CONVERTING TABLE.
Remarks on the following table for converting the degrees recorded in the dialling-book of an under-ground survey into the bearings. (See Table, page 495.) All practical men are aware of the difficulty, hazard, and delay that attend an attempt to obtain the bearing of every draft under-ground in a long and complicated survey. The best process is to record the degree or angle only at which the needle settles, and after the work is finished under-ground; then convert the various angles into the real bearing or true direction of each draft; and we may remark that the bear ings must be obtained if the work is to be mathematically proved. But, as it is not an easy matter to turn a long course of surveying into the bearings with an assurance of being correct, this table has been constructed for that express purpose; and its utility, simplicity, and perfection have been acknowledged by many practical men.
All circumferentors (dial, or miner's compass) are not graduated alike. In all cases, stands at the north point, and 180° at the south; but some are figured towards the right hand from the north point (which we call a "right-hand dial"), and others towards the left hand: so that a "right-hand dial" has 90° at the east point, and a "left-hand dial" has 90° at the west point. This diversity of graduation has often caused much perplexity and confusion among surveyors. The following table is contrived to suit both sorts of instruments, and is so plainly arranged and marked as to require but little explanation. It must be specially regarded that the table has been constructed upon the consideration that the eye of the surveyor has been applied to the south sight or vane standing against 180°: this must be invariably the case. Hence the north sight
must always •take the lead, and the young practitioner may here be told that in survey ing a level and making double, or fore and back, drafts at every station, that although his eye must be placed at the north sight, necessarily, for the back observation, yet, as the dial has not been turned, the needle will stand to the true degree for the record, and no confusion or liability to error can occur.
In converting an under-ground survey, or any other, from angles into bearings, it is obviously our first object to know the graduation of the instrument by which the work has been performed; and if it has been a "right-hand dial," and the first draft was on 167°, the bearing would be 13° west of south; but if it was done by a "left-hand dial," the bearing would be 13° east of south The only thing where a liability to error at all exists in obtaining the bearings by inspection from this table, and where caution is required, is in applying the fractions. of degrees when they occur in the drafts. On these occasions, observe that when the angle and bearing progress alike, as in all the left-hand side of the column, then the fraction must be added to the whole number of the bearing; but otherwise, as in the right-hand side, the fraction must be deducted from the whole number. Lastly, the following desirable proof may be resorted to:—If the cause has been correctly converted, the degree and bearing added together or subtracted from each other will make one of the following numbers: 0, 90, 180, 270, 360; and this may be done almost at a glance, after the survey has been converted into bearings.