placed on the principal level-tube, for the purpose of shewing if the instrument deviates from its horizontal position while the observer is noting the observation, the mirror being previously adjusted.

FRENCH REFLECTING LEVEL.

This little instrument consists of a small bit of looking-glass, set in a vertical metal plate, weighing about 11b, or perhaps much less. Its verticality is insured by its being suspended from a ring, which allows it to hang freely, but not turn round its axis of suspension. When the mirror is vertical, and held at an arm's length at the height of the eye, the image and eye are evidently in the same horizontal line; and any distant object coinciding with these must necessarily be on the same level. Hence it appears that this little portable instrument is very valuable in seeking the best direction to carry a road or canal through any portion of a country; as the relative heights of distant points are at once ascertained by merely turning round, holding the instrument at the same height all the while.

Its value, however, consists only in its portability and expedition in comparing heights, with a view to strike out the best route for roads, canals, or other watercourses. The person using it must ascertain the height of his eye, which is a constant quantity. It answers very well for military purposes, but is by no means adapted to levelling operations requiring minute details. This instrument is adjusted by planting a levelling-staff at a small distance behind the mirror, giving directions to raise or lower the vane, so as to make it to coincide

with the line of sight: reverse the mirror, and let the vane on the staff be again made to coincide with the line of sight, the staff remaining stationary; take a mean between these two heights, and adjust the mirror to this mean height, by means of a small screw attached to the instrument.

There are other levels used by civil and military engineers, artificers, &c.; such as the French waterlevel, much used on the Continent; the mason's level, the road maker's level, not unlike the last; and several others, too imperfect and limited in their use to deserve any notice in this place. Their use and application will at once be understood, from what has been said of the more perfect instruments already described.

ON INSTRUMENTAL PARALLAX.

Instrumental parallax is often a source of much error in surveying and levelling operations. Its cause is not difficult to be understood. Modern theory teaches us to believe that light consists in the vibration of an elastic fluid. Be this as it may, we are sure that the rays of light are the media through which the representation of an object is conveyed. We are also aware, from actual experiment, that when these rays impinge upon media of different densities, they become refracted or bent, which necessarily causes a convergence towards a particular point, called the focus.

This focus is situated in an imaginary plane, on which the picture of the object is supposed to be delineated. Now if this plane, containing the picture of the object, be not placed exactly in the focus, the points of the

object become undefined in consequence, and this consequence we call parallax. This parallax is great or small, according to the distance the imaginary plane is from the focus.

Let ABC represent an object, and abc its corresponding

E

B

:

points on the plane hi, upon which every point of the object, after passing through the lens E, is perfectly defined but if we suppose a plane to be situated at ef, the points abc become indefinite, and their position varies between the spaces (1, 2,) (3, 4,) (5, 6,) when the eye is turned all round when looking through the telescope.

The figure here represents the teleseope of a level :

F

2

it has two lenses; the one at B is called the eye-glass, and the one at F the object-glass.

If we view the staff A, or a point C in it, we shall find the situation of the point to vary with every change of view. The amount of variation will be denoted by a space limited by the figures 1, 2, which is the extent of the parallax. Besides this variation in the apparent

place of the point C, it will appear dim and ill defined. From what has been said, it is evident, that in order to obtain a clear and distinct representation of the object viewed, the eye and object-glasses must be slidden in and out till the points a and b are made to coincide; these points are called the foci of the telescope.

LEVELLING STAVES.

One of the oldest description of staves for levelling operations is that with a sliding vane, which is moved up and down by means of a chord passing through two pulleys, the one placed at the top of the staff, and the other at the bottom. This staff is sometimes graduated into feet and inches, and sometimes into feet and decimals of a foot. Its length is generally 12 feet. This staff sometimes consists of two or three sliding pieces, graduated on the face similar to the last. The vane is moved by the hand over the first piece: when the observation requires that the vane should be higher than the first division, put it to the top of this by the hand, and then slide this division up on the second division till it shall arive at 10 feet; and should not this reach to the required height, move up the second slide on the third, till the required height be attained by the vane. This staff is read by means of an index on the side. If it happens to go so high as the top of the first division of the staff, it will be 5 feet, to which is added the reading on the second division raised. Thus if the second slide be raised 3 feet 4 inches, then the reading will be 8 feet 4 inches. If the operation required the vane to be raised higher than the sum of the

heights of two slides, namely 10 feet, proceed as before by moving another division, adding the reading on this to 10 feet. This staff, though subject to error, and very difficult to manage in wet weather, is the best that could be used in connection with the old spirit-level.

It is but very rarely employed at present, except among some old practitioners, who still cling to what they have been so long accustomed to. The staves now, very deservedly, in general use, are those invented by Mr. Gravett. They are graduated into feet, tenths, and hundredths; and can be read distinctly at any distance not exceeding ten or eleven chains. As the subdivisions on the staff can be read distinctly by the observer, it requires no vane, and therefore the assistant has no duty to perform, except to hold the staff in an upright position. Mr. Gravett's staves consist of three pieces, with joints similar to a fishing-rod.

Mr. Sopwith has adopted Mr. Gravett's graduation, but applied a different mechanical arrangement, by which the pieces of which his staves are composed, draw out like a telescope, a spring catch retaining each joint in its place.

Mr. Brough has contrived a form of staff, on which the figures are inverted; it is 15 feet long; but when closed, measures only 5 feet; it is shod with iron, which is, no doubt, an improvement of no inconsiderable importance. The principal objections to the use of these staves, are the difficulty of pulling out the different slides of which they are composed, in damp weather, and the weakness of the upper joints. Want of sufficient strength in the upper joints, renders them very unsteady, and therefore difficult to be observed, even in very