611 



side, according as the light of day, or that of a lamp or 

 candle, is employed. 



The measuring rod employed in deflection experiments 

 is a compound bar of gun metal, formed of two bars, the 

 lower of which has its surface horizontal, and the upper ver- 

 tical. It is three feet in length,* and is graduated on its ver- 

 tical surface. It is placed upon the upper plate of the circle, 

 beneath the box, and at right angles to its longer sides ; and 

 it is so fixed that it may be removed with ease, and replaced 

 exactly in the same position. The support of the deflecting 

 magnet slides upon the upper bar, and is furnished with a 

 vernier, by means of which the distance of the two magnets 

 may be determined with accuracy and ease. 



The apparatus is furnished with two soft-iron hollow 

 cylinders, nine inches long, and three-fourths of an inch in 

 diameter, which fit in vertical sockets attached to the upper 

 plate of the circle. By this addition the instrument is con- 

 verted into an Induction Inclinometer, for the measurement 

 of the changes of the Inclination. By a slight addition to the 

 suspension apparatus, the instrument may likewise be used 

 as a Bifilar Magnetometer, for the measurement of the changes 

 of the Horizontal Force. These adaptations are, however, of 

 minor importance to the travelling observer, whose main con- 

 cern is with the absolute determinations ; and in a fixed ob- 

 servatory it is essential that there should be separate instru- 

 ments for the separate purposes. 



The most convenient order of the observations to be made 

 with this apparatus, when employed by the travelling ob- 

 server, is the following. 



1. Measurement of Absolute Declination. 



The copper box and measuring rod beiug removed, one 



• For the purposes of the travelling observer, it will be more convenient that 

 this rod shoulil be in two pieces. Two single bars, placed edgewise, will suffice. 



