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the true meridian : for the circle of inclination, being al- 
ways in the needle’s vertical plane, the edge of it will evi- 
dently point out upon the horizon, the variation E. or 
W. But at fea, when there is not too much motion, you 
turn the frame round, till the vertical circle is in the 
plane of the Sun’s rays; that is, till the fliadow of the 
one lide of it juft covers the other, and the edge of the 
circle of inclination will then give the magnetic ampli- 
tude, if the Sun is riling or fetting ; but the azimuth at 
all other times of the day, and the true amplitude or azi- 
muth being found in the ufual way, the difference is the 
variation. If the motion is confiderable, obferve the ex- 
tremes of the vibration, and take the mean for your 
magnetic amplitude or azimuth. When the Sun does 
not lliine fo bright as to give a lliadow,, you can fet the 
brafs circle in a line with his body, if he is at all vilible 
by your eye. The principal advantage at firft aimed 
at in this compafs, was to contrive a dipping-needle, 
which lliould be fufficient for making obfervations at 
fea. As thofe needles, to be of ufe, muft be placed, by 
fome means or tether, in fuch a manner as that all their 
vibrations lhall be made in the true magnetic meridian;, 
North and South, otherwife they are good for nothing. 
For if one of them is placed at right angles, acrofs the 
magnetic line, it will ftand perpendicularly up and down 
in any part of the world; the leaft dip, therefore, is al- 
ways in this magnetic line. But the only method of 
fetting a dipping-needle at fea, was to place it in a line 
with the common compafs needle; and this muft be 
very 
