Fig. 3, Plate XIII., is a ground plan of this district : that portion of it which is unshaded represents the surface, with the houses, roads, fences, &c. Those parts which are shaded are supposed to be excavated or re- moved in order to afford a view of the strata, as, in fact, every section requires such an assumed removal of the intervening objects. The strong dotted lines denote the course of several sections required to illustrate the subterranean formation and mining operations, and this, by the ordinary mode, could only be done by 5 or 6 separate and detached sections, in addition to the ground plan. By Isometrical Projection all these sections, as well as the ground plan, are combined in one drawing, as in Plate XII* every part of which admits of accurate measurement, and which conveys, at a single glance, a clear idea of the connection of the various parts of the district. It is obvious also that many more particulars might be introduced, and that pictorial effect might be employed to give additional force and truth to the representation. By this means, both the horizontal and vertical work- ings may be delineated together. The connection of levels of different ran- doms,* of shafts in various positions, the throw and hade of veins, &c., may all be clearly represented, and form a faithful geometrical delineation of the interior of mines, and of the geology of the district in which they are situated. On the Ground Plan of Holyfield Mine (Fig. 2, Plate XIII.), the workings, &c., are represented as they would appear if viewed from an extreme perpen- dicular height, and are consequently projected on a horizontal plane, or base. On this base, lines may be supposed to be drawn, forming geometrical squares, as at a a, in the same figure, for the purpose of reference. In Isome- trical Projection these squares assume the figure represented in Fig. 4, Plate XIII., and that portion of the Sun Vein Level which is represented on the horizontal squares from b to c is delineated on the Isometrical Squares in Fig. 4 at d e. By means of the Isometrical Squares, any plan may be represented with great ease and accuracy, and thus we obtain the ground plan drawn in Isometrical Projection, But now arises the distinguishing feature of this kind of projection ; the shaft, which on a common ground plan is only shown by its circumference, as at Fig. 5, can be deli-
- A mining phrase for the situation in which levels are driven in regard to height; thus,
a level above another is said to be driven at a higher random.
