8 
the determination of the magnetic intensity, and with the separate ad- 
justments required in erecting it. 
3. The constants to be determined—the magnitude of the added 
weight, and the radius of the pulley by which it acts—can be ascertained 
with more ease and certainty than those which are required in the 
method of vibrations, and are less liable to subsequent change. 
4, The observations themselves are less varied in character than the 
usual ones, and may be completed in a shorter time. 
The Rev. Dr. Luoyp also read a paper— 
ON AN IMPROVED FORM OF THE THEODOLITE MAGNETOMETER. 
Dr. Luorp exhibited to the Academy an improved form of the 
Theodolite Magnetometer, constructed under his direction by Mr. 
Jones of London, for the Magnetic Survey of the British Islands now 
in progress. 
The principle of the improvement consists in observing the celestial 
object, whose azimuth is known, by reflexion, and in transferring the 
necessary adjustments to the small mirror used for that purpose. A 
light gun-metal frame, 13 inches long, and 8 inches wide, is attached 
to the upper plate of the theodolite. N ear one end of this frame are 
two Y supports, placed longitudinally, to receive the observing tele- 
scope; and near the other are two similar supports, placed transversely, 
to receive the cylindrical axle to which the mirror is attached. The 
magnetometer box is placed between, over the centre of the divided 
circle. The telescope, accordingly, remains horizontal, and is always in 
adjustment for the observation of the collimator magnet; and the image 
of the celestial object is brought to the cross of wires in its focus, by 
turning the apparatus in azimuth, and, at the same time, causing the 
mirror to revolve. The axle is furnished with a slow motion for the 
purpose. 
There are three adjustments required :— 
1. The axle to which the mirror is attached must be horizontal when 
the instrument is levelled. This is tested by a small riding-level. It 
may be effected permanently, with sufficient exactness, by filing one of 
the Y’s. 
2. The mirror must be parallel to the axis of the cylindrical axle to 
which it is attached. This is tested by reversing the axle in its Y’s, 
and by noting the reflected division of a scale cut by the wire in the 
focus of the telescope, before and after reversal. The adjustment is 
effected by means of three screws at the back of the mirror. 
3. The line of collimation of the telescope must be perpendicular 
to the axis. This may be tested by observing a well-defined distant 
object in the horizon, first by reflexion, and afterwards directly; the 
deviation of the line of collimation from the normal to the mirror is half 
the supplement of the angle through which the telescope is moved. The 
adjustment may be most readily made by moving the wire-plate in the 
focus of the telescope. 
