Second Series.— Thermo-Multiplier. 449 
to that of the pile.* The length of the conic frustum which I 
employ is 12 inches, and its aperture 12 inches. 
7. It is well known that the deviations of galvanometers are 
not, generally speaking, proportional to the forces producing them, 
and that for the most part angular spaces at greater distances 
from zero correspond to increments of force greater than for 
equal spaces near zero. Thus to cause the needle to advance 
from 25° to 30° requires a force greater than to make it deviate 
from 0° to5°. Also the force indicated by a deviation of 30°, is 
more than six times the foree indicated by a deviation of 5°. M. 
Me txont has pointed out an ingenious method of comparing the 
values of the different parts of the scale. This consists in em- 
ploying two constant sources of heat to affect the opposite ex- 
tremities of the pile, and after observing their separate effects, 
noting their joint effect, which will not generally be equal to the 
arithmetical difference of the others. Thus let one source of 
heat force the needle in a positive direction to 30° on the scale, 
and a second source of heat acting separately produce a negative 
deviation of 25°, the effect of both acting at once will not be a po- 
sitive deviation of 5° merely, but probably will indicate some 
greater number, as 6° or 7°. Thus, a érue scale of degrees equal 
in value to those near zero may be constructed.. The execution 
of this investigation is not so simple as it appears, chiefly owing 
to a tendency of the zero point to shift during experiment, ap- 
parently owing to a permanent electro-magnetic condition of the 
conducting wire. It seems that this electro-magnetic condition 
* Suppose that we wish to have a conical reflector ABCD, such that the whole 
of the parallel rays which fall upon it shall 
reach some part of the surface AB of the 
pile, which is all that we want, we have this 
simple construction. Let the length of the 
trumpet-mouth AE be given. Make FB 
equal and parallel to it. Join FA, and pro- 
long the line to D, then is DAE the great- 
est inclination that the sides of the cone can have to answer ee purpose intended. 
