4 Mr Harris on Magnetic Intensity by the 
tain this object *. Plate I, Fig. 2, represents a portable appara- 
tus of this kind, which I have employed with considerable advan- 
tage. A brief explanation, with the assistance of Figs. 1, 2, 3, 
will render it easy to be understood. 
Fig. 1. A, represents a strong base, resembling somewhat in 
form the sector of a circle ; it is separable into parts, being held 
together by a single screw and nut at a. The diameter ach of 
this sector is to be directed as nearly as possible in the magnetic 
meridian. An oblong plane of wood P, Fig. 2, clamped at each 
end, and carrying the magnetic apparatus and air-pump, is placed 
upon this base, so as to be moveable for a short distance on a 
pin c, Figs. 1 and 2, concentric with the suspended bar and gra- 
duated card ; this point c is also the centre of the are mn, Fig. 1, 
which admits of a minute adjustment of the graduated card, in the 
line of the magnetic meridian, without disturbing the state of 
the exhausted receiver 77, Fig. 2. 
6. The magnetic bar m, Fig. 3, is suspended in a light frame 
of weod ff, attached to a circular block g. The upper and nar- 
row part of this frame ,f’, is separable from the lower portion f/f, 
and may be detached if required ; it is moveable with friction in 
a circular hole in the transverse bar which sustains it, and there 
are sight slits ff in the sides of the lower portion of the frame, 
which serve to adjust with precision, the position of the needle in 
one direction, by observing the suspension silk through them. 
The centre of the block ¢ is hollowed to about one half of 
its diameter, in order to admit of the operation of a sort of forked 
lever //, Fig. 4, moveable through an air-tight collar ; as also for 
the purpose of securing the block by means of a screw and nut, 
to a circular plate of finely polished slate, ss, Figs. 2, 3, 4. This 
plate of slate is extremely well adapted to the purposes of an air- 
pump plate, and is connected with the exhausting barrels by 
* Trans. Royal Society for 1831, p. 69. 
