TRANSIT THEODOLITE. 55 being made higher, the telescope is free to revolve on its axis, and we thus save the troublesome work of taking it out of the Y's, as is the case with the cradle, so that in prolonging a line from A to B (the instrument being at B) on to D it is only necessary to turn the telescope over, and, providing the instrument is in adjustment, you obtain a more accurate result in much less time. The transit, so far as the clamping and slow-motion screws of the lower and upper plates and the vertical circle (not an arc or semicircle), is similar to the cradle theodolite. The vertical circle, like the horizontal limb, is divided into 360 degrees and subdivisions, and has two microscopes in length of its diameter, which revolve round the horizontal axis. For special purposes it is fitted with a supplementary level x (Fig. 104) fixed by standards with jaws on to the horizontal spindle, this being an additional safeguard against the upper portion of the instrument being out of the horizontal. The larger instruments, such as are chiefly used in constructive works, are fitted with a small spirit lantern, resting on a bracket attached to one of the A frames. This is to throw rays of light into the telescope, through a crystal let into the arc, when observations are required to be taken in the dark. My own experience has been that such appendages are more trouble than they are worth, and the extra money would be better spent upon strengthening the working parts of the instrument. If it is required to work in the dark you can have a man with a lamp against the instrument, which answers the same and a better purpose, for if you are taking angles you can also use this lamp to read the vernier. Triangular Plate.-Figs. 104 and 105 show the tripod to be surmounted by a triangular plate, in which only three levelling screws are used, the parallel plates being dispensed with. The lower part of the screw has a shoulder which fits into a pear TRANSIT THEODOLITE. 57 shaped hollow slot. The advantage claimed for this method is that the instrument can be levelled with one hand only, whereas with four screws both hands are required. But I must confess I have a Fig. 105. strong prejudice in favour of the old system, and most transit theodolites are made with parallel plates and four screws. The cost of transit theodolites is as follows: three-inch, £23 four-inch, £26; five-inch, £28; six-inch, £32; seven-inch, £40; eight-inch, £58; and ten-inch, £125. Everest Theodolite.-So called after the name of its designer, the late Sir George Everest, of the Indian Trigonometrical Survey. The chief efficacy of this beautiful instrument (Figs. 106 and 107) is that the limb may be made of much greater diameter, and consequently there is scope for greater detail in the working parts and a more powerful telescope. There is only one horizontal plate divided into 360 deg., and instead of an upper plate three arms radiate from the vertical axis, thus Y Y being the verniers and the arm which clamps this arrangement to the limb a, having also a slow-motion or tangent screw with which to manipulate it. The standard is much stronger than in other theodolites, and, whilst the telescope is mounted somewhat in the same manner as the transit, yet it cannot revolve upon its axis. Instead of having a vertical circle the Everest has two arcs, м м, with an arm fixed to the telescope, using the same axis, and consequently travelling with it, at either end of which are the verniers. The Everest, so far as the arrangement for levelling is concerned, EVEREST THEODOLITE. 59 is similar to that already described, having only three screws, and no ball and socket, and when not required to be used upon a REESE LIBRARY UNIVERSITY Fig. 107. tripod is so constructed that it may rest upon a wall or any flat surface. The cost of an Everest theodolite is as follows, including case and tripod-stand: 4-inch, £21; 5-inch, £25; 6-inch, £28; 7-inch, £34; 8-inch, £39; and 10-inch, £54. Box Sextant.—This is an instrument, without which no surveyor should go into the field. It may be made to serve the purpose of an optical square. I have had an opportunity of testing its merits, as some years ago I had to make a survey of a large portion of the town of Sunderland for Parliamentary purposes, and for a length of seven miles, when a theodolite would have been impossible in consequence of the heavy traffic in the principal streets, I found this little instrument invaluable, and the results highly satisfactory. The box sextant is about 3 in. in diameter and 11⁄2 in. deep, and has a lid which completely covers it when not in use, but which can be screwed on to the bottom, and serves as a handle when taking observations. Fig. 108 is a view showing the chief features of the instrument, and also gives a fair idea of its internal arrangements. A graduated scale F from 0 deg. to 140 deg. with subdivisions, is engraved on a silver arc, and along this moves the |