The eye-piece K contains two lenses, sliding in a tube fixed to the telescope, for adjusting them to a distinct vision of the cross wires, or hairs, which are held in a proper state of tension in the frame t, and regulated to the axis, or line of sight, by four minute screws passing through the outside of the telescope.
We have before shown how the four screws act upon the two parallel plates, a a and e e, while the axis b can be set very nearly perpendicular ; then by the screw y the telescope can be set very nearly level. To turn the telescope about horizontally upon its axis 1, a screw m works in a fixed collar a ; its nut p is fastened upon an arm which projects from a clamp sf, embracing a collar upon the axis 4 and is tightened by a screw r; which being unscrewed, the clamp springs open, and the telescope, together with the level, moves round with freedom upon the axis 1, according to the pleasure of the operator. When the screw r is tightened, the clamp holds the telescope fast, but admits of a slight movement, either way, when acted upon by the mill headed screw az.
Our readers will readily perceive the simplicity of this level, beyond any others in use ; and will lament the demise of a gentleman, who, to profound theory, .added ingenious and skilful prac tice.
Where a very long and continued range of brickwork is to be raised, it is often advisable to use a water level, made by laying a ridge of mortar along the centre of the wall, and opening a very narrow channel throughout its centre longitudinally, so as to form a kind of trough; let the ends be stopped, and the trough be filled with water, as far as it will flow. The surface of the water will give a true horizontal level ; which, if continued for miles, would conform ex actly to the curvature of our globe. To continue the level along the rest of the trough, stop it at the place where the wa ter reached, and raising the adjacent part with more mortar, let the trough of the su perior level be filled, and thus in succes sion. The difference between the end of one trough, and the beginning of an other, will shew the respective levels ; from which parallels may be set off at any height above by plumb lines of equal length. This mode is often practised in
large works, such as fortifications, and when strata of masonry are to be regu larly disposed ; also to prevent those ir regular breaks, and partial connexions, that are almost inevitable where small sal. lows, or triangular levels, with plumb weights, are in use.
When no instrument can he obtained, and where it is not easy to draw an exact level by the foregoing means, take the hose of an engine, and having fixed one end at the spot whose level is to be sought, (on any opposite bank, for instance,) car ry the other end to the place where the corresponding height is to be established. Fill the hose with water until it ceases to require raising at the further end. When both ends show full to the brim, and that the water is retained at both, then they are on the same level : for it is a maxim in IlvatiosmaTics (which see), that water, or, indeed, any fluid heavier than atmos pheric air, will, when at liberty, always find its own level.
Where a succession of contiguous le vels are wanted, it will often be found convenient to use a small leaden pipe, of about half an inch bore, which should be applied as above described ; or even a common gutter, made of two pieces' of planks, like those under the eaves of houses, may be made to answer the pur pose, by supporting either end, until the water may come to a level in every part.
Where works of moderate extent are carried on, and where the perfect level of each stratum of materials is not anob ject of importance, the common brick layer's level, made in the form of an in verted T, thus j,, having i a plumb sus pended from the top, and received in an opening at the junction of the perpendi cular with the horizontal piece, will an swer well enough. The principle on which this acts, is, that as all weights have a tendency to gravitate towards the centre of the earth, so as the plumb line is a true perpendicular, any line, cutting that at right angles, must be a horizontal line at the point of intersec tion.