328 Prof. F. Rosotti's Experimental Researches 



wires of the same bobbin in the same direction. Moreover 

 the two bobbins were placed as near the magnetized disk as 

 possible ; and in addition I placed a strongly magnetized bar 

 underneath, for the purpose of neutralizing as completely as 

 possible the influence of the earth's magnetism on the disk. 

 In this manner the instrument was rendered extremely sensi- 

 tive. The telescope (furnished with cross-wires) and the 

 divided scale were placed at a distance of 3 metres from the 

 galvanometer. The scale was divided into fifths of a centi- 

 metre, and by means of the telescope could be read to the 

 tenth of a millimetre. When the readings of the variations 

 were made, it was necessary to pay attention to the differences 

 in the position of equilibrium of the disk, due to variation of 

 the magnetic declination of the earth ; on this account, before 

 passing the current, the division of the scale corresponding to 

 the cross-wire of the telescope was noticed ; and this served as 

 a starting-point in the elimination of successive variations. 



II. Experimental Determination of the Law of Radiation as a 

 function of the Temperature. 



Experiments betiveen 0° and 300°. — In order to make this 

 determination, I took first for radiating body Leslie's cube 

 filled with water, which I heated to different temperatures 

 by a flame, and finally kept it at the boiling-point. For tem- 

 peratures between 100° and 300° I employed an iron cube 

 filled with mercury. The cover of this cubical vessel was 

 pierced with three holes, through two of which passed the 

 stems of two thermometers, each of which was graduated up to 

 360°; through the third the handle of an iron stirrer passed. 

 At a short distance from the radiating face of the cube was 

 placed a diaphragm with a double wall ; in the centre of each 

 wall were circular holes with a diameter slightly less than that 

 of the opening in the case containing the pile. This precaution 

 was taken to insure that every part of the pile should receive an 

 equal amount of heat. The exterior walls of the screen were 

 blackened ; and between the pile and the first screen a second, 

 also with double sides, was placed with windows slightly larger 

 than those in the case. The two screens were placed parallel 

 to each other and to the face of the case by means of small 

 holes, in the same manner that the pile was directed towards 

 a radiating object. The experiments were made with the more 

 sensitive pile No. 2, and with no more resistance in the circuit 

 than that offered by the pile itself, the rheophors, and the wire 

 of the galvanometer. The following Table shows the results 

 of the experiments : — 



