Chains have been known to stretch as much as 3 inches in a day's work, this though trifling in one Chain, would be found of material consequence, after measuring 400 or 500 Chains during the day, amounting as such an error would, to nearly one Chain and a half in the whole distance measured. The rectifying of such errors in the Chain measurements is easily done by a calculation from Table A, (vide Appendix) but a correct Chain saves much trouble.

In the event of a Chain having lengthened, the correction, whatever it may be, to the Chain lines, must be made by addition, and vice versâ, if the Chain is short, by subtraction of the quantity required to rectify the error. Thus,

Supposing a Chain to have stretched 1 inch, a correction of one link additive in every 8 Chains measured, would be necessary to give the true length of any line measured with that Chain. Taking the measured line to be 64 Chains the correct length of the line would be 64 Chains + 64 Inches or 64 Chains, 8 Links, and the reverse 64 Chains - 64 Inches or 63 Chains, 92 Links, should the Chain be 1 Inch too short; such a correction is sufficient for all practical purposes.

It is a common practice, to allow Chainmen too much latitude in measuring lines, i. e., the Surveyor is satisfied to come up at the end of the line measured, count the number of links up to the station, depending entirely on the rear Chainman, for a correct account of the number of Chains measured. This, even were the account of Chains correct, (which is always doubtful) can never be a satisfactorily measured line. Unless the Surveyor follows in the rear of his Chainmen, and keeps a continued watch on them, the probabilities are, that his work will have to be measured over again.

A Surveyor should accustom himself to follow his rear Chainman and satisfy himself, as he is progressing, that he is measuring straight. To ensure the Chainmen proceeding in as straight a line as possible, it is always well, for the leading Chainman to check the direction of the rear Chainman, by keep

ing the latter, and the back station (on which there is invariably a flag) in a straight line with himself. The rear Chainman does this, as he directs the leading one, with the forward station, and thus by a mutual check, great accuracy is obtained.

Eleven arrows should be used, instead of ten as is generally the custom, for in the latter case, when the Chain arrives at the end of the tenth arrow, thus denoting 10 Chains as measured, the Chain is stopped and liable to be shifted; whereas with eleven arrows, one arrow always remains a fixture in the ground and is never brought into the account, thus preventing the possibility of the Chain being shifted whilst the other ten arrows are being taken to the leading Chainman.

It is usual to have steel rods of 6 or 11 feet, to test the Chains with, the former are the most portable, and the Superintendent of a Survey cannot be too careful in ascertaining that these rods are supplied of the exact lengths, and also, in insisting on their use.

Two rods of 6 feet each, are sufficient for a Survey party, and the method of using them is, to stretch the Chain pretty tight on a level piece of ground, fixing two stout pins made for the purpose, at each end in the handles of the Chain, then, by laying down the two rods from one end, keeping the second stationary, and taking up the first, and placing it beyond the second, then keeping that stationary and taking up the second, and placing it beyond the third and so on, until arrived at the end of the Chain, when eleven rods thus measured should be its length.

Experience, however, and the disagreeable necessity of doing work twice over, which is always irksome, can alone teach a Surveyor the necessity of measuring lines correctly, and of keeping a constant watchfulness over the length of his Chain.

Directions for using the Chain. Flags are first to be set up at the places whose distances are to be obtained; the place where the measurement is commenced may be called the first station, and that measured to, the second station. Two men hold the Chain, one at each end; the foremost or leader is

provided with eleven arrows, and a small hammer. These arrows are made of iron about one-third of an inch square pointed at one end, with a loose ring on the head, for the purpose of hanging in a hook suspended from the girdle of the Chainman. On the Chain being stretched in the direction of the second station the leader hammers an arrow firmly into the ground, the rear Chainman holding the other end at the first station; he then proceeds on in the direction of the second station, until the rear Chainman has arrived at the first arrow, when the latter directs the former in a line with the first station, and a second arrow is firmly driven in; the rear Chainman then takes up the first arrow, counts one Chain as measured, and proceeds on until the eleven arrows are expended, one of which remaining in the ground, the other ten are sent on to the leading Chainman. The exchange of the arrows is always notified by the rear Chainman calling out with a loud voice, so many Tens. The Surveyor here marks in his Field Book, that one change has been made, or 10 Chains or 1000 Links measured. The Chainmen then proceed onwards, until another change has been made, and entered, and so on, marking every change until the second station is arrived at, when the number of arrows in the hand of the rear Chainman will denote the number of Chains, which, together with the odd links and the number of changes that may have been made between the two stations, will make up the entire length of the line.

OF THE "CROSS STAFF" AND "OFFSET ROD."

When the boundary of a Survey has turns and bends in it, as is generally the case, it is not necessary to measure round every such turn and bend. The best and most usual way, is to proceed in a straight line from one principal corner to another, and when opposite to any bend in the boundary to measure the rectangular distance, termed the Offset from the chain line to the bend, noting the same,

together with the distance on the Chain line from whence such Offset was made. These Offsets are generally measured with an Offset Staff or Rod of ten or twenty links in length, ten or five of them making one Chain, and as they are all rectangular with the Chain line, they either form triangles or trapezea, of which the distance on the Chain line is the base and the Offset the perpendicular, to be calculated by the rules given under the head of "Areas," and added or deducted according as they are to the right or left of the Chain line. Great care is required on the part of the Surveyor in measuring Offsets, for unless the Offset is taken at right angles with the Chain line, the perpendicular measured for determining its area will be too long, and a correct result will not be obtained. A very convenient instrument called theCross-Staff," and which can be made up by any Bazar Carpenter, is used for the purpose of taking Offsets. It consists of a piece of wood, about 6 inches square and an inch and a half in thickness, fixed on the end of a staff about 5 feet in length, with an iron spike at the end, for the convenience of planting it in the ground. The square piece on the top has two slits ab and cd in it, about half an inch deep, at right angles with each other,

made with a common saw. This instrument being placed any where on the Chain line, if one slit is directed to the forward or back station, the other will of course give the perpendicular to the Chain line. A well practised Surveyor can, however, generally tell a right angle for an Offset, without the assistance of this instrument.

The best method of measuring Offsets, is, for the Offset man to walk along the boundary, and to give a signal to the Chain party, whenever he comes to a bend or corner, the Surveyor then places himself on the Chain line in a rectangular

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position with the Offset man, when the latter, measuring down towards him, gives in the length of the Offset in rods, and returns immediately to the boundary to take up the next bend. A good Offset man should never be taken off his work, for by constant practice he knows exactly when and where an Offset is required.

ON THE PERAMBULATOR.

THIS instrument is very useful for measuring roads, level plains, and every thing where expedition is required. It gives however a measure somewhat too long in going over uneven surfaces, which is one of its principal objections, and is therefore only applicable to road and route Surveys, where great accuracy is not essential.

The following figure represents the Perambulator, which consists of a wheel of wood A, shod or lined with iron to prevent the wear; a short axis is fixed to this wheel, which communicates motion by a long pinion fixed in one of the sides of the carriage B, to the wheel-work C, included in the box-part of the instrument. For portability the wheel A is separable.