For further information respecting the law for the inclosure of commons, waste lands, &c., the student may consult the following

ACTS OF PARLIAMENTS.

GENERAL ACT.

"An Act for consolidating in one Act, certain provisions usually inserted in Acts of Inclosure; and for facilitating the mode of proving the several facts, usually required on the passing of such Acts." (Act. 41 Geo. III., chap. 109. 1801.)

Since the General Act, the following Acts have been passed, relating to inclosures.

1. An Act to amend the Law respecting the inclosing of open fields, pastures, moors, commons, and waste lands in England." (Act 1 & 2 Geo. IV. chap. 23; April 19th, 1821.)

2. "An Act for facilitating the inclosure of open and arable fields in England and Wales." (Act 6 & 7 Will. IV. chap. 115; August 20th, 1836.)

3. "An Act to extend the Powers and Provisions of the several Acts relating to the enclosure of open and arable fields in England and Wales." (Act 3 & 4 Vict. chap. 31; July 23rd, 1840.)

4. "An Act to facilitate the enclosure and improvement of commons and lands held in common, the exchange of lands, and the division of intermixed lands; to provide remedies for defective or incomplete Executions, and for the non-execution of the Powers of general and local Inclosure Acts; and to provide for the revival of such Powers in certain cases." (Act 8 & 9 Vict. chap. 118; August 8th, 1845.)

5. "An Act to amend the Act to facilitate the inclosure and improvement of commons." (Act 9 & 10 Vict. chap. 70; August 26th, 1846.)

NOTE. All the above Acts may be procured through any bookseller in town or country.

ENGINEERING SURVEYING.

PART II.

LEVELLING.

CHAPTER I.

DEFINITION OF LEVELLING.

By the art of levelling the inequalities of the upper boundary of any section of the earth's surface may be shewn, and thence may be determined the several heights of any number of points in that boundary, above or below an assumed line, called a level line; though, in reality, this line is a great circle of the earth, and is such as would be derived from a section of the surface of still water.

LEVELLING INSTRUMENTS.

(1.) Levelling instruments all depend on the action of gravity of these the plumb-line, on which the mason's level depends, is the most simple; but it cannot be used in extensive operations, on account of its practical inconvenience. The fluid, or water level, in all its modifications, is also found inconvenient for extensive practice.

(2.) Spirit levels are now commonly used, as the most accurate instruments for finding the differences of level, or vertical distances between two stations: of these there are three, which we shall proceed to describe, namely, the Y level, Troughton's, and Gravatt's levels.

The foregoing figure represents this instrument. A is an achromatic telescope, resting on two supporters, which in shape resemble the letter Y; hence the name of the instrument. The lower ends of these supporters are let perpendicularly into a strong brass bar, which carries a compass box C. This compass is convenient for taking bearings, and has a contrivance for throwing the needle off its centre, when not in use. One of the Y supporters is fitted into a socket, and can be raised or lowered by the screw B.

Beneath the compass box, which is generally of one piece with the bar, is a conical axis passing through the upper of two parallel plates, and terminating in a ball supported by a socket. Immediately above the upper parallel plate is a collar, which can be made to embrace the conical axis tightly by turning the clamping screw E; and a slow horizontal motion can be given to the instrument by means of the tangent screw D. The two parallel plates are connected together by the ball and socket already mentioned, and are set firm by four mill-headed screws, which turn in sockets fixed to the lower plate, while their heads press against the under side of the upper plate, and thus serve the purpose of setting the instrument truly level.

Beneath the lower parallel plate is a female screw, adapted to the staff head, which is connected with brass joints to three mahogany legs, of exactly the same construction as those already described for supporting the theodolite.

The spirit level 77 is fixed to the telescope by a joint at one end, and a capstan-headed screw at the other, to raise or depress it for adjustment.

(3.) Previous to using this instrument the following adjustments must be attended to.

1. The Adjustments of the telescope for parallax and collima

tion.

2. The adjustment of the bubble tube.

3. The adjustment of the axis of the telescope perpendicularly to the vertical axis.

1. The adjustment for parallax and collimation have already been described, (p. 76,) being the same as those required for the theodolite.

2. The adjustment of the bubble tube.-Move the telescope till it lies in the direction of two of the parallel plate screws, and by giving motion to these screws bring the air-bubble to the centre of its run. Now reverse the telescope carefully in the Ys, that is, change the places of its ends; and should the bubble not settle in the same point of the tube as before, it

shews that the bubble tube is out of adjustment, and requires correcting. The end to which the bubble retires must then be noticed, and the bubble made to return one half the distance by turning the parallel plate screws, and the other half by turning the capstan-headed screws at the end of the bubble tube. The telescope must now again be reversed, and the operation repeated, until the bubble settles at the same point of the tube, in the centre of its run, in both positions of the instrument. The adjustment is then perfect, and the clips that confine the telescope in the Ys should be made fast.

3. The adjustment of the axis of the telescope perpendicularly to the vertical axis.-Place the telescope over two of the parallel plate screws, and move them, unscrewing one while screwing up the other, until the bubble of the level settles in the centre of its run; then turn the instrument half round on its vertical axis, so that the contrary ends of the telescope may be over the same two screws, and, if the bubble does not again settle in the same point as before, half the error must be corrected by turning the screw B, and the other half by turning the two parallel plate screws, over which the telescope is placed. Next turn the telescope a quarter round, that it may be over the other two screws, and repeat the same process with these two screws; and when, after a few trials, the bubble maintains the same position in the centre of its run, while the telescope is turned round on the vertical axis, this axis will be truly vertical; and the axis of the telescope being horizontal, by reason of the previous adjustment of the bubble tube, will be perpendicular to the vertical axis, and remain truly horizontal, while the telescope is turned completely round. The adjustment is therefore perfect.

(4.) In this level the telescope T rests close down upon the horizontal bar bb, the spirit level 7 is permanently fixed to the top of the telescope, and does not therefore admit of adjustment; and the compass box C is supported above the level by four small pillars, attached to the horizontal bar. This construction makes the instrument very firm and compact. Its parallel

plates and supporting parts are the same as those already described for the theodolite.

The diaphragm is furnished with three threads, two of them vertical, between which the levelling staff may be seen, and the third, horizontal, gives the reading of the staff by its coineidence with one of the graduations marked upon it. Sometimes a pearl micrometer scale is fixed on the diaphragm, instead of the wires. The central division on the scale, in this case, indicates the collimating point, and by its coincidence with a division of the levelling staff gives the required reading on the staff; and the scale serves the purpose of measuring distances approximately, and of determining stations nearly equidistant from the instrument, since at such equal distances the staff will subtend the same number of divisions on the micrometer scale.

The line of collimation is set perpendicular to the vertical axis, in the same manner as in the theodolite already described. This level is preferred by many, on account of its adjustments not being likely, after they are once perfected, to become deranged.

GRAVATT'S LEVEL:

(5.) This instrument is furnished with an object-glass of large aperture, and short focal length; and, sufficient ligh being thus obtained to admit of a higher magnifying power in the eye-glass, the advantages of a much larger instrument are secured, without the inconvenience of its length:-it is hence called "the dumpy." The diaphragm is carried by the internal tube a a, which is nearly equal in length to the external tube. The external tube TT is sprung at its aperture, and gives a steady and even motion to the internal tube a a, which is thrust out, and drawn in, to adjust the focus to objects at different distances by means of the milled headed screw A. The spirit level is placed above the telescope, and attached to it by capstan-headed screws, one at each end, by means of which the bubble can be brought to the centre of its run, as in the case of