THE COMPASS-NEEDLE ON THE DEVIATIONS OF THE COMPASS. 
163 
time at which it is made. But when this is not the case, from the iron of the ship 
being too near the compass, or from the correctors being from necessity brought too 
near the compass, or from the length of the compass-needle itself being excessive, an 
error is introduced which it is the object of this paper to consider. 
That the effect of this error has been for some time felt by practical compass adjusters, 
appears probable from the difficulties which are reported to have been experienced in 
correcting the deviation of certain ships by Mr. Aiey’s method, and from the advantage 
reported to have been derived in some of such cases from the use of compasses with 
two needles ; but we are not aware that the particular nature of the error to which we 
refer has been hitherto pointed out, or considered either experimentally or theoretically. 
Experiments. 
The attention of Mr. Evans was drawn to this subject by the observations made in 
the Great Eastern on her experimental voyage from the Thames to Portland, and after- 
wards while she was lying at Holyhead and Southampton*. The standard compass of 
this ship was fitted with needles of unusual length, viz. two needles of 11^ inches in 
length placed near each other. Its deviations had been carefully corrected by Mr. Geay 
of Liverpool, by magnets and soft iron, but after such corrections, and when the devia- 
tion was nearly corrected on the cardinal and quadrantal points, there were errors of 
between 5° and 6° on some of the intermediate points. The importance of this case has 
induced us to append a Table of the deviations (Appendix I.) of this compass on the 
points on which the deviations were observed as the ship swung to her moorings on the 
days of observation at Holyhead in October 1859, and in Southampton Water in June 
1860. These observations indicated the existence of a considerable error, which was 
neither semicircular nor quadrantal, and thus apparently of some source of error which 
had not been taken into account by Mr. Aiey in his plan of mechanical correction. In 
order to ascertain the cause of these apparently anomalous results, Mr. Evans instituted 
a series of experiments on the deviations produced on compass-needles of different 
lengths by magnets and soft iron placed in different positions with respect to them. 
For this purpose a bar magnet 12 inches long was placed on a horizontal table 
revolving round a fixed vertical axis, on which axis a compass was placed at different 
elevations. The magnet was placed at distances of 17 and 20 inches from the centre 
of the compass, and in one set of observations was turned endways, and in another 
sideways to the compass. Observations were made on single edge bar needles of 3, 6 
and 12 inches in length, and on Admiralty Standard compass cards of 7'6 and 3-8 inches 
diameter. Observations were also made of the deviations produced by two cylinders of 
soft iron arranged as if for correcting the quadrantal deviation, on a 7-^-inch single 
needle, and on an Admiralty Standard compass. 
• See Mr. Evans’s paper, entitled “ Eeduction and Discussion of the Deviations of the Compass observed 
on board of all the Iron-built Ships, and a selection of the Wood-built Steam-ships in H.M. Navy, and the 
Iron Steam-ship Great Eastern,” Philosophical Transactions, 18G0, p. 337j 
2 A 2 
