794

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

But in reality when we look at a distant object, our eye is in the direction of a tangent tothe surface of the earth, which is not the true level ; this follows the earth’s curvature,and in constructing a canal the bottom should not be a straight line, but concentric :any drops of water placed in succession upon such a curved line, from their beingequidistant from the centre of the earth, will remain in the position in which they areplaced.

In the levelling for the foundation of a building where the dimensions are small in com-parison to the circumference of the earth, they are generally treated as straight lines, andin practice this is sufficiently accurate; but all plummets gravitating towards the earth’scentre, it stands to reason that a succession of lines taken with the common level must bepolygonal.

Telescopes, used to distinguish distant objects, consist ofa number of glasses placed in a cylindrical tube, withtheir centres in a straight line.

The Eye-Glass is placed in a small tube, which can bedrawn out, and adjusted to different observations, and ithas in its focus a thread marked E, which serves toregulate the sight.

The Object Glass , at the other end of the telescope, isshown at C.

The Optical Axis is the line proceeding from theeye when looking through the glasses, and which passesthrough them at right angles. The glasses are spherical,and of a convex form, their centre being thicker thantheir edges.

The Focus of a Glass is the point in which the raysreflected from an object, having passed through a glass,unite in a point. In practice it is highly important thatthe cross hairs in the telescope should be properly adjusted,which is done by attaching them to a brass ring, andthe eye tube must be drawn out until these hairs appear to occupy the focus of the glass,and can be distinctly seen. The telescope has a screw attached to the instrument, onwhich it rests, and which elevates and depresses it in a manner to be directed to anyobject. The horizontal hair in the telescope being in apparent contact with the object,the vertical one must be treated in the same manner until it also cuts it.

The Spirit Level with a Telescope is made of brass about 12 or 18 inches in length,sometimes of a cylindrical form, and at others that of a parallelogram : it encloses a tele-scope D E, in which is a tube F, having an eye-glass, with a thread fastened in its focus,and which draws out to suit the various sights : at the other end of the telescope theobject-glass is inclosed in a small frame, and which can be moved either up or down by ascrew. This telescope is placed in the tube B C in a manner that it can be turned on its

Fig. 912.

Fig. 913.

Fig. 914.

axis half round, and back to its original position: against one side of the tube B C thespirit level H is attached at its two ends by the screws K and H ; at II are two rings, oneof which clasps the tube, and the other end is attached to the screw L, by which the leveiH I can be elevated or depressed at pleasure, so as to make the level agree with the visualray of the telescope : below the square tube BC is a plate of brass, which can be elevatedor depressed by the screws MN; to the centre of this plate the joint is attached, by whichthe level can be turned in any direction, and roughly adjusted.

To centre the Telescope , it must first be mounted on a stand, and pointed towards the object,in order to observe where the thread of the telescope cuts, and which need not be on a level