By a pair of oscillating engines placed on a frame running on rails in the usual way.
But oscillating between the two fields, it took part in neither.
This ray is also given a periodic motion of the same frequency by reflection from a separate oscillating mirror so as to make the two motions at right angles to one another, and thus we have depicted on the screen a bright line having the same form as the periodic current being tested.
The angle is gradually diminishing and the declination will in time again be o°, when it will slowly increase in an easterly direction, the north magnetic pole oscillating slowly around the North Pole.
Consequently, the face which the moon presents to the earth is subject to a corresponding variation, the globe as we see it slightly oscillating in a period nearly that of revolution.