answer very well for the purposes of the gardener and the farmer. The latter could use it for the construction of water cuts to irrigate his meadows, and for determining the range of his drains, &c. The gardener would find it useful in the formation of terraces, in making ornamental pieces of water, &c., &c.

Ex. 1. Suppose it were required to run a level through the ground indicated by the line A B, in fig. 71., from the point A.

Fig. 71.

A

2

B

Provide a few staves proportioned in length to the work in hand, and let them have cross pieces to slide up and down. Then having firmly fixed the staff in the ground to which the quadrant is attached, at the point a set the instrument in such a position as the plumb-line shall hang exactly parallel to the perpendicular limb of the quadrant: the upper limb will then be horizontal. This done, direct the eye through the sights, and, at the same time, let an assistant adjust the slides on each staff so as exactly to range with the line of vision. Then suppose the height a C to be five feet; measure five feet downwards from the upper side of the slide upon each staff; so shall the dotted line A B represent the level line required.

Ex. 2. Suppose the operation had been to determine a cut for a drain, to have a fall of 3 inches in every 20 feet. The distance between each staff in the above figure may be supposed to be 20 feet: then, 5 feet 3 inches would have to be measured down the first staff, 5 feet 6 inches down the second, 5 feet 9 inches down the third, &c., &c. The dotted

line a b, in fig. 71., would then represent the line parallel to the bottom of the intended drain.

Ex. 3. If a gardener had to form an ornamental piece of water, the process of finding the level for its bed would be exactly the same as the first example. And if he is pretty well master of what has been taught in the preceding pages, he would have no difficulty in determining to a great nicety the quantity of earth to be excavated, and consequently the cost of the job; for, referring to the figure illustrating these examples, he would only have to calculate with exactness the area of the section marked 1, 2, 3, and 4, and multiply this area by the width of the proposed cut for the cubic content to be excavated.

It was shown in page 171. that the height of the apparent above the true level is (at the distance of one mile) eight inches. Hence the distance or extent of the visible horizon is proportional in leagues to the square root of the observer's height in fathoms; that is, if the heights be 1, 4, 9, 16, &c. fathoms, the distances will be 1, 2, 3, 4, &c. leagues, or 3, 6, 9, 12, &c. miles; or, multiply the height in feet by the constant number 1.5, and extract the square root of the product for the distance in miles. Thus, if the height of the observer be 3262 feet, then √(3262 × 1·5)=69.95 miles = the distance at which an object can be seen on the horizon. Hence we learn that if a spring be on one side of a hill, and a house on a hill opposite, with a valley between them, and that the spring, seen from the house, appears by a levelling instrument to be on a level with the foundation of the house, at a mile distant, then is the spring eight inches above the true level of the house; and this difference would be barely sufficient for the water to be brought in pipes from the spring to the house, the pipes being laid all the way in the ground.

182

СНАР. Х.

PLANNING AND MAPPING.

PLANNING.-By a ground-plan is to be understood lines representing nothing more than the bases of objects; such as the space and direction occupied by the foundations of the walls of a house, the fences of a field, &c. The ground-plan of a single tree would be nothing more than a dot, or, if it was a large tree, a small circle. The ground-plan of a box edging is nothing more than a line; and the ground-plan of a gravel or grass walk is merely a space included between two lines. Fig. 72., drawn after the conventional mode, is the ground-plan of an estate, in which merely the fences, roads, watercourses, trees, &c., are indicated.

Mapping.-By a map of an estate is to be understood a drawing or delineation, in which is inserted, not merely the ground-plan of the lines and other objects, but a picture more or less perfect of the superstructure or elevation of these objects.

The first person, as far as we are aware, that made any great improvement on this mode was Mr. Horner, the author of An improved Method of delineating Estates, published in 1813, and illustrated by pictorial drawings, partly sold with the work, and partly sold separately, one of the latter being the parish of Clerkenwell. Mr. Horner was a land-surveyor very considerable practice, but, being a man of genius and of great energy, he occupied himself with a panoramic view of London, and was ultimately brought to ruin, along with a number of other persons, by the erection of the Colosseum for the exhibition of that panoramic view, and for other

in

extravagant objects. He died some years since at New York, U. S.

"That the art of land-surveying should have remained almost stationary since the rudest period," Mr. Horner observes, "is a fact which must excite our surprise, when we consider the advances which other arts analogous to it have made. In comparing some of the earliest delineations with those of the present day, we find that, in the former, a rude attempt has been made to give a character of perspective to what was called the map; trees, water, and houses, are delineated in them with some faint resemblance to nature; while, in the latter, those objects, as well as all others, are represented by mere emblems or signs of convention, quite as arbitrary as those of heraldry. The arts of surveying and of landscapepainting, which seem to have been united in former ages, are now distinct; and, as modern surveyors seldom study the principles of design, they content themselves with a strict adherence to precedent, and consider the embellishments of landscape, which they deem extrinsic, as exclusively belonging to the province of the painter. Hence, an estate surveyed in the days of Elizabeth, the whole surface of which has been materially altered by the renewed growth of timber, and by the general progress of vegetation, as well as by the hand of improvement, will, under the scale and pen of a modern surveyor, present a more formal, naked, dead appearance than it did in the plan drawn by his predecessor 200 years ago. Though it abounded with the most attractive natural beauties, it would be reduced to a skeleton formed by outlines of the fences and buildings, with a few indistinct though elaborate scratchings to signify the trees, and a number of parallel waving hair-strokes of a pen to represent water. So utterly do the instruments of a surveyor, like those of time, level all distinctions, that an estate consisting of one unvaried tract of enclosures fares as well under them as one enriched with every variety of picturesque and romantic scenery; nay, it fares better, for the delineation of it is not deformed by the rows and patches of stubs and dots introduced into that of its rival, to represent the shady woods and groups of trees which adorn