452 Rey. Mr. Lioyp on a New Method of Observation 
its distance from the centre. Now this quantity (which we may denominate the 
magnetic moment) varies with the temperature, and this variation must be taken into 
account before we can make any accurate inference from the formula. Let 7 denote 
the temperature of observation, 7’ a certain standard temperature, and o' the cor- 
responding value of c. ‘Then, assuming the changes of the magnetic moment to be 
proportional to the changes of temperature, we have 
a=o' [ l—a(r—r) ]5 (9) 
in which a is a constant whose value is to be determined by observation. 
In order to obtain the value of this constant, the Needles III and IV were sus- 
pended horizontally by a few filaments of silkworm’s thread, and vibrated in a large 
glass bell, the air of which was heated from beneath by means of a spirit lamp. The 
time of 100 vibrations was observed at the artificial temperature, and at the ordinary 
temperature of the room before and after. ‘The following are the results : 
Needle III. Needle IV. 
Hour Time ‘Temp. Hour ‘Lime ‘Temp. 
10" 28" 222.64 | 52°2 12° 28° | 238".00 | 58°2 
10 46 222.50 58.5 12 49 238.00 58.5 
1 42 222.80 63.5 3 39 238.45 58.9 
Hg 27 222.84 63.8 3 50 238.56 58.6 
Mean 222.70 61.0 Mean 238.25 58.5 
LLORES 222.97 90.3 2 4 238.80 78.8 
ES iN) 222.84 91.1 emis) 238.76 79.5 
12 29 223.28 92.3 Mean 238.78 79.2 
Mean 223.03 OND | 
Hence we have for Needle Ill 
A= 299070 sk — Ae Ol, SOee rt OO e 
and substituting in the formula 
—(so—«c) sy 2 (TT) 
Hira) MG) 
there isa= 00010. For Needle IV 
{MSOs Oops MNS MSOs 4 7—7 —20°.7 ; 
and a=.00021. But as these two needles were made at the same time by the same 
artist, and are therefore probably similar in temper, as they are in material and size, 
it is natural to suppose that the effects of temperature will be the same on both, and 
that the difference here observed is due to the uncertainties attending observations of 
this nature. Taking then the mean of the preceding results as the most probable 
value of the coefficient for both needles, we have 
a— .00016., 
a= 
