22 Bappicker—On the Influence of Magnetism on the Rate of a Chronometer. 
influence of terrestrial magnetism on the chronometer must exceed the influence of 
the ship’s iron on it. And though a modified action takes place where these two 
forces do not act in the same direction, yet the mfluence of both these forces 
on the balance (which vibrates horizontally) must be similar to that on the equally 
horizontal compass-needle. And, moreover, as the medium effect of the attraction of 
the ship’s iron om the compass within the parallels of Great Britain does not exceed 
5° of deviation on each side of the meridian (and is, probably, a little less), the 
force of deviation is represented by sin 5°, the directive force by sin 865°, the 
relation of which number is 1 to 11°35. As, therefore, ‘‘ the proportion of error 
due to the local attraction of the ship would appear not to exceed in these latitudes 
the eleventh or twelfth part of that resulting from the earth’s magnetism (while 
nearer to the equator it must be still less),” it will be of little service to compensate 
for the error of local attraction so long as the action of terrestrial magnetism 
remains uncorrected. In the polar seas the force of local attraction approaches the 
directive force much nearer than in the British seas, and in some situations very 
near the magnetic poles exceeds it; but as it acts ‘‘ without any increase of force,” 
except what may arise from the little augmentation of the earth’s magnetic 
intensity in those regions, the rate of a chronometer should be more equable in 
polar regions (could the effect of temperature be perfectly compensated), because 
the earth’s magnetism acts nearly at right-angles to the plane of the balance. A 
corroboration of these opinions Scoresby finds in Barlow’s experiments; and though 
he considers Varley’s instance an extreme one, still he thinks “that the influence 
of magnetism in the balances most probably affects the rate, in some degree, of 
almost every watch and chronometer.” 
It appears doubtful whether the above conjectures as to the influence of the 
ship’s iron would still hold good in the present time of iron steamers (cf. p. 42, 
Experiments of Delamarche and Ploix); and as observations in this respect are 
wanting, the following remarks of the Astronomer Royal (then Mr. Airy) may be of 
some importance. The first ones I find in Shadwell (No. 28), in two foot-notes on 
pp. 14 and 15 :—‘In a letter to the Hydrographer to the Admiralty on the liability 
of chronometers to be disturbed by the earth’s magnetism (Nov. 1857), Mr. Airy 
has recently stated that ‘of the hundreds of chronometers which have passed 
before us, and have been regularly put under magnetic trial, only one, viz. Buck- 
land’s (?) (No. 425) has been sensibly affected by the earth’s magnetism.’” And: 
“©... . Mr. Airy, in the communication to the Hydrographer before referred to 
(Noy. 1857), ‘On the magnitude of the magnetic forces in an iron ship,’ observes—“ I 
have never been in any part of a ship in which a chronometer would be placed in 
which the compass-needle was reversed, or in which it followed rudely the move- 
ments of the ship: there are places possessing this character, as very near the 
funnel, or very near some iron stanchions or knees, but nowhere where a chrono- 
meter would probably be placed. That is to say: the ship’s magnetic force 
