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those who are entirely deficient of actual practice, the Author proceeds to explain the construction, adjustment, and use of the principal surveying instruments; and as some knowledge of the application of Trigonometry is almost indispensable to a sound understanding of the full value of these instruments in the field, he has explained this application before proceeding with the more important branches of Surveying. It is only after the elucidation and discussion of all these matters, that he proceeds with similar practical details on the subject of those Road, Railway, Harbour, and other Surveys, which the young practitioner generally hopes to obtain the management of in the course of his future career.
LONDON: March 1868.
LAND AND MARINE
CHAINS AND CHAINING; GUNTER'S CHAIN, THE 100 FEET CHAIN, AND THE
THE FIELD BOOK;
THIS CHAPTER is written for those who are supposed to have had no practice whatever in Surveying, and, indeed, to know nothing whatever on the subject; numerous technical details are given, which, therefore, to be of any service, must be of a purely elementary character: the reader will be able to determine for himself how far it will be necessary for him to read the first few following pages; or whether he may skip them, and go on to other matter.
CHAINS AND CHAINING.
Chains. There are two chains used by the land surveyor, besides the décamètre chain-Gunter's chain, and the 100 feet chain the first, which is quite peculiar to this country, is 66 feet in length, but divided into 100 links, each of which measures, from centre to centre, 7.92 inches, every ten links being denoted by particular brass marks or fingers.' Where the only object of a survey is to prepare a plan of an estate, and take out the quantities of land in acres, roods, and perches, Gunter's chain is the most convenient.
The 100 feet chain is, as its name proclaims, 100 feet long; it is divided into 100 links, each of which of course measures 1 foot from centre to centre; every ten links is marked as in the former chain. This chain has gradually been growing in favour with the English engineering surveyor, and is used almost exclusively for work done abroad,
unless by some legal enactment we are obliged to make use of metric measurements, in which case we employ the décamètre chain, which is 32.8 feet, or 10 mètres long.
The Décamètre chain is also divided into 100 links, each of which measures a décimètre, equal to 0-328 of an English foot. The very short length of this chain is somewhat against it, and, to overcome this objection, two décamètre chains are very commonly fastened together; sometimes a double décamètre chain is used, which is 65.6 feet long; this also may be divided into 100 links of 0.656 length each from centre to centre.
Where it is necessary to make use of the décamètre chain, and there is none ready at hand, the Gunter's chain is sometimes made use of, each link nearly corresponding to the double décamètre, and as its whole length is only about 0.4 of a foot longer than the double décamètre chain, it is not very troublesome to make a corresponding allowance in the field work or in the plotting, at every five or ten chains; it is, at the least, a very useful makeshift, and as such has often been had recourse to. Where a double décamètre chain is thus employed, a corresponding scale must of course be made use of.
The advantages of a long chain over a short one are evident enough, inasmuch as the measure being longer, we have only to apply it fewer times in ascertaining the length of a line, than we must do with a shorter measure, thus avoiding so many chances of error. Thus, if we could measure a mile with a one mile measure, we should only have one chance of error; but if we measure the mile with the 66 feet chain, we have to apply the measure eighty times, giving eighty chances of error instead of one; and if we use the 100 feet chain, we shall only have to apply it 52.80 times, giving nearly 50 per cent. less chances of error than in using the Gunter's chain. Moreover, a considerable amount of time is saved in fixing the arrows in the ground, as in the one case we have only 52.8 instead of eighty to measure a mile.
Again, we may observe that the only advantage of the Gunter's chain is the ready means of ascertaining acreage, for which it was originally intended, but it is quite inappropriate for engineering purposes, inasmuch as to make use of links of 7.92 inches either for measuring or setting out any works, we must reduce feet or yards into the lengths of such links. We have known experienced surveyors use a double Gunter's chain, that is 132 feet long; in wet weather it is heavy work to drag it.
We believe that the retention of the Gunter's chain for