146 GAUSS ON A METHOD OF FACILITATING 
galvanic current: let the circuit be closed at a second which is 
to be reckoned as 08, reopened at 10S, and definitively closed at 
208. If we desire to measure the deflection produced by a mag- 
netic bar in a particular position, this position must be before- 
hand exactly and conveniently marked, and the bar being held 
vertically as the observer approaches, must be laid down in its 
place at the instant 0°,—taken up again at 108, and replaced de- 
finitively at 208. The result will be, that during the first 10 
seconds the magnetometer will move with an accelerated velocity 
from its original position towards that which corresponds to the 
deflection ; at the tenth second it will have reached a point pre- 
cisely midway between the two positions, and during the second 
10 seconds it will pass through the remaining half of the interval 
with a retarded movement, so that at the 20th second it will 
have taken up its new position, and all motion will have ceased. 
It is easy to perceive how the needle may be brought in the 
same way from one deflected position of rest to an opposite 
position, in which it shall also be at rest; namely, by causing 
the deflecting force first to act for the third part of a vibration 
in the opposite direction, then during an equal time again in the 
original direction, and then changing again. In galvanic cur- 
rents the alternations may be made almost instantaneously by 
the aid of a suitable commutator ;—with deflecting bars the effect 
may be produced by a rapid half-revolution (most conveniently, 
a horizontal half-revolution), bringing the north pole of the bar 
into the place of the south pole. It is equally evident, that after 
observing the deflection, the needle may at once be brought to 
rest in the magnetic meridian, by first suspending the action of 
the deflecting force for the third part of one vibration, then re- 
newing it for a similar interval, and then finally causing it to 
cease. 
3. The method described rests on the two following supposi- 
tions :— 
First, that in the oscillations of the needle, the distance from 
the middle point of the vibration (so long as that point ‘is 
itself stationary) is proportional to the sine of an angle which 
increases uniformly, and is augmented by 180° during the time 
of one vibration. 
Second, that the time of vibration is not altered by the addi- 
tional force. 
Inasmuch as these suppositions are not rigorously fulfilled, 
