CHAIN, in surveying, is a measure, consisting of a certain number of links of iron wire, usually a hundred; serving to take the dimensions of fields, &c.

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Chains. This is what Mersenne takes to be the arvipendium of the ancients.

The chain is of various dimensions, as the length or number of links varies: that commonly used in measuring land, called Gunter's chain, is in length four poles or perches; or sixty-six feet, or a hundred links; each link being seven inches \frac{1}{2}. Whence it is easy to reduce any number of those links to feet, or any number of feet to links.

This chain is entirely adapted to English measures; and its chief convenience is in finding readily the numbers contained in a given field. Where the proportions of square feet and acres differ, the chain, to have the same advantages as Gunter's chain, must also be varied. Thus in Scotland, the chain ought to be of 74 feet, or 24 Scotch ells, if no regard be had to the difference between the Scotch and English foot; but if regard be had to this difference, the Scotch chain ought to consist of 74\frac{1}{2} English feet, or 74 feet 4 inches and \frac{1}{2} of an inch. This chain being divided into an hundred links, each of these will be 8\frac{1}{2} inches.

That ordinarily used for large distances, is in length a hundred feet; each link one foot. For small parcels, as gardens, &c. is sometimes used a small chain of one pole, or sixteen feet and a half length; each link one inch \frac{1}{2}.

Some in lieu of chains use ropes; but these are liable to several irregularities; both from the different degrees of moisture, and of the force which stretches them. Schwenterus, in his Practical Geometry, tells us, he has observed a rope sixteen feet long, reduced to fifteen in an hour's time, by the mere falling of a hoar frost. To obviate these inconveniences, Wolfius directs, that the little strands whereof the rope consists be twisted contrary ways, and the rope dipped in boiling hot oil; and when dry, drawn through melted wax. A rope thus prepared, will not get or lose any thing in length, even though kept under water all day.