534 Royal Society :— 
magnetism of the hull or machinery predominates ; and it is inferred, . 
especially from the example of the ‘ Royal Charter,’ that large qua- 
drantal deviation and fluctuating sub-permanent magnetism (due to 
hull alone) are co-existent, and give rise to conditions of compass 
disturbance which are beyond prediction, and which have hitherto 
baffled inquiry and given a complexion to theoretical deductions 
varying as regarded from different points of view. 
Tn order to examine the change which the original magnetism of 
an iron ship undergoes after launching, a series of compass observa- 
tions were made in the steam-ship ‘ Great Eastern’ prior to her quit- 
ting the River Thames in 1859, and subsequently at Portland, Holy- 
head, and Southampton*—at the three first-named places within 
short periods of time of each other. 
The results, from an Admiralty Standard Compass placed in a 
position the least subject to influence from local masses of iron, were 
as follows :—In the first five days, from Deptford to Portland, the 
ship’s force had diminished from 0°585 to 0°480 [the earth’s force 
=1:000], or nearly one-fifth ; representing a decrease in the ‘‘ semi- 
circular” deviation from 35° 50! to 28° 45!; the direction of the 
force, or neutral points, approaching the fore-and-aft line by 10°, 
or changing from 47° on the starboard bow to 37°. 
At the expiration of the next six weeks, the ship in the interim 
having made the passage to Holyhead, the ship’s force diminished 
from 0°480 to 0°390, or about one-sixth, corresponding toa decrease 
of “semicircular” deviation from 28° 45! to 23° 0', the direction of 
the force changing from 37° to 32°. 
At Southampton, in June 1860, or nearly eight months after the 
experiments made at Holyhead, the force had further diminished 
from 0°390 to 0°235, or by one-half, corresponding to a decrease in 
the “ semicircular’ deviation from 23° 0! to 13° 30’; whilst the 
direction of the force approached the fore-and-aft line 25°, or from 
32° to 7°; the quadrantal deviation remaining nearly constant 
[+4+°] the whole time included in the various observations. 
The unvarying tendency of the direction of the ship’s force in the 
‘Great Eastern’ to assume a fore-and-aft line, supports the view that 
time, with the vibrations and concussions due to sea service, leads to 
a distribution of the magnetic lines, of the nature of a stable equili- 
brium depending on the average of the inducing forces to which the 
ship is exposed ; the respective sections of the hull having north and 
south polarity, being separated by lines approximating more nearly 
a horizontal plane and vertical axis through the body of the ship ; 
instead of the inclined axis and equatorial plane of separation due to 
the magnetic dip of the locality, and divergence from the magnetic 
meridian, of the hull while building. 
The practical information resulting from the example of the ‘Great 
Eastern’ is, that prior to a newly built iron ship being sent to sea, 
her head during equipment should be secured in an opposite direction 
to that in which she was built; and that the magnetic lines should 
* The observations at Southampton were made after the paper was communi- 
cated to the Royal Society, and are introduced by way of supplement. 
