492 Mr, Babbage and Mr. Herschel's account of the 
the shell, and the other 90® removed from it in the same 
place, this latter will be identical with Mr. Barlow's secon- 
dary axis. 
34. In what has been said, the velocity of rotation has 
been supposed commensurate to the velocity with which 
magnetism is propagated through the iron of the shell. But 
if we conceive in this, or in the general case, either the re- 
tentive power of the shell, disc, or lamina great, or the velo- 
city of motion excessive, it may be instructive to consider 
the modifications thus introduced into the effect. It is evident 
that the induced pole will lag farther and farther behind the 
magnet in proportion as either of these conditions obtains. In 
the case of rectilinear motion, this will, up to a certain point, 
increase the oblique action, and the dragging effect will be 
strengthened ; but if the velocity be excessive, or the reten- 
tive force considerable, as in steel, the pole may lag so far 
behind as to carry it altogether out of the sphere of the mag- 
net's attraction; and the magnetised portion, remaining 
within its limits, may have not had time enough to acquire a 
high degree of polarity. From both causes the drag (the 
expression, though uncouth, is convenient) should be weak- 
ened. In the case of circular motion this effect may go so 
far, that a complete circumference shall have been described 
before the polarity of any one point shall have been either 
completely induced, or completely destroyed. In this case 
the effect observed will be a general weakening of the total 
polarity of the disc or sphere ; and (supposing the latter of 
iron, or soft steel) a directive virtue on a small compass 
needle placed near it, not probably towards any particular 
place, but to a resultant imaginary point depending on the 
