"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 

 nanifest 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 a distance. 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. 



