.Heights of the principal Hills of Dent, fyc. Yorkshire. 85 



wires of the telescope (or line of collimation) was known to 

 be parallel to the cylindrical rings encircling its tube, when 

 the telescope could be made to revolve in its Ys without 

 causing the point of intersection to deviate from the distant 

 well-defined object which it had bisected. After this adjust- 

 ment, the line of collimation, provided the artist had made the 

 axis of the cradle parallel to the plane of contact of the two 

 circles, and the Ys at right angles to that axis, would neces- 

 sarily move in a vertical plane. As the remote signals could 

 not be distinctly seen on interposing the horizontal wire of the 

 telescope, the following method of observation with the verti- 

 cal wire only was resorted to. The point of intersection of the 

 wires being made to accurately bisect an object at a moderate 

 distance, one wire was rendered perfectly vertical by turning 

 the telescope to the right or left within its Ys, until either 

 extremity of the wire, on elevating or depressing the telescope, 

 would also pass through the middle of the object observed. 

 Thus adjusted, it would evidently be superfluous to bisect 

 constantly with any particular point of the vertical wire. To 

 guard, however, against the possibility of the wire not being 

 perfectly straight, a point, visually estimated to be equi-distant, 

 from some particular particle of dust, &c. adhering to it, and 

 the point of intersection by the other wire, was invariably 

 brought on the signal. { 



The graduations being numbered from 0° to 360°, it follows, 

 that the readings from the one vernier should differ exactly 

 1 80° from those given by the opposite one ; or it will prove either 

 that the vertical axis does not coincide with the centre of the 

 divided circle, or that the divisions of the latter are incorrect. 

 As both sources of error may exist, it will be proper to re- 

 gister for calculation the mean of the two readings. The or- 

 der of numeration of the graduations being from left to right, 

 the angle between any two of the observed signals is imme- 

 diately found from a similar register, being equal, either to the 

 difference of their respective readings, or to 360° minus that 

 difference, according as it falls short of, or exceeds 1 80°. 



The description of the instrument and its adjustments has 

 been so ample, that the mode of observing with it will be too 

 evident to require pointing out. It may, however, be needful 

 to remark, that notwithstanding the length of the vertical axis, 

 which is nearly four inches, it was found that a variation in 

 the degree of force with which the two circles were clamped, 

 caused a sensible deviation in the direction of the line of col- 

 limation ; a source of error which it was endeavoured to ob- 

 viate by always applying the utmost force to the clamp. Having 

 Completed single observations of the whole of the signals, it 



would, 



