of the tempe- 

 rature. The friction- by means of suitable apparatus, is pro- 

 duced with a determined velocity and precision, so that its in- 

 tensity is always known. The two bodies can easily be sepa- 

 rated instantaneously from each other, and can be immediately 

 subjected to the required examination. These are the methods 

 by which the experiments were made, and we now proceed to 

 the results. 



We begin by inquiring into the effect produced upon the 

 needle by the contact of one of the rubbed surfaces with one of 

 the faces of the pile ; in short, with the effect produced by the 

 heating of this face. 



Experiment proves that, whatsoever is the nature of the rub- 

 bed disk, whether it be a conductor or a non-conductor of heat, 

 the time which the needle requires to attain its maximum of 

 traversing, provided the traversing does not exceed 60°, is al- 

 ways 10". For a traversing to the extent of between 60° and 

 15°, it takes 9^% and 9", for deviations which reach from 75* to 

 90^ 



The needle, then, in this respect, corresponds to a pendulum 

 which oscillates, under the action of a weight, within narrow 

 limits, since the deviations are isochronous ; at the same time, 

 with this difference, that in the pendulum, when the amplitude 

 of the oscillation increases beyond a certain limit, the time of 

 oscillation also increases, whilst the contrary occurs in the ex« 



