OF RADIANT HEAT THROUGH DIFFERENT BODIES. H 



employed in the construction of the thermomultiplier, I'urnish suffi- 

 ciently simple means of solving the question in each particular case. 

 Indeed there is nothing easier than to keep the index of the galvano- 

 meter at any degree of deviation. All that is required for this purpose 

 is to place a lighted lamp at proper distance from either side of the 

 thermoelectrical pile. To prevent the possibility of mistake on this 

 point, let us suppose the axis of the pile to be pei-pendicular to the 

 magnetic meridian, and the communications so fully established that, 

 when the left or the right side of the pile is heated, a corresponding de- 

 viation will be exhibited by the galvanometer. Let there be now pro- 

 duced a sufficiently marked deviation by placing a lamp near enough 

 at the same side. Let this deviation be 44°. After having brought the 

 needle back to 0° by interposing a metallic screen, let us make it move 

 to the 42nd degree of deviation on the left, by means of a second lamp 

 placed on the other side. To bring the needle back again to the zero 

 point of the scale, we have only to stop the radiation by means of a 

 metallic screen, as before. 



It is natural to ask what will be the effect now produced by the heat 

 of both lamps being brought to bear simultaneously upon the opposite 

 sides of the pile. The calorific effects will be partially destroyed, and 

 the instrument will mark but their difference. If the same force were 

 always required to make the needles describe arcs containing the same 

 number of degrees, the index would stop at the second degree of devia- 

 tion to the right ; but we know that these effects continually increase to 

 the right and to the left of zero. The difference of two degrees just 

 now observed between the partial deviations of 44° and 42° was owing 

 to the application of a force greater than what is required to make the 

 index traverse the first two degrees of the scale. The position marked 

 2° will therefore be exceeded, and the more so in proportion as the first 

 force is greater than the second, and the arc described will, when com- 

 pared with the difference of the two deviations, mimediately give the 

 measure of the corresponding force. If, for instance, the needle stops 

 at 8° it will be inferred that the force required to make the needle pass 

 from 42° to 44° is four times greater than that required to make it pass 

 from zero to 2°. This effect would be five times greater if the needle 

 stopped at 10°, and so of the rest. 



I shall not attempt to conceal the fact, that in this process the propor- 

 tionality of the forces to the degrees in the arc employed as a compara- 

 tive measure is tacitly assumed. But the assumption is fully justified 

 by experience ; for we find tliat in galvanometers whose astatic system 

 has been brought to a high degree of perfection, the magnetic needles, 

 through the whole extent of the arc comprised between zero and the 

 twentieth degree nearly, describe arcs proportional to the action of the 

 electric current to v hich they arc subjected. To be coii\ inced of this, 



