Permanent Magnets. 59 
The curve in the figure exhibits the course of the values 
of M. In seven months it has diminished very nearly in the 
proportion of 12 to 11. 
These magnets are of what is called best cast steel, and are 
as hard as they possibly can be made. They were prepared 
about February 8th. 
It is incumbent on those who maintain that permanent 
magnets can be made subject only to insignificant changes, to 
submit their magnets to some long-continued series of tests of 
equal conclusiveness with the above. I hope to continue the 
series. 
The new value of the moment of inertia of the magnets was 
obtained by a series of forty experiments, in which the mag- 
nets were vibrated alternately alone*and with the standard 
brass bar which was made for the purpose. The wire used 
for suspension was better than that employed on the former 
occasion. ‘The torsion due to it was considerably less than 
that of the former wire, and was very nearly the same for all 
weights. 
The values of the moment of inertia arrived at were, 
Mia isi72. Wl £43369,. 
the old values were 
R= 13370. TL 213384; 
which were observed upon at the time as unsatisfactory. The 
correction due to this change is obtained by adding to the 
log.’s of both M and H the number ‘00111. Applying this 
correction to the ultimate value of H formerly obtained, we 
have :— 
Olitvalnerot Heo. ESOEL 
Corrected value» 9.1: ..-:,:7 8056 
for the beginning of March 1884. 
The mean of the values for March and September is 
"18038. 
The mean Greenwich value for the year, according to the 
formula in Everett’s ‘ Book of Units,’ is 
"13186. 
