to the Antarctic Regioni. 225 



one near each end. 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 forms a 

 moving collimator, and that its absolute position at any instant, as 

 well as its changes of position from one instant to another, may be 

 read oflF by a telescope at a distance. The aperture of the lens of 

 this collimator is 1|^ inch, and its focal length about 1 3 inches. Each 

 division of the scale is y/jjth part of an inch ; and the corresponding 

 angular quantity is about 43 seconds. 



To the suspension thread is attached a small cylindrical bar, the 

 ends of which are of smaller diameter, and support the stirrup which 

 carries the magnet. The apertures in the stirrup, by which it hangs 

 on the cylinder, are of the form of inverted Y s, so that the bearing 

 points are invariable. A second pair of apertures at the other side 

 of the magnet, serves for the purpose of inversal ; and care has been 

 taken to render the lines connecting the bearing points of each pair 

 of V s parallel, so that there may be no difference in the amount of 

 torsion of the thread in the two positions of the stirrup. The two 

 pairs of apertures are at different distances from the magnet, in 

 order that the line of collimation may remain nearly at the same 

 height on inversal, and thus it may not be necessary to alter the 

 length of the suspension thread. The stirrup, and the other sliding 

 pieces, are formed of gun metal. 



For the purpose of taking out the torsion of the suspension thread, 

 the apparatus is furnished vi^ith a detorsion bar, which (with its ap- 

 pendages) is of the same weight as the magnet. It is a rectangular 

 bar of gun-metal, furnished with a stirrup and collimator similar to 

 those of the magnet. A rectangular aperture in the middle receives 

 a small magnet, the use of which is to impart a slight directive force 

 to the suspended bar, and without which the final adjustment of 

 detorsion would be tedious and difficult. 



The frame-work of the instrument consists of two pillars of copper, 

 35 inches in height, firmly screwed to a massive marble base. These 

 pillars are connected by two cross pieces of wood, one at the top, 

 and the other 7 inches from the bottom. In the centre of the top 

 piece is the suspension apparatus, and a divided circle used in deter- 

 mining the amount of torsion of the thread. A glass tube (between 

 this and the middle of the lower cross piece) incloses the suspension 

 thread ; and a glass cap at top covers the suspension apparatus, and 

 completes the inclosure of the instrument. 



The box is cylindrical, its dimensions being 20 inches in diameter 

 by 7 inches in depth. It rests upon the marble slab, and encom- 

 passes the pillars ; and it is so contrived as to be raised, when ne- 

 cessary, for the purpose of manipulation. There are two apertures 

 in the box, opposite to each other. The aperture in front, used for 

 reading, is covered with a circular piece of parallel glass, attached 

 to a rectangular frame of wood which moves in dovetails ; the pris- 

 matic error of the glass (if any) is corrected by simply reversing the 

 slider in the dovetails. ITie opposite aperture is for the illumination 



Phil. Mao. S. 3. Vol. 15. No, 95. Sept. 1839. Q 



