Design of Theodolite Axes

Abstract

IN NATURE of September 15 a letter from Prof. A. F. C. Pollard points out that the cylindrical bearings adopted for the vertical axis of the Wild precision theodolite leads to a systematic error of the “order of two to four seconds” in the horizontal angles. We, in Egypt, have not had much experience with theodolites of the Wild design, but we have found that the substitution of cylindrical for the old Y-bearings for the telescope (trunnion) axis of English theodolites has not been found to be an improvement but very much the reverse. Whilst the instrument is new, the errors arising from this fault in design do not obtrude themselves, but as soon as the bearings become worn or one of the standards gets slightly bent, the telescope comes to rest in a different position every time it is raised or lowered in elevation. The error arising from this defect is of an accidental nature and may, in an extreme case, amount to one minute.