Mr. Bosanquet on Permanent Magnets. DY 
compounds. I therefore regard that expression as the best 
which commits one in the least degree to any very decided 
view of the inner construction of benzene, and such a repre- 
sentation is the simple hexagon. 
VII. Permanent Magnets —Il. On Magnetic Decay; with a 
Correction to the Value of H at Oxford. By R. H. M. 
Bosanquet, St. John’s College, Oxford. 
To the Hditors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
Hj decay of the magnetism of permanent magnets is a 
subject of the greatest practical importance. Some of 
the best makers of electrical measuring instruments continue 
to relie to a considerable extent on permanent magnets, and the 
change of these is not easily eliminated with accuracy. 
Among the methods employed for the discovery of the 
changes in permanent magnets, the direct determination of 
their moments, which accompanies the ordinary process for 
the determination of the horizontal component of the earth’s 
magnetism, seems to be free from objection. This is the 
method some further results of which I propose now to 
communicate. 
_ The existence of the decay or diminution of magnetic mo- 
ments has been pointed out by Joule*, and is alluded to in 
my former paper (Phil. Mag. xvii. p. 438). The values now 
dealt with form a sequel to those given in that paper. 
_ It is important to consider the justification offered for the 
continued use of magnets as standards, and this depends on 
the process used for the redetermination of the constants 
involved. 
One of the processes consists ultimately in the reference to 
a Clark’s cell, which is supposed to remain perfectly constant. 
The assumption is justified by the fact that two such cells 
similarly prepared at the same time do not vary relatively to- 
each other, though their values are not generally identical. 
It appears, however, that two such similar cells, being exposed 
always to the same influences, might be expected to make any 
changes they do make in common. At all events, without 
some further check, there is nothing in the shape of absoltite 
proof attainable by this method as to the constancy of the 
ultimate standard. 
* Reprint, i. p. 591. 
