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The various instruments used in Surveying may be conveniently arranged, into two general divisions.
(1.) NEEDLE instruments,—or such as owe their accuracy and value to the magnetic needle only, embracing the Plain and Vernier compasses, and the Vernier Transit.
(2.) ANGULAR instruments, including those in which the horizontal angles, are measured by a divided circle and verniers, as well as by the needle also; as the Railroad Compass, the Surveyors’ and Engineers’ Transits, &c.
In the present work we shall consider first, those instruments comprised in the first division, and, as in these the accuracy of the horizontal angles indicated, depends upon the delicacy of the needle, and the constancy with which it assumes a certain direction, termed the “magnetic meridian”. We shall here remark briefly upon the form, the length, and the movement of:
THE MAGNETIC NEEDLE.—The forms of the needle are almost infinitely varied, according to the taste or fancy of the maker or surveyor, but may be resolved into two general classes, one having the greatest breadth in a horizontal, the other in a vertical direction.
We have usually made our needles about one-twentieth of an inch broad. and one-third as thick, parallel from end to end, the north and south poles being distinguished from each other, by a small scollop on the north end.
Of course the form of the needle is always varied according to the choice of our customers, and without additional charge. The length of the needle varies in different instruments, from four to six or even seven inches, those of five and a half, or six inches long, being generally preferred by surveyors.
The movement of the needle, with the least possible friction, is secured by suspending it, by a steel or jewel centre, upon a hardened steel pivot, the point of which is made perfectly sharp and smooth.
The test of the delicacy of a magnetic needle is the number of horizontal vibrations, which it will make in a certain arc, before coming to rest —- besides this most surveyors prefer also to see a sort of quivering motion in a vertical direction.
This quality, which is manifested more in a horizontal, than in a vertical needle, and depends upon the near coincidence of the point of suspension with the centre of gravity of the needle, serves to show merely that the cap below is unobstructed.
Having now considered the different qualities of a good needle, we shall proceed to speak of those instruments of which it makes so important a part; of these, the most simple is that termed the PLAIN COMPASS.
The plain compass has a needle six inches long, a graduated circle, main plate, levels and sights, and is placed upon the brass head of the “Jacob staff.”
THE COMPASS CIRCLE in this, as in all our instruments, is divided to half degrees on its upper surface, the whole degree marks being also cut down on the inside circumference, and is figured from 0 to 90, on each side of the centre or “line of zeros.” The circle and face of the compass are silvered.
THE SPIRIT LEVELS are placed at right angles to each other so as to level the plate in all directions, and are balanced upon a pivot underneath the middle of the tube, so as to be adjustable by a common screw-driver.
THE SIGHTS, or standards, have fine slits cut through nearly their whole length, terminated at intervals by large circular apertures, through which the object sighted upon is more readily found. Sometimes a fine horse-hair or wire is substituted for one half the slit, and placed alternately with it on opposite sights.
TANGENT SCALE — The right and left hand edges of the sights of our compasses, have respectively an eye-piece, and a series of divisions, by which angles of elevation and depression, for a range of about twenty degrees each way, can be taken with considerable accuracy.
Such an arrangement is very properly termed a tangent scale, the divided edges of the north sight, being tangents to segments of circles having their centres at the eye-pieces, nnd their points of contact with the tangent lines at the zero divisions of the scale.
THE JACOB STAFF mountings which are furnished with all our compasses, and packed in the same case, consist of the brass head already mentioned, and an iron ferule or shoe pointed with steel, so as to be set firmly in the ground. The staff; to which the mountings should be securely fastened, is procured from any wheelwright, or selected by the surveyor himself from a sapling of the forest.
THE LEVELS — First bring the bubbles into the centre, by the pressure of the hand on different parts of the plate, and then turn the compass half way around; should the bubbles run to the end of the tubes, it would indicate that those ends were the highest lower them by tightening the screws immediately under, and loosening those under the lowest ends until, by estimation, the error is half removed; level the plate again, and repeat the first operation until the bubbles will remain in the center, during an entire revolution of the compass.
THE SIGHTS may next be tested by observing through the slits a fine hair or thread, made exactly vertical by a plumb. Should the hair appear on one side of the slit, the sight must be adjusted by filing off its under surface on that side which seems the highest.
THE NEEDLE is adjusted in the following manner: Having the eye nearly in the same plane with the graduated rim of the compass circle, with a small splinter of wood or a slender iron wire, bring one end of the needle in line with any prominent division of the circle, as the zero, or ninety degree mark, and notice if the other end corresponds with the degree on the opposite side; if it does, the needle is said to “cut” opposite degrees; if not, bend the centre-pin by applying a small brass wrench, furnished with our compasses, about one eighth of an inch below the point of the pin, until the ends of the needle are brought into line with the opposite degrees.
Then holding the needle in the same position, turn the compass half way around, and note whether it now cuts opposite degrees; if not, correct half the error by bending the needle, and the remainder by bending the centre-pin. The operation should be repeated until perfect reversion is secured in the first position. This being obtained, it may be tried on another quarter of the circle; if any error is there manifested, the correction must be made in the centre-pin only, the needle being already straightened by the previous operation.
When again made to cut, it should be tried on the other quarters of the circle, and corrections made in the same manner until the error is entirely removed, and the needle will reverse in every point of the divided surface.
The compass circle being graduated to half degrees, a little practice will enable the surveyor to read the bearings to quarters, or even finer—estimating with his eye the space bisected by the point of the needle, and as this is as low as the traverse table is usually calculated, it is the general practice.
Sometimes, however, a small vernier is placed upon the south end of the needle, and reads the circle to five minutes of a degree — the circle being in that case graduated to whole degrees. This contrivance, however, is quite objectionable on account of the additional weight imposed on the centre-pin and the difficulty of reading a vernier which is in constant vibration, and is, therefore but little used.
To take ANGLES of ELEVATION — Having first leveled the compass, bring the south end towards you, and place the eye at the little button, or eye piece, on the right side of the south sight, and with the hand fix a card on the front surface of the north sight, so that its top edge will be at right angles to the divided edge, and coincide with the zero mark; then sighting over the top of the card, note upon a flagstaff the height cut by the line of sight; then move the staff up the elevation, and carry the card along the sight until the line of sight again cuts the same height on the staff; read off the degrees, and half degrees passed over by the card, and we shall have the angle required.
FOR ANGLES OF DEPRESSION — Proceed in the same manner, using the eye-piece and divisions on the opposite sides of the sights, and reading from the top of the standards.
JACOB STAFF SOCKET — The compass is furnished with a ball spindle, or socket, upon which it turns, and by which it is levelled. The ball may be placed in a single or jacob staff socket, or in a compass tripod.
CLAMP SCREW — In the side of the hollow cylinder, or socket of the compass, which fits to the ball spindle, is a screw by which the instrument may be clamped to the spindle in any position.
SPRING CATCH — Besides the clamp screw, we now have fitted to the sockets of our compasses a little spring catch, which, as soon as the instrument is set upon the spindle, slips into a groove, and thus removes all danger of falling when the instrument is carried.
NEEDLE LIFTER—There is also underneath the main plate, a needle lifting screw which, by moving a concealed spring, raises the needle from the pivot; and thus prevents the blunting of the point in transportation. When the compass is not in use it is the practice of many surveyors to let down the needle upon the point of the centre-pin, and let it assume its position in the magnetic meridian, so as to retain or even increase its polarity. We would advise in addition, that after the needle has settled it should be raised against the glass, in order not to dull the point of suspension.
OUTKEEPER.—A small dial plate, having an index turned by a milled head underneath, is often used with this and the other compasses to keep tally in chaining. The dial is figured from 0 to 16, the index being moved one notch for every chain run.
ELECTRICITY — A little caution is necessary in handling the compass, that the glass covering be not excited by the friction of cloth, silk, or the hand, so as to attract the needle to its under surface. A brass cover is sometimes fitted over the glass of the compass, and serves to protect it from accident, as well as to prevent electric disturbance. When, however, the glass becomes electric, the fluid may be removed by breathing upon it, or touching different parts of its surface with the moistened finger. An ignorance of this apparently trifling matter has caused many errors and perplexities in the practice of the inexperienced surveyor.
To enable the surveyor to make such repairs as are possible without having recourse to an instrument maker, we here add a few simple directions.
1. THE NEEDLE.—It may sometimes happen that the needle has lost its polarity, and needs to be re-magnetized; this is effected in the following manner:
The operator being provided with an ordinary permanent magnet, and holding it before him, should pass with a gentle pressure each end of the needle from centre to extremity over the magnetic pole, describing before each pass a circle of about six inches radius, to which the surface of the pole is tangent, drawing the needle towards him, and taking care that the north and the south ends are applied to the opposite poles of the magnet.
Should the needle be returned in a path near the magnetic pole, the current induced by the contact of the needle and magnet, in the pass just described, would be reversed, and thus the magnetic virtue almost entirely neutralized at each operation. When the needle has been passed about twenty-five times in succession, in the manner just described, it may be considered as fully charged.
A fine brass wire is wound in two or three coils on the south end of the needle, and may be moved back or forth in order to counterpoise the varying weight of the north end.
2. THE CENTRE PIN.—This should occasionally be examined, and if much dulled, taken out with the brass wrench, already spoken of, or with a pair of plyers, and sharpened on a hard oil stone—the operator placing it in the end of a small stem of wood, or a pin vice, and delicately twirling it with the fingers as he moves it back and forth at an angle of about 30 deg. to the surface of the stone.
When the point is thus made so fine and sharp as to be invisible to the eye, it should be smoothed by rubbing it on the surface of a soft and clean piece of leather.
3. TO PUT IN A NEW GLASS — Unscrew the bezzle ring which holds it, and with the point of a knife blade spring out the little brass ring above the glass, remove the old glass and scrape out the putty; then if the new glass does not fit, smooth off its edges by holding it obliquely on the surface of a grind stone until it will enter the ring easily; then put in new putty, spring in the brass ring, and the operation will be complete.
4. To REPLACE A SPIRIT LEVEL — Take out the screws which hold it on the plate, pull off the brass ends of the tube, and with a knife blade scrape out the plaster from the tube; then with a stick made a little smaller than the diameter of the tube, and with its end hollowed out, so that it will bear only on the broad surface of the level vial, push out the old vial and replace it with a new one, taking care that the crowning side, which is usually marked with a file on the end of the vial, is placed on the upper side.
When the vial does not fit the tube it must be wedged up by putting under little slips of paper until it moves in snugly. After the vial is in its place, put around its ends a little boiled plaster, mixed with water to the consistency of putty, taking care not to allow any to cover the little tip of the glass, then slip in the brass ends and the operation will be completed.
A little beeswax, melted and dropped upon the ends of the vial, is equally as good as the boiled plaster, and often more easily obtained. We would here remark that an extra glass and level vials are always furnished, free of charge, with our instruments, whenever desired by the purchaser.
Three different sizes of this instrument are in common use, having respectively four, five and six-inch needles, and differing also in the length of the main plate, which in the four inch compass is twelve and a half inches long, and in the larger sizes, fifteen and a half inches. The six-inch needle compass is generally preferred.
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© 1997, 1998, Scientific Medical & Mechanical Antiques, Taneytown, MD.