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Every tenth link, from either end, is marked by a small attached brass plate, which is notched, to designate its number from the end. The division of the chain, into 100 equal parts, is very convenient, since the divisions, or links, are decimals of the whole chain, and in the calculations are treated as such.
1 chain 4 rods 66 feet 792 inches 100 links.
1 link is equal to 7.92 inches.
80 chains 320 rods = 1 mile.
19. Besides the chain, there are needed for measuring, ten marking-pins, which should be of iron, each about ten inches in length and an eighth of an inch in thickness. These pins should be strung upon an iron ring, and this ring should be attached to a belt, to be passed over the right shoulder, suspending the pins at the left side.
To measure a horizontal line.
20. The point where the measurement is to begin is usually located by a staff or stake temporarily placed for the purpose; or by some one of the many permanent marks by which the angles in a boundary are fixed.
The other extremity of the line must be provided with a staff or flag which can be easily seen.
The fore-chainman, with the pins, and one handle of the chain in his right, starts off on the line, drawing out the chain to its full length. Both chainmen now examine it carefully, to see if there are any "kinks" at the junction of the links. Having adjusted the chain for use, the fore-chainman resumes his place, to be directed by the hind-chainman, so
lished line. To facilitate this, and to insure the correct align
ment of the pin, at its proper distance, the chain and one pin should be held firmly in the right hand, as represented in the figure. While the pin is being aligned, it should be held by the fore-chainman as far from the body as possible, so that the view of the flag be left unobstructed. To accomplish this, and at the same time. draw the chain to the proper degree of tension, the right arm should be braced against the inside of the right knee.
The hind-chainman directs, by the simple orders "right" or "left," according as the pin, held as described, is to be carried to the right or left to bring it into line with the flag. When the pin is truly in line, the chain at the same time being drawn straight, the order "down" is given, when the fore-chainman bringing his left hand to bear on the top of the pin, forces it vertically into the ground, and resumes his course to the length of another chain.
After one or two chains have been measured, on any line, the fore-chainman can, by glancing back to the station just
left, place the pin nearly in the right position: the exact aligning should be left, however, to the hind-chainman.
When the distance is more than ten chains, the pins, when exhausted, should be returned to the fore-chainman-the distance noted-and the chaining recommenced at the place of the tenth pin.
All distances should be measured horizontally: Hence, when the ground slopes, one end of the chain must be elevated. Each chainman should be provided with a small plumbline, so that the elevated end of the chain may be held directly over the proper point.
When the raised end of the chain is only two, or even three feet above the ground, it will suffice, in many cases, to use a marking-pin, held lightly by the point, between the thumb and finger, instead of a plumb-line. When the chaining is on a steep inclination, other precautions should be observed.
Suppose the chaining to be up hill. The fore-chainman draws the chain out to its full length, as in any other case, and then returns to within such a distance of the hindchainman, that when the chain is drawn out to that length. horizontally, it shall not be too high to be held conveniently. The hind-chainman holds his end of the chain carefully over the point or station, by means of the plumb-line, while
The point fixed in this manner, by the fore-chainman, must not be marked by a marking-pin, but by a small peg or nail. At the order, "Down," the fore-chainman does not go forward in the usual manner, but waits until the hind-chainman comes up and takes the chain by the precise point held, the moment. before, to the ground.
This point is now held above the peg by the hind-chainman, who uses the plumb, as before, and aligns the fore-chainman, who has taken hold of the chain a few links farther on, and is holding it to the ground. These short distances are not recorded.
In chaining down-hill, the method is essentially the same. The fore-chainman uses the plumb, and determines by it where the peg is to be placed.
At the end of a course, the part of a chain is measured by drawing the chain only to the flag, where it is held by the forechainman, until the hind-chainman comes forward to the last pin, and counts the links.
In measuring up the hill from A to C, or down the hill from C to A, we measure the horizontal distances a b, c d, and f C, and their sum is the horizontal distance between A and C.
Two staves are often used with the chain, in the measurement of lines. Each of these should be about six feet in length, and have a spike in the lower end to aid in holding it firmly, and a horizontal strip of iron to prevent the chain from slipping off: each staff is to be passed through the ring, at the end of the chain.
21. As the length of the chain may vary, from heat or cold, or become changed from other causes, it should be compared from time to time, with a standard, kept for the purpose.
To facilitate this comparison, let two stakes be driven in the ground, distant from each other one chain, and let nails be driven in the heads of the stakes to mark the exact length of the standard.
Marks made upon the coping of a wall will answer the same purpose. If it is found that any line has been measured with a chain, either too short or too long, the measured distance may be corrected by the following proportion:
As the length of the standard : the length of the chain
the measured distance
the true distance.
For the correction of areas we have this proportion :
As the square of the length of the standard : the square of the length of the chain, the area found
: the true area.
MEASUREMENT OF ANGLES.
22. We come next to the measurement of angles, and for this purpose, several instruments are used. The one, however, which affords the most accurate results, and which indeed can alone be relied on for nice or extended operations, is called a Theodolite. This instrument, only, will be described at present; others will be subsequently explained.