that this instrument is very heavy, is rather difficult of manipulation because of inaccessibility of leveling and clampingscrews, and is in fact too cumbersome for convenient use, excepting where travel is easy.

The Johnson plane-table (Fig. 49), so named after its inventor, Mr. Willard D. Johnson, is used by the United States Geological Survey, and though not quite as rigid as the Coast Survey type, is sufficiently so for all practical purposes and is much lighter, more portable, and more easily manipulated. The movement is also more compact and less liable to derangement or injury. It consists of a split tripod securely attached to the head as in the case of the Coast Survey tripod, but the leveling and horizontal movements are entirely unique in surveying-instruments, being essentially an adaptation of the ball-and-socket principle, so made as to furnish the largest practicable amount of bearing surface.

They consist of two cups, one inside the other, the inner surface of the one and the outer surface of the other being

FIG. 50.-JOHNSON PLANE-TABLE MOVEMENT.

ground to fit as accurately as possible. The interior cup consists of two parts or rings, one outside the other, one controlling the movement in level, and the other that in azimuth (Fig. 50). From each of these there projects beneath the movement a screw, and each screw is clamped by a wing nut. These cups and rings are bound together and to the tripod

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head by the two nuts, and are attached to the plane-table board by screwing it over a center axis or pin projecting from the upper surface of the upper cup. The instrument is first leveled, not by leveling-screws, but by the ball-and-socket motion given by the pair of cups which are clamped by the upper screw when the board is level, the latter being still left free to revolve horizontally for orientation and being clamped by the lower screw. There is no tangent screw for slow motion in azimuth, it being possible owing to the long lever-arm furnished by the outer edges of the board to move it with sufficient slowness for all practicable purposes.

59. Telescopic Alidades.-Alidades used with the more rigid plane-tables differ in form according to the character of the work to be executed. Where the instrument is used chiefly in triangulation, the alidade should be of the most approved type and the rule should be of sufficient length to permit its being used as a straight-edge in drawing lines from one extreme of the board to the other. In practice this rarely exceeds 25 inches in length. In using the plane-table in sketching or traversing a smaller alidade and one having a straight-edge not exceeding eighteen inches in length will be found more portable and better adapted to the work required.

The alidade generally employed by the U. S. Coast and Geodetic Survey consists of a brass or steel straight-edge 2 by 3 inches in width and 12 to 15 inches long, from and perpendicular to which rises a brass column 3 inches in height, surmounted by Y's in which rest the transverse axes of the telescope. To one end of the axis is firmly attached an arm of thirty degrees, graduated to minutes on either side of a central zero, the accompanying vernier being attached to the Y support. On the telescope-tube are turned two shoulders on which rests a striding-level. There is a clamp and tangent screw for slow motion for moving the telescope in vertical arc, and on the straight-edge are two small spirit-levels at right

angles to each other. A déclinatoire accompanies the alidade and is carried in a separate box or is sometimes attached as a part of the striding-level. The déclinatoire box is oblong, with the sides parallel to the north and south lines and graduated to about 5 minutes on either side of the zero.

The chief difference between this alidade and the one used by the United States Geological Survey (Fig. 49) is that the straight-edge of the latter is 18 or 24 inches in length, with one edge beveled and graduated to the scale of mapwork. The telescope is on a standard 4 inches high, has a focal distance of 15 inches and a power of 20 diameters, with an objective of 18 inches diameter. The telescope revolves horizontally in a sleeve, with a stop for adjustment of vertical collimation. Instead of two small levels attached to the straight-edge, a single detached circular level is carried by the topographer.

The smaller telescopic alidade used by the United States Geological Survey (Fig. 51) on traverse and stadia work is

more like the Coast Survey alidade in having a shorter telescope and focal distance and a shorter straight-edge. The vertical arc, instead of being graduated on the side and reading against a vernier as is customary with other surveying in

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struments, is a sector, which, instead of pointing downwards, points towards the rear or eyepiece of the telescope and is graduated on its outer surface. This is read against a vernier fixed in such position that the reading may be made from one position of the observer at the eyepiece without his moving to the side of the plane-table as in ordinary instruments.

60. Adjustments of Telescopic Alidade. There are practically no adjustments to the plane-table and alidade excepting the adjustments of the latter for striding-level and collimation. Adjustments or tests may be made of the straightness of the fiducial edge of the rule by drawing a line along it, and reversing it, placing the opposite ends upon the marked points and again drawing a line; if the two lines do not coincide, the edge is not true. It is not necessary that the two edges of the straight-edge be exactly parallel, if care is always taken in using the instrument to draw along but one edge.

Attached levels when used may be adjusted by placing the alidade in the middle of the table, marking its edges on the paper, and bringing the bubble to center by means of the leveling apparatus; then it is reversed 180 degrees, and if the bubble be not in the center it is corrected one-half by leveling the table, and the other half by adjusting the screws of the attached levels.

The striding-level is adjusted by placing the alidade in the center of the table, leveling the telescope by the vertical tangent screw, then reversing the level upon the telescope. If the bubble come not to the exact center of the tube, half of the error is to be adjusted by the screws in the level, and the other half by releveling the telescope with the tangent screw.

In addition to the above there are a few other adjustments, as that of making the line of collimation perpendicular to the axis of revolution of the telescope, and of making the latter parallel to the plane of the rule, and for parallax, and to correct the zero of vertical arc, etc. None of these need, how

ever, be described, as in the alidade as now made the bearings of the axes are unchangeable and there is either a means of setting the vernier at zero or an index error is to be read.

61. Gannett Plane-table.-Where rough traverses are run in connection with the making of small-scale maps, and a firm board is unnecessary since the telescopic alidade is not used, an exceedingly convenient and portable plane-table is that employed in the U. S. Geological Survey and known as the Gannett plane-table (Fig. 52), after its originator, Mr. Henry

FIG. 52.-GANNETT TRAVERSE PLANE-TABLE AND SIGHT ALIDADE. Gannett. The tripod of this instrument is very light, consisting of three straight legs made of single pieces of wood. These are shod with metal tips and attached by bolts and nuts to the head, which is a simple plate 3 inches in diameter. The board, which is 15 inches square by of an inch in thickness, is a well-seasoned piece of pine and is attached to the tripod by a single center screw. There is no leveling apparatus, the instrument being leveled by means of the tripod legs, and there is no means of clamping the instrument in azimuth, the movement in azimuth being controlled by friction, and the board being held in place by friction due to the tightness with