Horizontal Intensity of the Earth's Magnetism. 119 



one ; for the conducting-wires, in order to ensure suspending- 

 power and conductivity, had to be copper wires about 1 millim. 

 in^diameter. As the ordinary mode of suspension is attended 

 with difficulties in the case of these strong wires, a solid cord 

 was constructed of the two wires by plaiting them with silk in 

 the manner represented in fig. 4, so that they were ™ , 



firmly connected with, while insulated from, each 

 other. In constructing this cord, 170 centims. in 

 length, to pass the shuttle containing the silk cord 

 about 5000 times through the wires was a some- 

 what tedious operation. But the complete cord was 

 remarkably firm ; and while it was not possible by 

 repeatedly stretching the two hard copper wires to 

 get rid of the great curvatures in each, when thus joined they 

 exhibited an almost faultlessly straight figure. 



In order that the ends of the wire, where they are Fig-. 5. 

 screwed and soldered into the pins of the upper axis, 

 may not be liable to bend, a sheath is passed over the 

 rod, the section of which is seen in fig. 5. 



The insulation of the two wires, as well as of all the 

 parts of the suspension, was tested with an extremely de- 

 licate galvanometer, and was found to be perfect. 



Nothing need be said in reference to the tangent -compass, were 

 it not that it had been constructed in a manner which is extremely 

 simple and might be frequently imitated with advantage. It 

 consists of a wooden disk of about 800 millims. diameter, upon 

 two grooves in the rim of which a thick copper wire was wound 

 in two coils. Suitable incisions in the disk give space for a small 

 torsion-circle, the needle with a mirror (for the readings), and a 

 powerful damper for the latter. The part of the wire projecting 

 at the top is twisted to form a cord, as in the bifilar galvanometer, 

 and, fastened to the ceiling, serves as a support for the whole in- 

 strument, which thus, held also laterally only by two clamps, has 

 great solidity produced by the simplest means. 



The connecting-wires of both instruments, together with the 

 wires from the battery, are, everywhere, in order to exclude local 

 action, placed in pairs near each other, and proceed to a commu- 

 tator near the observing-telescope, by which the current in each 

 branch of the conduction may be reversed. 



The data which must be ascertained from the instruments 

 once for all are the following : — 



(1) The surface enclosed within the coils of the bifilar galva- 

 nometer. It is obtained from the length of the covered wire 

 (177,318 millims.), and from the number (84) of coils formed 

 by this length, taking into account that the coils, owing to 

 gravity, have passed from the original circular shape into ellipses 



