MAGNETIC INCLINATION AT GOTTINGEN. 625 
and again lowered upon the agate planes, and which are so 
fastened to the raising frame as to admit of a small adjustment, 
must be so regulated, that after raising the needle, 
5. Its axis may be normal to the line of intersection (4.), 
whereby, in combination with (2.), it will also be normal to the 
plane of the vertical circle, and at the same time 
6. Coincide with the vertical diameter of the circle. 
Conditions 1, 2 and 4 taken together are equivalent to a single 
one, viz. that the axis of rotation being accurately vertical, a 
horizontal plane shall be a tangent to both the agate planes in 
the direction of their length, it being assumed that the plane of 
the vertical circle is parallel with the axis of rotation, so that 
they both become vertical together. This may be regarded as 
the seventh condition, which is usually tacitly assumed in de- 
pendance on the skill of the artist, and for which the instrument 
as supplied offers no means of trial and eventual correction. 
4. 
For the observations in this Memoir, in the absence of other 
means of testing the above-mentioned conditions, I proceeded 
as follows :— 
For the first condition I used the plane glass of an artificial 
horizon, which (after the frame, &c. were taken away) was placed 
_ on the agates with the ground surface uppermost. If the con- 
dition is not fulfilled, the glass touches only one agate throughout 
its length, and the other only at one end; this will be detected 
by the eye if the error is not very small; but greater precision 
and certainty will be obtained by placing on the glass a level, 
showing whether it has two different planes of contact or only 
one. It will be easily seen, that by the aid of this level, after the 
first condition is fulfilled, the second and fourth may also be 
tested. 
In order to examine the fifth condition, the needle must be 
brought into two different positions of equilibrium, such that 
the ends of the axle being similarly placed on the supports, 
(or the same face of the needle being to the front,) the 
needle, having only a moderate inclination from the horizontal 
line, may have that end which was to the left in one posi- 
tion, to the right in the other position. These two positions are 
most conveniently obtained by loading the needle suitably, and 
the fulfilment or otherwise of the conditions is recognised by 
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