THE DIPPING NEEDLE. 79 



oval or otherwise ill-formed pivot can be entirely re- 

 moved. To make these pivots turn with the least pos- 

 sible friction is of the utmost importance : and for this 

 object, the pivots must not turn in Ys like those of a 

 transit-instrument, but mu^t roll upon two edges of a 

 very hard substance, usually agate. In the direction 

 parallel to the plane in which the needle moves, these 

 edges must be straight and perfectly horizontal ; in the 

 vertical section at right angles to that plane, or in the 

 direction of the needle-axis, the section of each edge is 

 rounded ; a form very desirable for permitting the es- 

 cape of particles of dust, &c. Great attention is required 

 for the satisfactory polishing of the edges. When due 

 care is given to these preparations, the friction is ex- 

 tremely small. 



It is necessary now to describe the method of ob- 

 serving the position of the needle. 



The needles employed are always pointed : and, till 

 within a few years past, the needle was allowed to swing 

 within a graduated ring of brass, and the divisions op- 

 posite to the points of the needle were read. The read- 

 ing was very rough, and there was risk of error from 

 the close proximity of the needle to the brass, which is 

 seldom perfectly free from iron. Lately, a far superior 

 form has been introduced, known as the Kew pattern 

 (from the circumstance that it was invented and first 

 used at the Kew Observatory). A view of that in- 

 strument is given in Figure 30. Several auxiliary parts, 

 unimportant to the general principle, are omitted in 

 this drawing. There is no metal near the needle : the 



