Page:Scientific Memoirs, Vol. 2 (1841).djvu/105

Rh Of course, however, these observations could not furnish an accurate determination of the amount of error introduced, as the declination may have altered during the interval, which amounted to more than an hour.

In the graphic representation the second half of the Marburg curve has been drawn on a reduced scale, the reduced divisions representing 28 on the Marburg scale.

I may here mention a second case of a similar kind. The time of vibration of the magnet bar in Breslau, which, in March 1836, amounted to nearly 3·2 seconds, had from that period to November gradually increased, making altogether an increase of about 0″·4. This is no unusual circumstance, as all magnetic bars in the course of time lose some part of their force, though in very various degrees dependent on the unequal tempering of the steel and other circumstances. But, from November 1836, to January 1837, a decrease in the time of vibration of 1″·27 took place. Prof, von Boguslawski, who informed me of this remarkable circumstance, seemed inclined to attribute it in part to an increased intensity of the terrestrial magnetism. I did not doubt, however, that the cause must be sought in the immediate neighbourhood of the magnet bar, probably in some impediment to its free motion, and this supposition was verified by the following letter of M. Boguslawski:— "You were right in your supposition as to the cause of the alteration in the time of vibration. By a slight accidental displacement of the box, the edge of the small aperture through which the suspension thread passes, had been brought near the thread, though by no means into contact with it. However, some of the finer fibres of the silk must have been touched thereby, for when it was again made to pass quite through the centre of the aperture, the time of vibration was found almost identical with that formerly observed."

This is perhaps the place for some remarks on the movements themselves, which are here represented during six terms.

In the three summer terms, (Plates VI. VII. and VIII.) notwithstanding all the great anomalies, the regular diurnal movement is clearly seen in the curves, ascending during the hours