52 GAUSS AND WEUER OX TERRP:aTIlI AL MAGNETISM. 



meiitof the observations, the telescope must be directed towards 

 the mark, and this examination must be repeated from time to 

 time ; and if a deviation is indicated in the optical axis, it must be 

 again brought back to its original vertical plane. If the precau- 

 tion is taken to note two other divisions on the wall, one on 

 either side of the mark, thej' will furnish the means of estima- 

 ting the amount of the requisite coiTcction. But it should be 

 remembered that these divisions, though they may be made to 

 correspond exactly with the divisions of the scale, Avill not have 

 exactly the same value in seconds. If no such auxiliary marks 

 have been made, the amount of the connection must be judged of 

 by the eye, in parts of the divisions of the scale itself. 



The obsen^ations are made at the vertical thread ; the hori- 

 zontal thread serving merely to indicate nearly the middle of the 

 former. In order that it should make no difference whether the 

 parts of the scale appear somewhat higher or lower in the field 

 of view, the cross threads must have such a position, that a 

 fixed object, seen on their crossing, remains accurately on the 

 vertical thread, when the telescope is moved somewhat up and 

 down. The mark also serves for this verification, which, how- 

 ever, need not be frequently repeated when the position is left 

 unchanged. 



The plumb-line suspended from the centre of the object-glass 

 must be so near the scale that the image of both may appear with 

 the same distinctness in the telescope, and that thus the division 

 covered by the line may be precisely determined. The scale 

 must be so placed that its zero must correspond with the plumb- 

 line, or the division which does so correspond must be taken as 

 an arbitrary zero. The verifpng the undisturbed state of the 

 scale should be repeated from time to time in the course of the 



whether it was not hetter to place this mark on an insulated pedestal in the 

 interior of the room, than on an exterior wall cxi)osed to the weather. The 

 latter was decided on, as otherwise either the distance of the ohserver from 

 the needle must have been diminished, — or the advantage of seeing distinctly 

 the mark and the scale with the same position of the eye-piece be given up, — 

 or the room must have been made of a greater length, which was not possible 

 in the place fixed on. To have a separate foundation for a mark was regarded 

 for many reasons as objectionable. Moreover, the fear that the place of the 

 mark might be perceptibly altered by tlie influence of the weather on the wall, 

 was regarded as of little importnnce, considering the solid construction of the 

 building, and the si lall height of the mark above the foundation ; and espe- 

 cially as it was in our power to repeat, as frequently as desired, the measure- 

 ment of the angle between the mark and a church spire seen through the 

 northern window. The experience of three years justifies the propriety of 

 this arrangement. 



