MARYLAND GEOLOGICAL SURVEY 451

eliminated. To overcome the clip of the north end of the needle
downwards, the ordinary compass needle has attached to the south
arm of the needle a bit of brass wire at the proper distance from the.
pivot, so that the needle will swing horizontally. In the instrument
before us this is accomplished by fastening the, fibres to an arm ex-
tending about one-half of an inch above the body of the stirrup, thus
causing the point of suspension to be raised so high above the centre
of gravity of the magnet that the latter is little affected by the dip and
will hang sensibly horizontal in ordinary latitudes.

On the south end of the magnet is a graduated scale, divided into
60 equal parts, one division being equal to two minutes of are, and on
the north end is a small lens of such shape as to bring the scale into
focus in the small observing telescope when the latter has been focused
on a distant mark. This telescope, as will be noticed, is mounted
eccentrically on the same support as the magnet box. It is provided
with collimation and wye adjustments. The instrument is leveled by
means of the striding level resting on the pivots of the telescope. The
azimuth or reference mark may be sighted with the telescope by
looking directly through the box, the magnet having either been
removed or lowered out of the way, and by turning aside the glass
window and reflector placed at the south end of the box to throw the
necessary light on the graduated scale. On the north end of the box
is simply a round opening—no glass window—and hence both the
mark and the scale can be observed directly without looking through
any glass windows. To shut out air currents the dark hood is fastened
to the box and fitted tightly over the telescope tube.

To eliminate the error in the declination that may be due to non-
coincidence of the geometric and of the magnetic axes of the magnet,
the magnet is turned around 180° in the stirrup, so that the part
which was on top before is now at the bottom. The scale at the south
end will now be inverted and the figures read from right to left. The
mean of the readings taken with magnet erect and with magnet in-
verted determines the magnetic axis.

The small tube on the right of the glass tube containing the suspen-
sion fibres is a thermometer for noting the temperature inside the box