neated so as to exhibit its depth, and the several strata sunk through, as at Fig. 6, Plate XIII. The levels of Holyfield Vein and Holyfield Sun Vein, which, on a common ground plan, appear on one plane, may be shown in their proper situations, one above and the other below, the stratum of Limestone in which the mine is principally wrought. Such parts of the surface as have been correctly levelled can be exactly delineated, and whatever accessions of this kind are gained can at any time be added on an Isometrical Plan. Hence, if it is desired to sink a shaft, or drive a level to meet other workings a clear and intelligible record of the relative positions required is at hand, supplying the place of a numerous and unconnected series of Plans and Sections. True it is that when the parts of a mine become very numerous and complicated, the Isometrical Plan will appear so likewise, but this is an imperfection common to all plans, though certainly not so much to those which include only one plane, instead of a number of planes in all directions.
From what is here said, it must not be supposed that the advantages of this perspective are such as to supersede the use of other plans; on the contrary, it must be kept in mind that the common ground plan and vertical section are among the very clearest and most perfect aids which drawing can afford to science: the application of one common scale in every direction of them renders them so easily understood, and so much within the reach of even the most ordinary understanding, that no other kind of projection can excel them as a popular and useful mode of delineating subterranean features. But as an auxiliary to these, and as possessing peculiar advantages, Isometrical Projection claims a share of attention, and may be employed with success in a variety of instances. Its minuter details require an eye and hand more intimately familiar with the principles of its projection than ordinary plans, but its general features are easily understood. Those who are at all accustomed to geological plans will have little difficulty in understanding the example given in the Plan of the mining district at Nentsberry Greens, Plate XII The surface, with its walls, fields, roads and houses appears as if viewed obliquely from a great distance, and where the surface is supposed to be removed, a view of the strata is afforded in numerous vertical sections. By a perpendicular line the thickness of the strata can be measured by the same scale in any part of the
