As, from local attractions or other causes, the polarity of the needle may not be constant, it is scarcely to be expected that the needle should, when the telescope is directed back to a preceding station, be exactly coincident with the north and south line in the compass box ; yet a near approach to such coincidence will serve to detect the existence of considerable errors in the observed angles; and a com plete verification of the whole series of operations will be obtained, should the observed bearing of n from the meridian line n As at the last station A, that is, the angle nn be found to agree with the observed bearing of A from the meridian line n us at the first station n. When this agreement takes place the work is said to close accurately.
The survey of a road or an enclosure, by following the course of the former, or the contour of the latter, may be performed by simply observing with a surveying-compass or a circumferentor the bearings of the several station-lines from the magnetic meridian, and measuring their lengths ; and one of these instruments is generally employed when great accuracy is not required.
The plane table, which is also occasionally employed for surveying ground, is a square board fitted upon a tripod-stand and furnished with compass, and with an alidade, or ruler carrying "sights " at the extremities. Drawing-paper is made fast to the board or table, and the instrument being set up at any part of the ground which may be thought convenient, a point is marked on the paper to represent the place. The landed° Is next turned about that point, so that the line of the sights may be directed to any remarkable objects whose situations are to be determined, and lines are drawn by the edge of the ruler in its several positions ; then the distance from the instrument to some one of those objects being measured, and laid down on its line of direction by a convenient scale, the place of that object on the paper is obtained. The table is then removed to that object, and fixed by the needle in the compass-box, so that its edges may be parallel to their former positions ; that is, till the alidade placed on the line joining the places of the two objects of tho paper is in a direction tending to the former place of the instrument. In this position, the alidade being turned about the point which represents the actual place of the Instrument on the ground, lines are drawn as before along the edge of the miler, towards the several objects which had been observed at the preceding station; the intersections of these lines with the others will determine the places of the objects on the paper.
The length of every line which in to be measured must be obtained in a direction parallel to the horizon between its extremities ; and the determination of this length is generally a work of considerable difficulty on account of the inequalities of the ground.
Where great precision is required, it would be proper that the direction of the line to be measured should be indicated by pickets previously planted at intervals along it ; a cord may be stretched tight between the two first pickets, and the measurement may be performed by means of a graduated deal-rod 15 or 20 feet in length, which should be applied successively to the cord, the place of each extremity of the rod being marked by a pin pressed into the cord. But when the ground is nearly level, a measuring-chain is laid upon the ground itself in the direction of the line to be measured, the leading man pressing into the ground, at the end of each ehain's length, an iron-pin, which being taken up by the person who follows, the number of pins so taken up genes to show the number of chains in the length of the line measured. In ascending or descending any gentle elevation of the ground, tho chain should be held up at the lower end till It is in a horizontal position, as nearly as the chain-holder can estimate it ; and a plummet being suspended from that extremity, so as to touch the ground vertically under it, the measurement thus obtained is in general sufficiently near the required horizontal length of the line. When the slope of the ground is too great to admit of this simple method being put in practice, the chain must be stretched on the ground, and then the angle at which it is inclined to the horizon being found by some instrument (a small spirit-level furnished with a graduated arc), the horizontal value of the chain's length must be computed. And if, at the same time, the vertical height of one end of the chain above the other be also computed, there will be afforded sufficient data for determining on paper the form of a vertical section of the ground in the direction of the measured line.