Art. LXI.—On the Use of the Semicircular Protractor, with a Description of an Improved Form of that Instrument.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 3, 1870, Page 316

Art. LXI.—On the Use of the Semicircular Protractor, with a Description of an Improved Form of that Instrument. By James Stewart, C.E., Assoc. Inst. C.E. [Read before the Auckland Institute, November 7, 1870.] Any method by which a series of lines forming given angles with each other, or bearings with the meridian, can be expeditiously and accurately plotted, ought to be of interest to many besides the professional surveyor or draughtsman. It is presumed that this will be granted so readily, that the writer deems it unnecessary to apologize for bringing before the Institute a subject of apparent professional detail, or to enumerate the pursuits, in facilitating which the Semicircular Protractor can materially assist. So far as the writer knows, the protractors in almost universal use are circular, and are either of lithographed paper or more elaborate instruments of brass. In either case the method of using them is the same. The instrument is fixed down on the drawing paper, and all the bearings required are marked

off diametrically, and subsequently run into their positions with a good parallel ruler. The inconvenience of this method is not much felt in plotting compact surveys, although even in such cases the use of the Semicircular Protractor confers great advantages; but in the case of long and narrow surveys, such as railway work almost always presents, the inconvenience and liability to error attending the use of the circular instrument is such that the other method seems, in comparison, perfect. For about eight years the writer used, as occasion required, the old method, and then determined to adapt to more extended use than its inventor seemed to have intended the instrument described by Mr. Howlett, as quoted in Heather's Treatise on Matheinatical Instruments, in Weale's well known series. Several alterations, however, from Mr. Howlett's design suggested themselves, and the improved protractor was made by Messrs. Elliott Brothers, of London, from drawings supplied by the writer, and its use by him for the last eleven years, in every variety of work, has demonstrated its great superiority over the old method—in points of accuracy, comfort in using, rapidity with which work can be laid down, and the great facility which it offers in tracing the occurrence of any error in plotting. Mr. Howlett fastened the drawing paper to a board, and used a T square, whose edge represented the meridian, hence any angle to which the arm of the protractor was set could at once be transferred to any position on the paper, by keeping its meridional edge in contact with the square, and moving both on the paper until one edge of the arm coincided with the point required. But long rolls of paper cannot be used with a T square or drawing board, and the writer uses in such cases the same parallel ruler which is required in using the circular protractor. If the work lies mainly north and south, one meridian line is drawn in lightly in blue; if it lies otherwise, several meridian lines may be required at various intervals, laid down with great care. The edge of the heavy brass parallel ruler, then, represents the meridian, and its truth can be compared with the nearest line at any moment. The protractor is moved along the edge of the ruler until in position, just as with the use of the T square. Of course, when the size of the map or plan is such as to admit of the paper being fastened on a board, the T square forms the most perfect parallel ruler, and should be used in preference. The operations which must be gone through when using the circular protractor—consisting of laying down the bearings, reading off the particular one required (sometimes among a crowd of others of nearly the same angle), setting the ruler, and running it into place—may, by the method here advocated, be said to be comprised in one movement, such is the comparative and absolute ease attending its use. While setting the vernier arm, the instrument is held in the hand, and in the most suitable light. This, besides being a source of much comfort, is

conducive to accuracy, such as is seldom attained by any other method. Those who are accustomed to examine the accuracy of plotted traverses will appreciate, without further remarks, a method by which any bearing may be checked at a glance, and each traverse followed out with great rapidity and accuracy with no more preliminary work than ruling one or more meridian lines. Two forms of this instrument have hitherto been made — Hewlett's (mentioned above) and Metcalf's—but neither seem to the writer to present simplicity and compactness enough for general use, while, in both, the vernier is set at an angle of about 30 degrees with the plane of the graduation, causing great disturbance of light and shade, and, in part, sacrificing one advantage of the system. In the instrument designed by the writer, [a drawing of which was exhibited at the meeting] the vernier is in the same plane as the graduated limb, and is distinct from the arm, working on the latter in dovetailed grooves, and held to the limb by a light spring; the arm is parallel in the sides from the centre joint to the extremity, excepting only the part embracing the the graduated limb and vernier, which part is reduced to the smallest and most compact dimensions. In deference to the custom on all protractors, the centre is transparent, and means provided by which a line truly radial with the centre may be marked off, but simple angles may be easier laid down, precisely as bearings, using, for the sake of handiness, an ordinary short parallel ruler. No doubt very much lies in habit, but as the writer has given both methods fair and extended trials, he feels justified in awarding the special advantages above named to the instrument which, under the same circumstances, has enabled him to perform work the most satisfactory in all points of view.