On the Adjustments of the Needle of a Tangent-galvanometer. 135 



The frequent occurrence of crystals in which the planes n are 

 short, as in the adjoining figure, may explain 

 statements as to zircon being found at this 

 locality; such crystals having much resem- 

 blance to the terminal planes of a prism of 

 zircon : no crystal of that mineral has at least 

 been found either by jNIr. Dudgeon, or by the 

 late JNIr. Copland of Blackwood, who had pre- s 



viously carefully surveyed all the Criffel district. 



The form ycrnv (rig. 5) was found with rutile and chlorite 

 in quartz at Ci'aig Cailleach in Perthshire ; the crystals were of 

 a straw-yellow colour. 



I have followed Dana in giving to the figures of these crystals 

 a position in which y is the base of the fundamental prism, and 

 r is the prismatic plane; a consideration of the Perthshire and 

 St. Gothra'd crystals seems to indicate this to be the natural 

 position : the cleavage of the New Abbey crystals appeared to be 

 parallel to r and /, but very indistinct. 



XVIII. On the Adjustments of the Needle of a Tangent-galvano- 

 meter. By G. Johnstone Stoney, A.M., M.R.I. A., Secre- 

 tary to the Queen's University in Ireland*. 



MBRAVAIS has already publishedf the corrections ari- 

 • sing from the length of the needle of a tangent-galva- 

 nometer, so constructed that the magnetic centre of the needle is 

 at any given point of the axis of a circular current placed in the 

 plane of the magnetic mei'idian. Herr Bosscha, adopting the 

 ordinary form of galvanometer, in which the centre of the needle 

 coincides with the centre of the current, has added J the correc- 

 tion to be applied for the breadth of the metal band, which is 

 often used in these instruments instead of a wire as the guide to 

 the current. Since, however, the magnetic centre of a needle 

 seldom coincides with its point of suspension, and as the usual 

 mode of sup])orting it by fibres of silk forces us to depend for 

 the accuracy of the position of its point of suspension on adjust- 

 ments to be made each time the instrument is used, it was found 

 necessary to inquire also into the effects of the kinds of derange- 

 ment to be apprehended from either of these causes ; and as 

 others may have felt the same want as the author, it may per- 

 haps prove useful to publish the following results. 



Conceive the circular current to be placed in the plane of the 



* Communicated by the Author. 



t Coinptes Ren (/us, vol. xxxvi. ]). 19.'<. 



+ Pofrjfcnilorft's Annnlen, vol. ci. p. ^I'JJ . 



