522 DR. LAMONT ON THE MAGNETIC OBSERVATORY 
The mean interval B — A = 14! 26-12 = 200 vibrations, or 
1 vibration = 4-3306 ; the corresponding intensity by the dif- 
ferential instrument being 40°1. 
The interval C — B is = 10! 49"-71 = 150 vibrations, or 1 
vibration = 4!-3314 : the corresponding intensity being 39°7. 
The arc of vibration was always so smali that a reduction to 
infinitely small arcs is unnecessary. The two determinations 
being reduced to the same intensity, we have 4'"3306 and 
4"-3313. 
The deviation of the single determinations from the mean 
amounts to ;535,th of the whole. When the access of air is 
carefully prevented* greater accuracy is usually attained. 
b. Determination of the Moment of Inertia of the Deflecting 
Magnet. 
\ 
d nated | E 
Tine of tion tensity Pempere-[Time of wbestion a otenaty [ema | Han 
“ a ° a” ° 
Oct. 25 43003 10 | +60 132289 55 | +5°8 le 
Nov. 4 43187 38-4 | 46:3 13°4139 425 | +61 Il. 
... 25]  4:3402 468 | 44-2 13-3634 455 | +42] 1 
. 26| 4:3376 47-4 | 42-4 133575 | 465 | 424] 1 
In reference to the calculation it must be remarked, 
1. When the intensity increases one unit the time of vibration 
without the ring diminishes 0':00047. 
2. When the temperature increases 1° the time of vibration 
increases 0!-00167. 
3. For ring I. we have the log. of the moment 
Gf INGHIR .) mio’ +) snes) eaghe nse 
For-ring li) 60 oi ‘ « wows O15 95S 
4. The torsion force of the dicen G is in re first experiment, 
without ring 0:00087 
with ring . . 0°0034 
In more recent experiments . . . . without ring 0°0007 
with ring . . 0°0028 
Both in these moments and in the moment of the bar the ex- 
pansion of the metal by reason of temperature must be take 
into account. Calculation gives the log. of the moment of in 
ertia of magnet V. at 13° Reaumur, from single measurements 
as follows :— 
* In several of the determinations subsequently given, this condition ha 
not been sufliciently attended to. I first became convinced of the necessity 0 
completely cutting off the access of the air from without by the results them: 
selves. 
