INTRODUCTION. xlv 
§ 7. INCLINOMETER. 
52. The Dip Instrument is by Roprnson of London. The vertical circle, 94 
inches in diameter, is divided to 10’; and 1’ is estimated with the aid of lenses at- 
tached to the instrument. The horizontal circle, 6 inches in diameter, with a ver- 
nier, reads to 1’. There are two needles, marked No. 1 and No. 2, and one end of 
each needle is marked A, the other end, B. 
Till 1843, the instrument occupied the pillar I, in the plan, next to the decli- 
nometer, and it was necessary to remove the declination magnet during observations 
of dip. 
The reading of the horizontal circle, for the vertical circle in the magnetic 
meridian, was obtained with the aid of a horizontal needle carried on a pivot, whose 
arms rest on the Ys of the instrument. 
53. The dipping needle is observed in four positions, with one end dipping, 
namely, the circle being in the meridian, with its graduated face east and west, the 
marked side of the needle being with the graduated face and from it; as each extremity 
of the needle is observed, there are thus eight readings obtained. The poles being 
changed, and the other end dipping, other eight readings are similarly obtained. 
The mean of the sixteen readings is taken for the dip. In this volume, the mean 
of each eight readings is given. 
In changing the poles, the needle was placed on a small wooden block having 
a hole to receive the axle, and received eight strokes on each face from two small 
magnets. 
The needle, No. 1, was generally used. The level of the agate planes was 
occasionally verified by means of a small level. This had to be done always with 
the door shut, as it is in observations ; the level varied with the door open. 
54. Observations were made in different azimuths on April 18. and May 2. 
1843, in order to determine the correction due to the vertical copper circle and to 
the axle of the needle. 
1 and »{ being the inclinations observed in two planes, at right angles to each 
other, the true dip 4is deduced by the formula 
cot? @ = cot? » + cot? 
The following tables contain the observations and results. 
In reading these observations, the circle was considered to be graduated from 
0° to 180°, commencing with the right side of the horizontal diameter, counting down, 
and from the left side of the same diameter, counting up. 
MAG. AND MET. OBS. VOL. I. ™ 
