Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/318

278 portion of the Mine. In like manner, the ground plan shows the relative position of the horizontal workings, but gives no idea as regards their rela- tive heights. The Holyfield Vein Level, which is ten fathoms higher than the Sun Vein Level cannot be delineated otherwise than it would be if it were exactly on the same plane with the other. If, therefore, it was deemed necessary to record the relative situation and extent of these workings, several separate plans and sections would be required, each of which would accurately represent one portion of the Mine ; but from so many detached plans it would be difficult to form a clear idea of the relative position of the whole. The clearness and sim- plicity of the common ground plan and section arise from the circum- stance, that one scale is applicable to every part and in every direction of them, an advantage exceedingly great and which no other kind of projection possesses. The disadvantages consist chiefly in the number of separate drawings required, and the impossibility of showing with correctness any objects that are not exactly on the plane represented or parallel with it. As a common plan and section is a surface viewed from an infinite dis- tance, so also the isometrical projection varies from common perspective, in supposing the eye placed at an infinite distance in that direction which coincides with the diagonal of a cube produced. An interesting paper on this mode of drawing, by Professor Farish, of Cambridge, is contained in the Transactions of the Cambridge Philosophical Society, in which its nature and application are elucidated by several examples. Professor Farish, who was the first to point out the peculiar properties of Isometrical Drawing, observes, that it may be advantageously employed for representing the surface of mountainous countries and the geological features of a district. As for the latter purpose it possesses several advantages, the object of the present paper is to offer some illustration of its practical application to geological and mining plans, accompanied with a descriptive notice of the mining district selected as the subject of illustration. The manor of Alston Moor, so well known for its abundant produce of mineral treasures, is hollowed by the Rivers Tyne and Nent into two prin- cipal vallies, which unite near the town of Alston. The vale of Nent, extending from this point of junction in a south-east direction for about