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The 



UNIVERSITY OF TORONTO LIBRARY 



116 



ransit. 



HCC 1871. 



NOTE. While a bevel-limb graduation of a horizontal circle can be somewhat more readily seen than one on a horizontal 

 surface, it is well to remember, before ordering instruments to be made so, that there are very serious objections to their 

 general adoption for engineers' field-instruments. As will be seen in the annexed cuts, the sharper and therefore more 

 delicate edges of the soft solid silver, necessitated by the bevel at the junction of limb and verniers, are much more liable 

 to injury and wear in field use than the common horizontal graduation, where the same edges are carried down nearly rect- 

 angularly below the graduated surface. Thus, while a bevel possesses some advantages when neiv and luell constructed,- 

 it soon becomes impaired by slight dents and the edges rounded by brush or finger when dust and oxyd must be removed 

 at certain times. It then can be read only with difficulty and becomes a source of great annoyance, particularly as the eye 

 looks squarely at it, thus defeating the very object sought and rendering the instrument almost unfit for good work, although 

 otherwise in good condition. We say this with an experience of twenty-five years to back up. To make it plain we must 

 have recourse to the diagrams. Fig. c is the cross-section of a horizontal limb and vernier as commonly made. It is obvious 

 that the fine silver edges at the junction of limb and verniers are in this form better protected from wear, and also that, 

 when slightly rounded by wear, or when the graduated surfaces are not in the same plane, the eye, being stationed at an 

 angle of about 45 to the limb, requiring an observer to glance along the graduated lines, will more readily see and esti- 

 mate differences in the reading of limb and verniers, thereby enabling him to obtain closer results, as verified by 

 the superior results in triangulation obtained ivith horizontal graduations over bevel ones formerly in Togve. Fig. a is a 

 cross-section of a bevel limb, showing the sharper edges of the graduated surfaces. When new and properly made the edges 

 of limb and verniers appear the sarre as those in Fig. e and a c, but when worn off they will leave a big circular space between 

 them, making the reading of an angle all the more difficult and uncertain. There are other reasons, against their general adop- 

 tion, such as that the standards for the telescope have in bevel instruments not the stability so important in angle-measuring 

 instruments unless they are mounted on a special horizontal base provided for them, (instead of on the bevel surface of the 

 upper plate) which means a great increase in weight, or that the distance between them be quite short, resulting in a shortening 

 of transverse axis of the telescope, as in the cut above, thereby increasing the instability of the same, particularly if the tele- 

 scope is of modern power and length, besides reducing the size of compass, length of plate levels, and with this the degree 

 of sensitiveness of the latter. In both these instances the standards are apt to change their distance apart affecting 

 thf adjustment cf the line of collimation for near distances. The greater expense of repairing, in case of accident, is also 

 an item that should be considered. It is often double or triple that of a horizontal graduation. To our rjiind bevel gradu- 

 ation should be confined to exceptional cases and to the larsrer instruments read bv micrometer microsc 



