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SECT. III.

Containing a particular Description of the several Instruments used in Surveying, with their respective Uses. And first,

OF THE CHAIN.

TH

HE stationary distance, or mearings of ground, are measured either by Gunter's chain of four poles or perches, which consist of 100 links; (and this is the most natural division) or by one of 50 links, which contains two poles or perches; but because the length of a perch differs in many places, therefore the lengths of chains and their respective links will differ also.

The English statute perch is 5 yards, the twopole chain is 11 yards, and the four-pole one is 22 yards: hence the length of a link in a statute-chain is 7.92 inches.

There are other perches used in different parts of England, as the perch of woodland measure; which is 6 yards; that of church land-measure, which is 7 yards (or the same with the plantation-perch) and the forest measure perch, which is 8 yards.

The Irish or plantation perch, is 7 yards, as before; the two-pole chain is 14; and the four-pole one is 28

yards: hence the length of a link in a plantation chain is 10.08 inches.

The Scotch perch is 18 feet, or 6 yards, or 6 Scot's ells. In the shire of Cunningham in Scotland, their perch is 183 feet, and this perch is used in some few places in the north part of Ireland, as the statute perch is in some other parts.

For the more ready reckoning the links of a fourpole chain, there is a large ring, or sometimes a round piece of brass fixed at every 10 links; and at 50 links, or in the middle, there are two large rings. In such chains as have a brass piece at every 10 links there is the figure 1 on the first piece, 2 on the second, 3 on the third, &c. to 9. By leading therefore that end of the chain forward, which has the least number next it, he who carries the hinder end may easily determine any number of links: thus, if he has the brass piece number 8, next to him, and 6 links more in a distance, that distance is 86 links.

After the same manlarge ring of a chain

ner 10 may be counted for every which has not brass pieces on it; and the number of links is thus readily determined.

The two-pole chain has a large ring at every 10 links, and in its middle, or at 25 links, there are two large rings; so that any number of links may be the more readily counted off, as before.

The surveyor should be careful to have his chain measured before he proceeds on business, for the rings

is thereby increased, so that no one can be too circumspect in this point.

In measuring a stationary distance, there is an object fixed in the extreme point of the line to be measured; this is a direction for the hinder chainman to govern the foremost one by, in order that the distance may be measured in a right line; for if the hinder chainman causes the other to cover the object, it is plain the foremost is then in a right line towards it. For this reason it is necessary to have a person that can be relied on, at the hinder end of the chain, in order to keep the foremost man in a right line; and a surveyor who has no such person, should chain himself. The inaccuracies of most surveyors arise from bad chaining, that is, from straying out of the right line, as well as from other omissions of the hinder chainman: no person, therefore, should be admitted at the hinder end of the chain of whose abilities in this respect, the surveyor was not previously satisfied and convinced; since the success of the survey, in a great measure, depends on his care and skill.

In setting out to measure any stationary distance, the foreman of the chain carries with him 10 iron pegs pointed, each about 10 inches long; and when he has stretched the chain to its full length, he at the extremity thereof sticks one of those pegs perpendicularly in the ground; and leaving it there, he draws on the chain till the hinder man checks him when he arrives at that peg: the chain being again stretched, the foreman sticks down another peg, and the hindman takes up the former; and thus they proceed at every chain's length contained in the line to be measured, counting

the surplus links contained between the last peg, and the object at the termination of the line, as before: so that the number of pegs taken up by the hinder chainman, expresses the number of chains; to which, if the odd links be annexed, the distance line required in chains and links is obtained, which must be registered in the field book, as will hereafter be shewn.

If the distance exceeds 10, 20, 30, &c. chains, when the leader's pegs are all exhausted, the hinder chainman, at the extremity of the 10 chains, delivers him all the pegs; from whence they proceed to measure as before, till the leader's pegs are again exhausted, and the hinder chainman at the extremity of these 10 chains again delivers him the pegs; from whence they proceed to measure the whole distance line in the like manner; then it is plain, that the number of pegs the hinder chainman has, being added to 10, if he had delivered all the pegs once to the leader, or to 20 if twice, or to 30 if thrice, &c. will give the number of chains in that distance; to which if the surplus links be added, the length of the stationary distance is known in chains and links.

It is customary, and indeed necessary, to have red, or other coloured cloth fixed to the top of each peg, that the hinder man at the chain may the more readily find them; otherwise, in chaining thro' corn, high grass, briars, rushes, pototoes, &c. it would be extremely difficult to find the pegs which the leader puts down by this means no time is lost, which otherwise

It will be necessary here to observe, that all slant or inclined surfaces, as sides of hills, are measured horizontally, and not on the plane or surface of the hill, and is thus effected:

Let ABC be a hill, the hindmost chainman is to hold the end of the chain perpendicularly over the point A (which he can the better effect with a plummet and line, than by letting a stone drop, which is most usual) as d is over A, while the leader puts down his peg ate the eye can direct the horizontal position near enough, but if greater accuracy were required, a quadrant applied to the chain, would settle that. In the same manner the rest may be chained up and down; but in going down it is plain the leader of the chain must hold up the end thereof, and the plummet thence suspended, will mark the point where he is to stick his peg. The figure is sufficient to render the whole evident; and to shew that the sum of the chains will be the horizontal measure of the base of the hill; for de=Ao, fg=op, hi=pq, &c. therefore de+fg+hi, &c. =Ao+op+pq, &c. =AC, the base of the hill. If a whole chain cannot be carried horizontally, half a one, or less, may, and the sum of these half chains, or links, will give the base, as before.