461 
examination of these curves it appeared, as had been antici- 
pated, that the maxima and minima of the irregular movements 
usually succeeded each other with very great rapidity, their 
interval being, on the average, about 40 seconds, and their 
magnitude varying in these observations from 10” to 60”. 
The observations were resumed on the 23rd of October, 
and continued to the 26th, two series of an hour’s duration 
being made each day, in the hope of detecting some law go- 
verning the movements. No such law however could be traced, 
nor did there appear to be any connexion between the curves 
representing the march of the changes, at different hours of 
the same day, or at the same hour on successive days. 
The observed variations in these observations being small, 
it was suggested by an experienced friend, to whom Prof. 
Lloyd showed the results, that they were not true magneti¢ 
changes, but merely the errors of observation incidental to 
vibratory movement. In order to test this supposition, an 
unmagnetic bar (the brass detorsion bar of Gauss’s appa- 
ratus) was substituted for the magnet, to which it cor- 
responded in dimension ; and being provided with a mirror, 
was suspended by two parallel threads, and made to vibrate. 
The time of vibration was adjusted, by varying the interval 
of the threads, so as to differ little from that of the magnet. 
The successive elongations were then observed, as in the 
case of the magnet, and the means of each pair taken. The. 
variations in these means (which could arise from errors of 
observation or mechanical changes only) bore no com- 
parison whatever in magnitude to the corresponding varia- 
tions of position of the magnetic bar, thus showing that the 
latter were truly the results ofthe operation of magnetic 
‘forces. 
The rapidity with which these changes thus appeared to 
follow each other, held out the hope that they might be em- 
ployed in determining differences of longitude; and it only 
remained to ascertain, for that purpose, whether variations so 
