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It may perhaps be thought that too much stress has been laid. upon forms, in the above description of the details of an extensive survey; but method is a most essential part of an undertaking of such magnitude; and without excellent preliminary arrangements to insure uniformity in all the most trifling details, the work never could go on creditably. In topographical surveys on a smaller scale, where the boundaries of parishes, &c., are not to be shown, or the contents of various portions to be calculated, the same rigid attention to minutiæ is not requisite; but before closing this branch of the subject, it is only necessary, as a proof of the mass of valuable statistical and geological information that can be collected during the progress of a national trigonometrical survey, and which is quite out of the reach of any individual, to turn to the first volume of “The Ordnance Survey of the County of Londonderry." If this valuable accompaniment to the field operations could have been continued throughout every county, Ireland would be possessed of more available local knowledge than is on record in any part of the world.
The following brief hints may be found useful in filling-in the detail of a survey with the chain and theodolite.
The field-book should be kept in ink in the field, and have a distinctive letter marked on it as a reference; every day's work should be dated, and the names of those employed entered. On an extensive survey it is also necessary that every book should be kept on precisely the same system, that one person might find no difficulty in plotting from the book of another.
The theodolites should be constantly examined and adjusted, and the chains compared every day with a standard chain, or marks laid down from one for that purpose, and their errors, if any, either corrected or entered in the field-book, to be allowed for in plotting. The offsets should be numerous, and minute in proportion to the scale upon which the survey is to be plotted*, and the names of all
* From one to two chains should be the maximum length of offsets where the contents of inclosures are to be computed, or even laid down on a large scale. These limits must of course be extended in filling in the interior in less accurate surveys, or which are to be plotted on a very small scale. As drawing-paper is very much stretched when mounted on a board, and partially contracts when cut off, and as it is always liable to change from the