STANDARD - Survey Instrument Antique Center!

C. Li. Berger & Sons' Bevel-Limb Transit.
The above cut represents our Bevel Transit as made by us to order since 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 rectangularly
below the graduated surface. Thus, while a bevel possesses some advantages when ne-u and well 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 estimate
differences in the reading of limb and verniers, thereby enabling him to obtain closer results, as verified by
the superior results in triangulation obtained with horizontal graduations over bevel onesformerly in vogue. 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 same as those in Fig. c 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 adoption,
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 telescope
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
the adjustment of 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 mind bevel graduation
should be confined to exceptional cases and to the larger instruments read by micrometer microscopes.