ADDENDUM. lxix 
ascending and for descending temperatures ; on the whole, however, they indicate 
that the coefficient deduced from such rapid changes is greater for the same mean 
temperature (about 65°), when the temperature is increasing than when it is 
diminishing. The greater part of the observations were made without any inter- 
mediate temperature, so that there are not sufficient data to determine whether the 
difference is greater at high mean temperatures than at low mean temperatures. 
Mr S. H. Curistiz found that the temperature coefficient increases with the 
temperature, the increase becoming more rapid for temperatures above 80°; and 
that beyond 100°, a portion of the magnetism is permanently lost.* The previous 
results seem to indicate that (in such experiments at least), a portion of the magnet- 
ism is lost even at the lower temperatures for certain kinds of steel.+ 
The observations of deflection with the circle magnetometers have been reduced 
by the formula (see Introduction, p. xx.) 
ct sine 5 
Adit 
where 
shes sin %, 1 + k (ba — by) 
eae 3m 1—g¢(t,—t) 
1 — — sin u, 
m 
u, being the observed deflection corrected for declination change (see column 9, De- 
flection, p. 349-351). 
The observations of June 23, 1846, and of January 2, 1847, were made with 
Professor ForBEs’ 6-inch circle magnetometer ; the remaining observations were made 
with Sir THOMAS BRISBANE’s instrument, the dimensions of the magnets employed 
will be found, p. 351: the values of one scale division for the suspended magnets 
are as follow :— 
June 23, 1846. Jan. 2, 1847. May 31 and June 15, 1847. Sept. 11 and 13, 1847. 
0995 0-995 1025 2-017 
The solid bars are about 0-3 inch diameter, and the collimator bars about 0:4 
inch diameter. 
All the observations, excepting those of September 11 and 13, were reduced by 
the method of least squares, the equations of condition having the form 
1 1 Baek 
Lt Gah tege Qin s7sinu= =0 
* Philosophical Transactions, 1825, p. 63. 
} The previous results seem to indicate that while it will always be necessary to determine the 
temperature coefficient for the magnetometers from the observations with the magnet in its box, 
yet it will be desirable to determine it also by the usual method of hot and cold water experiments. 
MAG. AND MET. OBS. 1845 AnD 1846. s 
