236 Manipulations of the Dipping Compass. 
the centre of the graduated circle, by the same rotation which 
brings it to the true dip. ‘To restore the pivots to their place, two 
Vs and two inclined planes* raised by a lever engage them at 
their sides and at their ends, adjust them laterally and longitudi- 
nally, raise them just clear of the agate edges, and, by a reverse 
motion, lower and deposit them, and with them the needle in 
their proper places. Now the most objectionable anomaly has 
been, that when the same needle has been once adjusted by the 
Vs, read, and again simply raised by the Vs, and lowered appa- 
rently in the same place, and read again, there would be a dis- 
cordance in the readings, often amounting to five minutes of a 
degree. JI felt the full weight of these anomalies in my first at- 
tempts to determine the dip, and found tomy mortification, that 
the mean results of the usual eight reversals and sixteen readings, 
with two different needles, would sometimes differ as much as 
six minutes of a degree. 
_I finally reflected that if the pivots conld always be made to 
re-sit on exactly the same points, the readings must always agree, 
and that the apparent anomalies must arise from slight imperfec- 
tions of form, and slight and imperceptible, but really essential, 
changes of position of the points of support to the pivots. With 
this view I endeavored by all possible means, so to use the appa- 
ratus as to bring the bearings at the same points, and especially 
‘that the pivots should perform their rotations on the same trans- 
verse section; or, in other words, that the same ring or circle of 
theireireumferenee, should always rest on the agate edges. To 
accomplish this, the following points received attention. 
1. The Vs and inclined planes, which raise and fix the needle 
to its place, were so er as to. allow of no shake or lateral 
Motion of their own.+ 
"2 The distance between ie agate edge and the inclined 
plane, opposed to the end of the pivot operating as an end check, 
was made as nearly equal as possible. 
“3. Although the pivots had some “end chase” or longitudinal 
freedom’ between the end checks yet in use, each pivot was al- 
bei o_o over oe wee ‘schema end check by means of a 
Me 
*# TH he oe eee £ * 
— 
‘opposite the end of the pivots sch ine tonlly 
two parts of a Viv separated.» inde © a 
+ I have lately examined one. of Gambey 3 instruments, which was decidedly 
faulty in this particular. 
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