ON THE PRINCIPLES AND PRACTICE OF LEVELLING, SHOWING ITS APPLICATION TO PURPOSES OF RAILWAY ENGINEERING AND THE CONSTRUCTION OF ROADS, &c. BY FREDERICK W. SIMMS, F. G. S., M. INST. C. E., CIVIL ENGINEER. FIFTH EDITION, REVISED AND CORRECTED. WITH THE ADDITION OF MR. LAW'S PRACTICAL EXAMPLES FOR SETTING OUT RAILWAY CURVES. With three Lithograph Plates and several Wood-Cuts. NEW YORK: D. VAN NOSTRAND, PUBLISHER, 1870. APPLICATION OF A SECTION TO PRACTICAL PURPOSES TO DRAW LINES OF INTENDED SURFACE ON SECTIONS, TO ARRANGE JUDI- ON SELECTING A LINE OF COUNTRY FOR A ROAD OR RAILWAY. A TREATISE ON LEVELLING. PART I. ON THE PRINCIPLES OF LEVELLING. LEVELLING is the art of tracing a line at the surface of the earth which shall cut the directions of gravity everywhere at right angles. If the earth were an extended plane, all lines representing the direction of gravity at every point on its surface would be parallel to each other; but, in consequence of its figure being that of a sphere or globe,* they everywhere converge to a point within the sphere which is equidistant from all parts of its surface; or, in other words, the direction of gravity invariably tends towards the centre of the earth, and may be considered as represented by a * The figure of the earth is not exactly that of a sphere, but of an oblate spheroid flattened at the poles; the length of the equatorial diameter being 7924 miles, and that of the polar diameter 7898 miles. For our present purpose, it is sufficiently correct to consider it as a sphere. |