ON THE THREE REPORTS OF THE LIVERPOOL COMPASS COMMITTEE. 91 
or vertical iron magnetized by induction, drawing the north end of the needle 
downwards in the northern hemisphere ; the horizontal line a rod, such as that 
in fig, 3, No. 2, which would give +D, and which, when the ship’s head is 
Fig. 4. 
Fgh atten: ‘oleae mont 
north or south, will have no effect till the ship heels, when its upper (weather) 
end will attract the north point of the compass. Lach rod in the figure will 
therefore cause a deviation of the north point of the needle to the weather 
side. In order to correct this, the vertical magnetism must either act 
upwards, or the transverse magnetism must be such as would be caused by 
a horizontal transverse rod on each side of the compass, the formula indi- 
eating the relation which must exist between the vertical and the transverse 
horizontal magnetism in order that the heeling error may be zero, 
The 4th Part of the ‘Manual’ contains charts of the lines of equal 
variation, equal dip, and equal horizontal force over the globe; the first for 
the purpose of enabling the navigator at sea to determine the deviation by 
astronomical observations; the two latter to throw light on the changes which 
the deviations undergo on a lengthened voyage, and to enable the navigator 
to anticipate the changes which will take place on a change of geographical 
position. 
Of the Appendices, one (No. 2) contains a short account of the method 
proposed by Mr. Airy for the mechanical correction of the semicircular and 
quadrantal deviation, and a notice of a method lately proposed by Mr. Evans 
for the correction of the quadrantal deviation when excessive. No. 3 is on 
the mathematical theory of the deviations of the compass, being the deduction 
from Poisson’s equations of such formule as may be most conveniently ap- 
plied to the analysis of the tables of deviations derived from actual obser- 
vation. 
There is a graphical method of representing the magnetic state of a ship 
as regards deviation, described in pp. 106 and 107, which we may shortly 
describe. : 
If from the centre of a compass, in any part of the ship, we draw a 
horizontal line, representing in amount and direction the ship’s disturbing 
force on the north end of the needle of that compass, the ends of ali the 
lines so drawn will, as is shown in this appendix, trace out an ellipse. If 
the soft iron of the ship be symmetrically distributed, so that @ and & are 
| Zero, the construction of this ellipse is simplified, as its axes are then parallel 
| and perpendicular to the fore-and-aft lines of the ship. The position of the 
_ centre of the ellipse gives the amount of the force to head, and force to side, 
| which cause the semicircular deviation. The fore-and-aft and transverse 
| 
