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to the focus of the lens, contains a cross of wires. The rays 

 proceeding from the cross are, under these circumstances, 

 refracted parallel to one another, and to the line connecting 

 it with the centre of the lens, — which may be denominated 

 the line of collimation of the instrument. If, then, a teles- 

 cope be placed so as to receive any portion of this parallel 

 beam, the cross will be seen in the direction of the line just 

 mentioned ; and that, independently of the exact adjustment 

 of the telescope. 



The frame-work of the apparatus consists of two pillars 

 of copper, firmly attached to a massive slab of marble. — 

 The height of the pillars is eighteen inches ; they are con- 

 nected by two cross pieces of wood, one at the top, and the 

 other about five inches from the bottom. In the centre of 

 the top piece is the suspension apparatus, the plan of which 

 has been adopted from the torsion balance of Coulomb, as 

 described by M. Pouillet. It is provided with a divided 

 circle, for the purpose of determining the amount of torsion 

 of the thread, and of correcting it. 



The magnetic bar, suspended by parallel silk fibres, is 

 enclosed in a rectangular wooden box, to preserve it from the 

 agitation of the air. A glass tube, between the two cross 

 pieces, surrounds the suspension thread, and completes the 

 enclosure of the instrument. 



The box is entirely distinct from the rest of the appara- 

 tus ; it is made in two halves, which are joined at the sides by 

 dovetails ; so that it may be put on when the needle has been 

 fully adjusted. There is a circular window at each end of 

 the box. That nearest the observer is made of parallel 

 glass ; and is contained in a frame which has a motion of 

 rotation in its own plane. By a revolution of 180°, the pris- 

 matic error, if any, is corrected. The window at the other 

 end of the box is for the purpose of illumination. In 

 order to determine the internal temperature, the box is 

 provided with a small thermometer, the bulb of which 



