Inclinometer. 
liii 
The quantities a (#) are obtained from the observations corrected by the co- 
efficient q corresponding to the value q = 0*000247 (as in this volume), but are re- 
duced to scale divisions of the common value 0*00013. In one case only does the 
resulting coefficient exceed that by the temperature experiments. The final results 
are as follow : — 
JA(T) = - 55°-l ; XA(i) = 634°-8, 2 A (x) = + 157-8 Sc. div. ; error of adopted coefficient == + 0-25 Sc. div. 
If the three days, January 23, March 22, and March 23, on which the sign of 2 a (T) 
is the same as the sign of 2 a (t), be rejected, we shall have 
2 A (T) = — 253°-3 ; JA(() = 362°- 7, 2 A (x) = + 46-3 Sc. div.; error of adopted coefficient = + 0-13 Sc. div. 
75. It is not necessary to reason on these results ; the remarkable agreement 
of the partial results by all the methods is at once evident. A cause of the differ- 
ence from the result by temperature experiments has been already pointed out, 
namely, the probable effect of temperature in diminishing the elasticity of the sus- 
pending wires. This source of error might be avoided by a silk suspension, but 
another of a much graver nature would be introduced, namely, the effect of varying 
humidity, which could not be eliminated. 
76. All the observations of the bifilar magnetometer have been corrected by 
the equivalents in scale divisions of the coefficient q = 0*000247 ; the coefficients in 
scale division are given No. 39. 
§ 8. Inclinometer. 
77. The dip instrument was made by the late Mr Robinson of London. The 
vertical circle is 9^- inches in diameter; it is divided to 10', the graduations counting 
from 0° on the horizontal to 90° on the vertical ; 1' is estimated with the aid of 
lenses attached to a glazed case ; the vertical circle turns with a copper framework 
on a vertical axis, centred in a horizontal circle ; the latter is 6 inches in diameter, 
is divided to 30' and is read to 1' by means of a vernier. A sliding framework 
carrying Ys moves within that bearing the agate planes on which the axle of the 
needle rests, the Ys serve to lift and lower the needle on the agates, but they have 
been found to act very irregularly, at times giving the needle a pitch in a certain 
direction. A level screwed to the basement plate indicates the horizontality of the 
agates ; this was, however, also verified occasionally by means of a small level placed 
upon them; it was found that the level varied according as the door of the case 
inclosing the instrument was shut or open ; it was, therefore, always tested with 
the door shut, as it is during observations. 
78. The reading of the horizontal circle, when the vertical circle is in the mag- 
netic meridian, was obtained with the aid of a horizontal needle, carried on a pivot 
whose arms rest on the agate planes. There are two dipping needles, numbered 1 
and 2, and one end of each needle is marked A, the other end is marked B ; all the 
MAG. AND MET. OBS. 1843. O 
I 
