f Mr Huygens's invention, consists of a telescope a, fig. 11, in form of a cylinder, going through a ferril, in which it is fastened by the middle. This ferril has two flat branches b, c, one above, and the other below; at the ends whereof are fastened little moving pieces, which carry two rings, by one of which the telescope is suspended to an hook at the end of the screw 3, and by the other a pretty heavy weight is suspended, in order to keep the telescope in equilibrium. This weight hangs in the box 5, which is almost filled with linseed oil, oil of walnuts, or other matter that will not easily coagulate, for more aptly settling the balance of the weight and telescope. The instrument carries two telescopes close and very parallel to each other; the eye-glass of the one being against the object-glass of the other, that one may see each way without turning the level. In the focus of the object-glass of each telescope must a little hair be strained horizontally, to be raised and lowered as occasion requires by a little screw. If the tube of the telescope be not found level when suspended, a ferril or ring, 4, is put on it, and is to be slid along till it fixes to a level. The hook on which the instrument is hung is fixed to a flat wooden cross; at the ends of each arm whereof there is a hook serving to keep the telescope from too much agitation in using or carriage. To the said flat cross is applied another hollow one, that serves as a case for the instrument; but the two ends are left open, that the telescope may be secured from the weather and always in a condition to be used. The foot of this instrument is a round brass plate, to which are fastened three brass ferrils, moveable by means of joints whereon are put slaves, and on this foot is placed the box.

Fig. 12, marked I, is a balance-level; which being suspended by the ring, the two sights, when in equilibrium, will be horizontal, or in a level.

Spirit Level. The most accurate levelling instrument, and that possessed of the greatest essential advantages in use, is the spirit-level; which was first constructed by the late Mr Sisson, and to which some small additions and improvements have been since made. The following is a description of one of the best of these levels, as made by the principal mathematical instrument makers.

Fig. 13, is a representation of the instrument mounted on its complete slaves, copied (except the letters) from Mr Adams's Graphical Essays, Plate xvii. fig. 3. The telescope (ABC) is made from 15 inches to 2 feet in length, as may be required. It is achromatic, of the best kind, and shows the objects erect. In the focus of the eye-glasses are exceedingly fine cross wires, the intersection of which is evidently shown to be perfectly in the axis of the tube; for by turning it round on its two supporters DE, and looking through the telescope, the intersection of the wires will constantly cut the same part of the object viewed. By turning the screws at the side of the telescope, the object-glass at g is moved; and thus the telescope is exactly adapted to the eye. If these cross wires are at any time out of their adjustment, which is discovered by their intersection not cutting the same part of the object during the revolution of the telescope on its axis, they are easily adjusted by means of the four screws bbb, placed on the telescope about an inch from the end for the eye. These screws act in perpendicular directions to one another, by unfastening one and tightening the other opposite to the wire, so that if connected with it, it may be moved either way at pleasure; and in this manner the other wire perpendicular to it may be moved, and thus the intersection of the wires brought exactly in the axis of the tube.

To the telescope is fixed, by two small screws cc, the level tube containing the spirits, with a small bubble of air: This bubble of air, when the instrument is well adjusted, will settle exactly in the same place, in or near the middle of its tube, whether the telescope be reversed or not on the supporters, which in this case are kept unmoved.

It is evident, that the axis of the telescope, or the intersection of the wires, as before shown, must be in this In this facile mode of adjustment consists the new improvement of the instrument; and it is hereby capable of being adjusted by only one station and one object, which will at the same time determine it to be in a true level. If by change of weather, accident, or otherwise, the instrument should have lost its level adjustment or state, it may thus be readily restored and readjusted at the first station; which is an advantage none of the instruments formerly made have been capable of. The two supporters DE, on which the level rests and turns, are shaped like the letter Y. The telescope rests within the upper part of them; and the inner sides of each of these Ys are tangents to the cylindric tube of the telescope, which is turned to a true cylinder, and each touches it but at one place only.

The lower end of these supporters are inserted into a strong brass plate (FE), and so as to stand perpendicularly on it. One is kept fast by a tightening screw G, and to the other is applied a fine threaded screw H, to adjust the tube when on its supporters to a true level. To the supporter D is sometimes applied a line of tangents as far as 12 degrees, in order to take an angle of depression or elevation to that extent. Between the supporters is also sometimes fixed a compass-box I, divided into 360 degrees, and again into four 90°; having a centre pin and needle, and trigger, at d, to throw off the needle from the centre when not used; so in this manner it constitutes a perfect circumference, connected with all the foregoing improvements. This plate is fixed on a conical brass ferrule K, which is adapted to the bell-metal frustum of a cone at top of the brass head of the staves, having a ball and socket, with three bell-metal joints, two strong brass parallel plates LL, the four screws eee for adjusting the horizontal motion, a regulating screw M to this motion, and a fastening screw N to lighten it on the cone when necessary. The fastening screw N, and the regulating screw M, by which the whole instrument is moved with accuracy through a small space in an horizontal direction, was an addition of Mr Ramsden's.

The manner of adjusting the spirit-level at the first station. The whole level being now placed steadily on its staves, it must be rendered parallel to the axis of the telescope before you adjust the horizontal motion. To this end the telescope must be placed in a line with two of the screws ee, and then levelled thereby till the bubble of air in the spirit-tube keeps its position in the middle, while turned about to three points, making nearly right angles at the centre to one another.

The horizontal motion being thus adjusted, the rims ff of the Ys are to be opened, the telescope taken off and laid the contrary way upon the supporters. If the bubble of air then rests exactly the same, the level and telescope are adjusted rightly to one another; but if the bubble does not remain the same, the end to which the air bubble goes must be noticed, and the distance of it from the telescope altered; correcting one half the error by the screws ee, and the other half by the screws ee.

Now the intersection of the wires being directed to any distant object, it may be one of the vanes of the staves hereafter described; if they continue to be against it precisely while the telescope is turned round Levelling, or its Ys, it proves, as before mentioned, that the axis of the telescope coincides with the intersection of the wires, and that the instrument will give the true level direction.

The operation of levelling being of a very accurate and important nature, and the best instrument when out of its adjustment being of little use, it is quite necessary that every person using such an instrument should have it readily in his power to correct it; and the one above described appears to be the best adapted for that purpose of any hitherto contrived.