SURVEYING WITHOUT THE MAGNETIC NEEDLE.
This is a comparatively new method of mine surveying. "Necessity is the mother of invention," and the introduction of railroads and "tram-ways" in mines drove the surveyor to seek some substitute for the needle,—which the attraction of the iron rendered useless,—and he has succeeded.
In some mines the presence of iron ores also attracts the needle, while, generally, no correct survey can be made under ground in our coal-mines without the circumferentor or vernier scale on the outside of the compass or theodolite.
The method of surveying on this principle differs from the magnetic method chiefly in one particular, namely, that in every fresh draft the position of the bearing must be ascertained by the back observation in the direction of the sights, and the angle made at the old station must be obtained and preserved at the new station; and this is evident, because we have no magnet for our guide. For example:—Suppose we are surveying over a railway in a level, and the last observation was 259°; after measuring the length, the instrument is removed and carried forward to the place of the light where the angle was taken, and a mark and light left at the old station. Then, after the instrument has been adjusted in its true place, the next act of the surveyor is to place the centre of the vernier on 259°, as it stood at the old station; and, if the instrument does not move by rack-work, he must keep all firm with his hands, and turn the head toward the last station until the candle is seen through the sights. He then removes behind the instru ment, and moves the sights in the direction for the next draft, where the assistant is holding a light for the purpose (the graduation being fixed), and this new draft gives (say) 270e, showing a difference between the two drafts of 111°. Although this process is somewhat tedious in description, it is simple in practice, and the history of one draft is as well as a hundred; and we may observe that, with proper care and judgment, this is the most perfect method of surveying, because there is no risk of attraction; and, as the circle is much larger than the inside plate, and the divisions more distinct, together with the vernier scale being applied, the angle can be read off to one or two minutes,—a nicety which cannot be attained by the needle in the common way. It is hardly neces sary to state that, in order to obtain the bearing, there must be at least one draft in the traverse where the needle must be brought into play, and this draft will determine the polarity or direction of the whole.
Further, let it be remarked that a survey may be resolved into bearings, and worked trigonometrically, when this method is used, as by the needle.
Suppose a case that we are about to survey over a railway, but there is space enough clear of iron for the first draft; and, taking the observation with the needle, we find the north point (a right-hand compass) stand at 176l°; we then fix the outer circle with the vernier precisely at the same point, and then, throwing off the needle, perform all the remainder of the traverse by means of the outer circle. Hence it will be evident, then, if the outward circle is also graduated towards the right hand, that the whole course will come under the immediate operation of the " converting table," as if the work had been performed with the needle; and if' the graduation should be reversed, the "left-hand" bearings will apply accordingly, regard being had to inversion in both cases.
This instrument is also well adapted for taking the bearing of slopes having a lift of iron pumps,—a job that has often baffled the skill and ingenuity of engineers and occasioned numerous and most serious errors.
The operation may be performed thus. Suppose we are in the first lift, and from thence to the second the slope dips 3 feet in 6 northerly. By applying the instrument at some point in the level near the slope (but far enough away to be free from attraction by the pumps), we find the bearing by the needle, to a point opposite the slope, to be due west, and the vernier on the outer rim standing at 90°. We then remove the instru ment to the slope, where the light was held, and adjust the back observation as before directed, having 90° on the outer rim, and the needle thrown off as useless, because we are now close to the pumps. A light is to be carried down the slope as far as it can be seen, and, after the graduated circle has been screwed fast, the rack is applied, and the sights turned until we cut the candle in the bottom of the slope. This being done, we examine and read off the degree against the point of the vernier, which proves to be (say) 1871°. Now, as when the instrument stood in a due west position the outer circle stood at 90°, and in taking the bearing it stood at 1871°, therefore, by subtracting 90° from 1874° we find the gain to the right hand of west is 97e, and, the dip being northerly, the true bearing of the shaft is 74. east of north.
The imperative call for accuracy in cases of this kind will be seen when it is con sidered that the diagonal part of this slope is upwards of 240 feet, and the dip 3 feet in 6: consequently, the whole base is more than 120 feet, and an error in the bearing has the same effect on the survey as if it had been made in taking a horizontal draft of 120 feet long, and on which an error of 4° would throw the end of the line nearly 9 feet too far either to the right or left.
Should a surveyor be called to do a job of this kind, in the absence of a suitable instrument he may accomplish it in the following manner. Let him fix a cross-staff in such a position that through one pair of sights he can see the candle in the slope, and in the line of the other pair he has the compass fixed in the level, out of the way of the attraction: consequently, the light in the slope and the compass in the level are two objects forming a right angle with his cross-staff. He then requests his assistant to look at a light held immediately over the head of his cross-staff through the sights of the compass, and he finds this (say) 12° north of west; and, as the bearing of the shaft is exactly at right angles with this line, if the dip is northerly the bearing of the shaft will be 12° east of north; if southerly, 12° west of south. The best cross-staffs, or instruments for the express purpose of taking right angles, are now made of a hollow, cylindrical shape, of brass, with cuts or apertures for taking the observation; but a substitute may be used, on a pinch, by drawing two lines at right angles on a board about six inches square and an inch thick. Let these lines be cut half an inch deep with a fine saw, and then fix the board on a three-feet stand: if the lines are truly drawn and cut, this rough instrument will serve until a better one can be procured.