332 
employed in the determination of these elements and their 
changes. 
The magnet of the declination instrument is a rectangular 
bar, fifteen inches long, suspended by parallel silk fibres, 
and enclosed in a box to protect it from the agitation of the 
air. In addition to the stirrup, by which the bar is sus- 
pended, it is likewise furnished with two sliding pieces, one 
near each end of the bar. One of these pieces contains an 
achromatic lens, and the other a finely divided scale of glass ; 
the scale being adjusted to the focus of the lens, it is 
manifest that the apparatus constitutes a moving collima- 
tor, and that its absolute position at any instant, as well as 
its changes of position from one instant to another, may be 
read off by a telescope at adistance. ‘The stirrup is so con- 
trived as to enable the observer to invert the bar, and thus, 
by the mean of the two readings, to determine the point of 
the scale corresponding to its magnetic axis. 
The framework of the instrument consists of two pillars 
of copper, thirty-five inches in height, firmly screwed to a 
massive slab of marble. These pillars are connected by 
two cross pieces of wood—one at the top, and the other 
seven inches from the bottom. In the centre of the top 
piece is the suspension apparatus, and a divided circle used 
in determining the amount of torsion of the thread. A glass 
tube, between this and the middle of the lower cross-piece, 
encloses the suspension thread; and a glass cap at top 
covers the suspension apparatus, and completes the en- 
closure of the instrument. 
The box is cylindrical, and has two apertures opposite 
to each other. The aperture in front, used for reading, is 
covered by a circular piece of parallel glass, attached to a 
rectangular frame of wood which moves in dovetails; the 
prismatic error of the glass (if any) is corrected by simply 
reversing the slider in the dovetails. The opposite aperture 
is used for the purpose of illuminating the scale. 
