He M. MELLONI ON THE FREE TRANSMISSION 
it is by no means necessary to review in succession all the degrees that 
contribute to the formation of this are. The application of our method 
to the angles of 20° and 10° will be quite sufficient. This being done, 
we shall find an equal quantity between their difference and the effect 
produced by the simultaneous action of the moving forces. In other 
words, let us produce a deviation of 20° to the right and one of 10° to 
the left: let us then simultaneously expose the two opposite faces of the 
pile to the two radiations which produce these galvanometric indica- 
tions: the index will move to the right, and stop precisely at 10°. 
Hence we infer that the force necessary to make the needle describe 
the are comprised between 10° and 20° is equal to the force required 
to make it pass over the first ten degrees of the scale. Thus the pro- 
portion of the degrees to the forces is perceptible as far as the 20th de- 
gree on each side of zero. 
This fact seems opposed to the inference which might have been 
made in examining the nature of the galvanometric action ; for, in the 
successive rotation of the astatic system, the poles of the magnetic 
needles depart from the mean line of the electric currents. The inten- 
sity of the repulsive forces, therefore, decreases in proportion as the 
angle of deviation increases. Whence we should conclude that the 
effort necessary to make the needles exceed a given are should change 
as soon as the first degrees of the scale are passed. This would un- 
doubtedly take place if all the electric currents lay in a vertical plane 
passing through the line marked 0°; but the cireumvolutions of the me- 
tallic wire which is wound on the frame placed under the graduated 
circle are distributed to a certain extent on each side of this plain. In 
the galvanometer which I have employed in my experiments, they cover 
the two opposite ares of 76°, the chords of which are perpendicular to 
the line marked 0°. Thus so long as the oscillations take place within 
certain limits there will always be electric currents situated on each side 
of the needles. Now when the intensity of these currents is extremely 
feeble, their sensible effect on the needles must cease at a very short 
distance. Let us suppose this distance to be 18° of the division of the 
galvanometer intended to show the degrees of electric action which 
cause the deviations to the right and left for the first 20° of the scale. 
These degrees of action must be extremely feeble in a very delicate gal- 
vanometer. If, during these oscillations, the system of the needles is 
confined within the two initial ares of 20°, it is clear that it will always 
be subject to the same action, whatever may be the position in which it 
is placed; for there will always be near its plane a series of currents 
extending to 18° on each side, even when the system will occupy the 
extreme limits. The influence of the currents that are further distant 
will, according to our hypothesis, be nothing. As the moving force 
will therefore have a constant value, we shall have to consider only the 
