At f, 320 further forward in his first line, he will find himself in front of the upper part of Queen's Square; this line also bearing 282° N.W, is now to be measured.

Proceeding 360 links further, he comes to the lower part of the square, which line, at the right, also bears 282° N. W.

When atf, in front of the upper part of Queen's Square, he was also opposite to King Street, on the other hand, which bore 102° S. E. He may now proceed to the measuring of this street, noting every particular offset in his progress, as before directed. So also must he proceed with every other street comprehended in his survey: to advert to the particulars of all which, in a work of this kind, would be deemed a tedious repetition.

Having, on his survey-book, all the street-work, wherein the front of each house is noted down, he must go within, and set off the back of each house, and its garden-plot, in which he would do well to have a light chain of 50 links only; or a tape line and box, divided into links.

The survey of a large tract of country, as a county, &c., must be conducted on similar principles; the surveyor of which would, however, do well in making frequent observations with his theodolite, from various stations, on particular distant objects; the which he may plot off by intersections, or by some of the rules of inaccessible distances, laid down in a former part of this work.

By distant objects are meant such as are found within the limits of the survey; as towers, castles, beacons, windmills, remarkable houses, &c. These being set off with the plotting instruments, as before

directed, will serve as useful checks to the other parts of the work. In the progress of the road-survey of a county, regard must be had to the passing of every hill, dale, wood, park, brook, river, bridge, ford, seacoast, rock, sand, and other objects deserving of notification; from all which a faithful delineation of the face of the country is to be drawn.

In all these points, nothing can exceed the accuracy and elegance of the general survey of the kingdom, now carrying on by the honourable Board of Ordnance, and placed at first under the direction of General Roy, afterwards of Colonel Mudge, and now of Colonel Colby; an undertaking that will constitute a monument of the improved state of our practical science, and reflect high honour on the abilities, the industry, and the taste of.those who are engaged in it.

Thus, by precepts and examples, have I led the pupil through the various parts of land-measuring; and may his judgment, his industry, and his integrity, go hand-in-hand through his whole practice, and so entitle him to a liberal remuneration of his labours.

PART XIII.

(BY THE EDITOR.)

ON LEVELLING.

INSTRUMENTS USED IN LEVELLING.

THE two principal instruments required in levelling are, a spirit-level, or glass tube, partially filled with alcohol, or some other fluid, and attached to a telescope, having fine hairs or wires stretched across its focus; and a levelling staff divided into feet and hundredths, and furnished with a sliding vane, for marking the height at which the telescope points, and measuring the difference of level between that instrument and the station on which the staff is rested.

The spirit-level differs in nothing essential from the telescope with its attached spirit-tube, which have been already mentioned (at page 191.) in the description of the theodolite. That instrument is, in fact, frequently employed as a substitute for the spirit-level, although it is, in general, very inferior to it in steadiness, power, and accuracy.

Figure 1. represents the most common form of the spirit-level, which is denominated the Y level, from the forked or angular shape of the two supporters of the telescope. A is a glass tube, partly filled with spirit, the bubble or space occupied with air, indicating, by its position in the middle of the tube, the hori

Fig. 2.

zontality of the instrument. a is a screw for making the visual axis of the telescope parallel to the glass tube. B is a compass-box, which is useful for taking the bearings of the course of levelling, although it is not essential to the operation itself. C is a milled-head, by turning which the object-glass is adjusted to its proper distance from the cross-wires, dependent on the distance of the object under observation. Y Y are the two supporters of the telescope, one of which admits of a vertical motion, by turning the milled-head D.

Figure 2. represents the common levelling-staff and vane. The staff is usually made of two pieces of mahogany, one of which slides on the other by means of a dove-tailed groove, so as to admit of being drawn out to a length of about eleven feet.

The face of the staff is divided from the bottom upwards into feet and hundredths as far as six feet, which is the usual length of the staff before being drawn out; the graduations are afterwards continued on the back of the staff downwards.

The vane is made to slide up and down the staff, with just sufficient friction to retain it in any situation in which it may be placed. A white horizontal line, bounded by two black ones, is placed across the middle of the vane, as the mark to be bisected by the horizontal wire of the telescope in taking an observation. In the centre of this line is an aperture, through which the divisions of the staff may be seen, for the purpose of reading off the height of the vane.

There are three principal adjustments required in the spirit-level itself, besides that which must be made in taking an observation.

The first is to make the line of collimation, or optical axis, coincident with the axis of the brass tube of the telescope. This is necessary in order to adapt it for observing objects at very different distances from itself. When the object viewed is near the telescope, the divergency of the rays incident on the object-glass causes the image to be formed farther back towards the eye than when they come from a greater distance, and are consequently more nearly parallel. The object-glass must be moved outwards, by turning the milled-head C when the object is a near one, and in the contrary direction when it is a remote one, in order to bring its image into contact with the cross wires, which ought to be made quite perfect in every observation. It is necessary, therefore, that the intersection of the cross wires, and the optical centre of