CHARACTER OE THE ARMOUR-PLATED SHIPS OE THE ROYAL NAVY. 267 
simpler construction than the Standard, not being fitted with the azimuth circle, and 
generally having only two needles, but they are of little inferior accuracy, magnetic 
power and delicacy. The two needles are arranged so as to obviate the sextantal error 
above alluded to. 
The Tables of deviations of these compasses have in all cases been most satisfactory, 
and on those points on which the directive force is very much diminished, they con- 
tinue to give satisfactory indications which compasses of inferior workmanship would 
wholly fail to do. 
The observations of horizontal force were made by vibrating a small flat lenticular 
needle 2f inches long and ^ inch broad, fitted with a sapphire cap, on a pivot of its own, 
made to screw into the socket of the pivot of the Standard Compass, and comparing the 
time of vibration with that of the same needle vibrated on shore. 
The observations of vertical force were made by vibrating a dipping-needle of 2f 
inches, placed in the position of the compass, the needle being made to vibrate in a 
vertical plane at right angles to the magnetic meridian. The observation might of 
course be made by vibrating the needle in the plane of the meridian and observing the 
dip ; and in low dips that method is probably the best. In so high a dip as that of 
England, vibrations in the east and west plane are sufficiently accurate, and enable us 
to dispense with observations of dip. 
In the selection of these instruments it has been found of great importance that they 
should be light, portable, easily and quickly fixed in position, capable of being placed 
in the exact position of the compass, should admit of observations being made quickly 
and in rough and boisterous weather, and should be such that each separate observation 
should give a useful result. 
When the observer can command favourable circumstances of observation, as in the 
case of observations made in a ship on the stocks, it is possible that instruments of 
greater nicety may give more exact results, but for the ordinary observations which can 
be made in the process of swinging a ship, we have every reason to be satisfied with the 
results obtained from the instruments we have described. 
As the formulae made use of in the reductions are nowhere published except in the 
‘ Admiralty Manual,’ it seems necessary here to give them with a brief indication of the 
manner in which they are obtained. 
The effect of the iron of a ship on the compass-needle is assumed to be due partly to 
the transient magnetism induced in the soft iron by the magnetism of the earth, and 
partly to the permanent magnetism of the hard iron. Simple physical considerations 
show that the components of the first in any three directions in the ship are linear 
functions of the components of the earth’s magnetism in the same directions, the last is 
expressed by constant forces acting in the same three directions. 
If, therefore, the components of the earth’s force on the compass be X in the direc- 
