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fully levelled with a spirit-level, pickets being left at convenient intervals for the contours to start from. The surveyor to be employed in tracing these contours is furnished with the altitudes of the pickets, or those of bench-marks out of the direct line between the trigonometrical points if they have been so left in preference, from which he has to level up or down to the contour height from whence he is to commence. With a theodolite or spirit-level he then traces the contour lines round the hill features in the manner already described, levelling to certain other bench-marks, whose positions have been given to him, but of whose altitude's he is not informed, in order that a check may be established upon his work; the position of the contour lines being recorded in a field-book, with reference to the measured detail of the houses, fences, &c., in a close country; or by transverse lines in open uncultivated ground.

The whole of the altitudes for the foundation of the contour lines are determined by levelling with the spirit-level; the calculated heights obtained by angles of elevation and depression during the progress of the survey, not being considered sufficiently accurate for the work as it is now performed :—the vertical distances between the contour lines thus traced out on the Ordnance Survey (now published on the scale of 6 inches to 1 mile, or 880 feet to 1 inch), varies—according as the character of the ground is steep or flat-from about 50 to 250 feet. These contours are, however, all interpolated with intermediate horizontal lines, run with the water level at the constant fixed vertical intervals of 25 feet.

By assuming the level of the sea as the datum plane from which these progressive series of contours are to reckon, the altitudes of the several horizontal sections above that point are at once represented, which is a more useful and practical arrangement than the system adopted by the French (who first introduced this method of delineating ground), of fixing upon some imaginary plane of comparison above the highest parts of the plan, similar to the mode still practised with ordinary sections.

On surveys, where pretensions are not made to such extreme mathematical precision, horizontal sections at distant vertical intervals, may be traced with the theodolite or spirit-level, and the

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These Contours should have been represented by dotted lines.

Plate. 9.

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HORIZONTAL CONTOURS,

Traced at equidistant vertical intervals of 25 feet.

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