330 OBSERVATIONS ON THE MAGNETIC DIP AND DECLINATION. 
whom it was made. The whole instrument is of brass. The azimuth circle is divided 
by the aid of a vernier to read to minutes. The vertical or dip circle is eight inches in 
diameter, divided, on a silver-washed surface, to read to fifteen minutes of arc. - But, by 
the aid of a magnifying lens, the readings may easily be estimated to the nearest two 
minutes, 
In the centre of the dip circle are two polished semi-cylindrical agate supports for the 
axis of the needle to rest upon. ‘The central position of the needle is adjusted by two 
brass Y’s, which can be raised or lowered, when necessary, to relieve the axis of the 
needle from the agates, or to adjust its bearing upon them. They also serve to shorten 
the vibrations of the needles, in order to diminish the time necessary for the readings. 
The instrument was supplied with two needles, each 7,4%, inches long. The one 
designated as needle No. 1, is a parallelogram in shape, with rounded ends, marked by a 
fine line at each end, to indicate the centre of form. 'The other, designated as needle 
No. 2, is of the acute lozenge shape. The axes of these needles are of nicely polished 
steel. The box in which the instrument is packed is furnished with a pair of bar mag- 
nets, for reversing the poles of the needles; a magnifying lens, with a silvered reflector, 
for reading the angles; and the necessary adjusting tools. There is also a needle, three 
inches long, in a rectangular brass box, with a glass cover, with fixtures for attaching it 
upon the azimuth plate, parallel to the plane of the dip circle, for the purpose of bringing 
the latter to coincide with the magnetic meridian. When this is done, this small needle 
is removed. There are two short, but delicate, spirit-levels attached, at right angles to 
one another, to the moveable azimuth plate. These are levelled by three foot-screws, 
which support the instrument upon its stand. The needles are protected from the action 
of the wind by a glass cover, placed over the vertical or dip circle, the side next to the 
graduated face being readily opened for the necessary arrangements of the needles, 
reversing them on their axes, &c. 
The needles are marked, upon one of the flat sides, and near one end, with the figures 
1 and 2, respectively. This simple mark upon each needle serves to designate its num- 
ber, its face, and its magnetic state. It is sufficient to refer to this number to designate 
the needle observed upon, and all the changes of position of which it is susceptible. In 
making the observations, the following method was always pursued ; namely,—Both ends of 
the needle were read, with the face of the dip circle to the east, and also to the west, and 
with the face or marked side of the needle twice to the east and twice to the west; that 
is, once each way, by a change of direction of the face of the instrument, and once each 
way, again, by a reversal of the needle on its axis. A mean of all the readings was then 
taken and noted. The poles of the needle were then changed, and the same course 
repeated, and a mean of all the readings noted. A mean of these two results was taken, 
for the dip indicated by the needle. ‘The same operation was then performed with the 
other needle, and a mean of the final results of the two needles was adopted as the mag- 
netic dip for the station. Care was always taken, in selecting positions for observation, 
both for the dip and the declination, to have them beyond the reach of influence upon 
the needles, arising from the iron in buildings, or from any other local cause. The 
magnets—all iron or steel in the shape of keys, penknives, the cooking utensils belonging 
to the camp, é&c.—were always kept remote from the instrument during the observations, 
