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206 DESCRIPTION ET USAGE 
paper, and records the « absolute inclination » ; — butif 
an accidental cause, for example a current of air, should 
impart some small rotation to the wheel, 1t will always 
endeavour to return to rest al its true zero, because the 
mean surface of the sea is horizontal, and the mean 
position of the ship is, vertical : and the period of oscil- 
lation of the wheel is so long that the mean operation 
of the friction tends to restrain the motions of departure 
of the wheel from ïts true mean position, more than it 
restrains the motions of approach towards it. 
If the wheel has, by any such cause, been a little 
disturbed, the record 1s not thereby vitiated — indeed 
itis not sensibly injured — because the « period » of the 
disturbance 1s so long that it does not become in any 
degree mixed with the motions of the ship : but the 
oscillations of the ship, instead of being refered to a 
Straight axis or zero line, are recorded on both sides of 
a curved axis. We at first possess the « lines of oscilla- 
tion »; we then circumseribe the extremities of the curves 
of oscillation by fair flowing lines, making the opposite 
members of the curves as nearly symetrical as we can ; we 
then draw a series of verticals through the extremities 
of the several oscillations and bisect them. If we have 
performed the operation correctly, we shall find that 
the iocus of the bisections will be generally straight, 
and, in the case we suppose, nearly a eurve of sines 
(sinusoïd), the « period » of which is 70”. 
But even in the former case we shall sometimes find 
deviations from exact straightness ; for example, if the 
ship has for a time been inelined by the force of the wind, 
and until we have separated this extraneous inclination 
