"معاملات الجمعية الجغرافية في بومباي. من يناير ١٨٦٣ حتى ديسمبر ١٨٦٤. (تحرير السكرتير.) المجلد XVII." [‎٢‎٠] (٥٢٤/١٨٣)

20
DESCRIPTION OF A MAGNETIMETER.
graphical position per chart. Under the turntable there is a tano’ent
screw for regulating the motion, and the horizontal is preserved by
small moveable weights on this table.
When intended for use, the instrument is taken in a convenient
place for observing a distant object, it is properly balanced, the needle
made to correspond, and the lower needle so placed as to deflect to an
angle equal to the dip of the place. A set of sights are taken at the
distant object, with the upper compass, to make sure there is no deviation
produced on it by the iron bar or lower needle ; should there be any,
it is reduced simply by increasing the length of the pedestal. After
all the adjustments are complete, the table is moved until the vertical
iron bar is at right angles to the natural position of the lower needle*
the upper compass showing the magnetic meridian the deflection is
noted: the table is then moved round 180°, the second deflection noted,
taking care the plumb line is at zero : in the event of the iron having
any retentive or sub-permanent magnetism, the bar is reversed end
for end, and the experiments repeated, the mean of the whole is taken,
‘and the difference between it and the dip of position is noted -f or —
for the correction, when used to determine geographical position.
It is obvious that, after the corrections are once made, the observations
are of a very simple kind, and should there be any doubt about the cor
rectness of the dip-lines on the chart, it can still be made of much use
to steamers and vessels constantly plying on the same route; for
instance, a vessel leaving here for Aden, making careful observations,
would have the same deflection to guide her returning to and making
the same port in thick weather in the monsoon.
The description tends to make the observations appear tedious, but
they are much more in theory than in practice, and perhaps some
will say, ‘'Would not the dipping needle do all this?” but it is well
known, a good dipping needle is a difficult thing to be got, very ex
pensive, and easily injured, and does not come to a state of rest
so quickly as the needle I have been endeavouring to describe;
with an ordinary divided arc at hand, any person with a little mechan
ical genius can construct this instrument, and the errors occasioned
in the dipping needle from friction, dust, &c., so difficult to overcome,
is reduced in this plan to the very small inequality that can exist on the
flne points ol the two pins employed to support the magnetic needles.
The following is a set of observations on this outward voyage