Inclinometer. liii 



The quantities a (<a^) are obtained from the observations corrected by the co- 

 efficient q corresponding to the value q — '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 : — 



S A (T) = - 55°-l ; S A (f) = 634°-8, S A (a) = + 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 (^), be rejected, we shall have 



S A (T) = — 253°-3 ; S A (0 = 362°-7, 2 A (a) = + 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 diifer- 

 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. Inclinometee. 



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. 



