46 Bappicker—On the Influence of Magnetism on the Rate of a Chrorometer. 
apparatus of Scoresby—viz. for the turning of the chronometer—quite unnecessary ; 
and that this idea of compensating the terrestrial magnetism is effective we see 
from Airy’s compensation (No. 22), which is based upon exactly the same con- 
sideration, and perhaps suggested by the above remark of Scoresby. 
Airy’s rule is: ‘‘ The action of terrestial magnetism upon a magnetic chrono- 
meter may be annihilated by placing the chronometer upon the top of a compass- ~ 
box, whose needle is perfectly free, provided that its elevation above the compass 
is properly adjusted.” This elevation of the magnetic part of the chronometer is 
that, in which a small compass-needle, placed above the large compass, loses its 
directive power. When, accordingly, the chronometer Brockbanks (cf. p. 36, ff.) 
was placed above a large compass in such a way that its balance (where the mag- 
netism was assumed to lie) was as nearly as possible at the same height with the 
small compass-needle, the result was (I reproduce only the mean rates) : 
Free. Placed on the Compass. 
XII pointing N. — 4°64 : : — 6°90 
a EK. —8 70 ; : — 8:10 (one day only.) 
i Gl SOK GIG ee oe Coola 
bs We cab oe tO or 
the effect of the compass was therefore not sufficient. When it was raised by { of 
an inch, the rates became 
XII pointing N. — 7°96 
- E+ — 9-20 
. Sy i 054 
' We b= 841) 
The error still lying in the same direction, the compass was again raised by + of 
an inch, and it was observed, ; 
XII pointing N. — 9°24 
i E. — 9°41 
i Shi up Ono 
7 Wistit 2e10%03, 
or the chronometer is somewhat supercorrected. The small remaining discre- 
pancy does not follow the same law as the original discordance, which Airy 
supposes to be due to the influence of the steel parts of the chronometer upon the 
compass, so that it did not entirely neutralize the terrestrial magnetism. It 
appears, therefore, to be advisable to make use of strong compasses which bear a 
greater distance from the chronometer. Airy is surprised at the necessity of 
bringing the large compass nearer to the chronometer than to the small compass- 
