APPLICATION OF THE CONVERTING TABLE.
It may be remarked that the table is equally applicable for changing bearings into angles if required. For example :—An observation was made with a right-hand dial, and the bearing found to be 27° 17' E. of N.: at what degree did the needle point ? Ans. 332° 43'; and if proof is required, it will be seen that the sum of these degrees and minutes is 360°.
In pressing on our young mining friends the advantage of adopting a perfect system, we advise that in preparing a course of dialling for trigonometrical solution, by changing the angles into bearings, care should be taken that all the drafts should be made either to exceed 45°, or that they should all stand below, or at least not exceed, that half quadrant. Our reason for being urgent on this matter is, that there may be a uniformity in placing the sides in the traverse table after the draft has been com puted. And let it be particularly noticed that, if the bearings are not suffered to exceed, 45°, that the last expression of the bearing will signify the longer of the two sides. That is, suppose a draft taken under ground was 287e, measuring 45 feet 8 inches; now, looking at the "converting table," we see that, if this draft was taken with a "left-hand dial," the bearing is 174° north of west (or N. of W.), and the two sides will be found by computation to be 13 feet 7 inches, and 43 feet 7 inches. Query, into what columns respectively must these numbers be placed? As the bearing was north of west, and our system states that "the last expression of the bearing will signify the longer of the two sides;" consequently the longer side (43 feet 7 inches) must be placed in the "west" column, and 13 feet 7 inches in the north column.
If this order is followed up, it will render the working of traverses (which is the most important operation in mine surveying) a plain, pleasing, and satisfactory exer cise. In this edition we would needs bring forward every thing likely to promote the
advancement of the young mining officer in this paramount branch of his profession, and therefore give him to understand that, in traversing, there must be a regular course from beginning to end.
We shall make ourselves understood in this matter, by taking a case where a person makes a survey for the purpose of ascertaining the length and bearing of a level driven on an east and west lode; and, for some convenient purpose, he begins his surveying at some point about the middle of the level, and surveys from thence to the eastern end; he then returns to the station or start at the middle of the level, and continues on to the western end, and thus completes the survey.
Now, if he were to proceed to work the traverse from his note-book in this state, his results would appear as if his level were almost without length or bearings, as his eastings would be balanced by his westings, &c.
In order to go systematically to work in this case, his first operation must be to reverse the order of one or the other of the surveys; that is, if he pleases to let the first remain, which is the eastern, and would accommodate the western part to suit the other, he must alter or reverse all the drafts, by converting (say) 16° south of west into 16° north of east, and so of all the rest.
In winding up this course of instruction, we will take a short survey, and go through with it at length, and the student may accompany us if he pleases; for we are still of the opinion that practical teaching is the best.