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 acb 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 arc 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 r r, Fig. 2. 



6. The magnetic bar m, Fig. 3, is suspended in a light frame 

 of woodff, attached to a circular block q. The upper and nar- 

 row part of this frame f, is separable from the lower portion ff, 

 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 q is hollowed to about one half of 

 its diameter, in order to admit of the operation of a sort of forked 

 lever 1 1, Fig. 4, moveable through an air-tight collar ; as also for 

 the purpose of securing the block b) r means of a screw and nut, 

 to a circular plate of finely polished slate, s s, 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- 



