Levelling
A scientific knowledge of Levelling, sufficiently exact for land surveying, is rarely expected of a gardener; but, in the execution of ordinary ground work, an acquaintance with the principles, and the method of applying them according to requirements, will be of the utmost importance. Levelling is a term similarly applied to the equalising of soil, either on a horizontal or an inclined plane surface. When the preparation of land, by digging, for the reception of seeds or crops, is the only object in view, the surface will generally be rendered level enough with the spade or rake, if their use is guided under the eye of a practical workman. In the formation of an edging of any description, the making or gravelling of walks, turf-laying on lawns, draining operations, &c., some method, in accordance with scientific teaching, must be adopted, if satisfactory results would be attained. The difference in permanent work as above named, executed on a definite plan, regarding the Levelling or the natural inclination of the soil, and that performed in a haphazard way, is widely marked, even on its completion, and more so when tested, afterwards, by heavy rains. Where the general surface of garden land is flat, or nearly so, Levelling is of greater importance, in order to dispose of any superfluous rain or other water which may collect. A slight incline will cause water to flow, but an obstruction to the same extent will similarly check its course; and, as inequalities of this sort cannot with certainty be avoided where the uses of levels are ignored, their value in securing a uniform surface on any plane will be readily seen. Reference may first be made to the theodolite, a rather expensive instrument, principally used in land surveying, for measuring the horizontal or the vertical angle between two distant objects. It consists of a small telescope, which may be raised or lowered according as the disposition of the ground may require, the angle in either direction being ascertained by two graduated circles, which are attached. The instrument is provided with a spirit-level and adjusting screws, for fixing its proper position. On ascertaining the apparent level at the opposite end, through the telescope, the amount of rise or fall in the distance will be indicated, and a calculation will show how much it is in a given distance. Far less expensive, and, moreover, invaluable instruments for Levelling purposes generally, are a good spirit-level, an ordinary straight-edge, and three borning-rods. The spirit-level, in its simple form, consists of a glass cylinder tube, filled with spirit, except a very small space, which is occupied by a bubble of air. The tube is sometimes fixed in the centre of a straight-edge; but this plan is not to be recommended, on account of the liability of the latter to become crooked. A more certain method is that of fixing it exactly in the middle of a piece of hard wood, having a plane surface on all sides. The air-bubble, being lighter than spirit, will rise in the tube, whenever either end is placed above the horizontal line. A straight-edge should be made of a strip of wood, not liable to warp easily, and its edges should be planed straight and even as frequently as they get the least otherwise. Borning-rods are about 4ft. long, and have a strip of wood placed exactly at right angles across their tops, and painted respectively red, white, and either black or blue, in order to distinguish each readily from the other. When made of an equal length, and placed in line, all the tops must run evenly when viewed from either end. Sometimes one rod is made longer than the others, and a very small hole is bored through the strip at the exact height of the others, so that the eye may not be misguided. By sufficient practice, Levelling may be very exactly carried out with the borning-rods, after the spirit-level and straight-edge have aided in fixing the proper position for the pegs at the two ends. An insertion of ordinary wooden pegs, at distances of about 8ft. apart, will be a sufficient guide for forming a dead level surface, or for equalising a fall throughout a given length. There are other forms of levels, but those referred to will be found sufficient for all gardening purposes.