OF RADIANT HEAT THROUGH DIFFERENT BODIES. Il 
employed in the construction of the thermomultiplier, furnish 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 perpendicular 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 ares 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 
9° will therefore be exceeded, and the more so in proportion as the first 
force is greater than the second, and the are described will, when com- 
pared with the difference of the two deviations, immediately 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 are employed as a compara~ 
tive measure is tacitly assumed. But the assumption is fully justified 
by experience ; for we find that 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 which they are subjected. To be convinced of this, 
