328 M. MELLOKI OV THE POLARIZATION OF HEAT. 



with a smaller aperture. A longitudinal opening made from end to 

 end in the superior surface of the case enables us to see whether the 

 axis of the pile is placed in the direction of the radiation. Bj' gently 

 moving the case in one direction or another, the circular aperture is 

 brought into the proper position relatively to the pile, which continues 

 to occupy the same part of the table on which it has been placed. 



By means of this double case, the air surrounding the thermoseopic 

 body is kept perfectly calm. At least the motions produced in it by 

 its slow variations of temperature react upon the two sides of the pile 

 with such uniformity that the index of the thermomultiplier stands 

 exactly at the zero of the scale, and when a specific deviation has been 

 produced by the influence of an exterior calorific radiation, returns to 

 that point in some minutes after the communication of the rays has 

 been intercepted by means of a metallic screen. 



Those who are in the habit of frequently using astatic galvanometers 

 must have rentarked, no doubt, that these instruments, whatever may 

 be the solidity of the table on Avhich they stand, suffer, in consequence 

 of the observer's changing liis place or of the passing of vehicles in the 

 neighbourhood, a sliglit tremulous motion, which, being transmitted to 

 the suspension thread and to the needles, causes them to oscillate like 

 a pendulum for a period of greater or less duration. For the purpose 

 of preventing these eccentric oscillations, which prove so embarrassing 

 in very delicate observations, we have but to fix the instrument on the 

 marble slab of a chimney or any other horizontal plane firmly fastened 

 to one of the solid walls of the building in which we are carrying on 

 our operations. The wire will then always preserve its vertical direc- 

 tion ; the needles will be no longer susceptible of any other than hori- 

 zontal movements ; and the index will deviate with so much regularity 

 under the action of the electric currents, that, upon seeing it, one might 

 fancy that, instead of being suspended by a thread, it moves on a 

 pivot. 



In the particular instance under consideration, the deviation com- 

 mences as soon as the rays of the calorific source, at a constant tempe- 

 rature, have reached the anterior face of the pile through the aperture 

 in the receptacle. The motion of the needles is at fii*st very slow, but 

 becomes gradually accelerated, and, having attained its maximum of ve- 

 locity, is again retarded by imperceptible gradations, until at last it 

 ceases altogether: the needles then return gently toAvards zero, de- 

 scribe an arc of some degrees, resume the direction of the first motion, 

 and, after having made three or four oscillations successively decreasing 

 in extent, stand still and take a fixed position. Now this steady devi- 

 ation is always a little inferior to the deviation indicated by the needles 

 ftt their first departure from the point of repose. The difi'erence varies 



