228 



ON A NEW MAGNETICAL INSTRUMENT. 



ences between each of these readings and the reading belonging to 

 the vertical position of the bar, converted into angular measure ; the 

 fifth, the actual deflections ; the sixth, the calculated deflections, as 

 deduced by the formula given above ; and the seventh, the differences. 

 In order to derive the numbers of the fifth column from those 

 of the fourth, it is necessary to know the deflection corresponding* 

 to the vertical position of the bar. This angle is determined by 

 placing the bar vertically, with its acting pole above and below 

 successively, and noting the readings of the horizontal circle, when 

 the same division of the moveable scale, reflected by the mirror, 

 was brought to coincide with the fixed wire of the telescope. 

 The differences between each of these readings, and the similar 

 reading when the bar is removed, are double the deflections 

 corresponding to the two positions of the bar ; and, when they 

 are nearly equal, the mean of these deflections may be taken as 

 that due to the induced force. 



FIRST OBSERVATION. 



Acting end of bar a south pole, reading = 14 8', deflection = 17 0' 

 north pole, ... 82 51, = 17 22 



Bar removed, . . 48 7, mean =1711 



