56 



With the plane-table such a map is constructed on the ground t>y at once drawing 

 upon the paper, which is spread upon the table, the angles subtended by dilVcrrnt 

 objects, and determining by intersections their relative positions, instead of reading 

 off the angles on graduated instruments and afterwards plotting the lines by means 

 of a protractor, as is done in other methods of surveying. The practice with the 

 plane-table has in this respect a great advantage in directness and precision. The 

 istances and of vertical angles are used, in conjunction with the 



method of intersections, to obtain all the data for representing the horizontal and 

 vertical features on the map, which is drawn in the field with pencil, the details 

 being filled in according to established conventional signs. 



"Adjustments. Troin the nature of the service in some sections of the coun- 

 try the plane-table is often necessarily subjected to rough usage, and there is a 

 constant liability to a disturbance of the adjustments ; still, in careful hands, a well 

 made instrument may be used under very unfavorable conditions for a long time 

 without being perceptibly affected. One should not fail, however, to make occa- 

 sional examinations, and while at work, if any difficulty be encountered which cannot 

 otherwise be accounted for, it should lead directly to a scrutiny of the adjustments. 



'*!. The fiducial edge of the rule. This should be a true, straight edge. Place 

 the rule upon a smooth surface and draw a line along the edge, marking also the 

 lines at the ends of the rule. Reverse the rule, and place the opposite ends upon 

 the marked points, and again draw the line. If the two lines coincide, no adjust- 

 ment is necessary; if not, the edge must be made true. 



4 'There is one deviation from a straight line, which, by a very rare possibility, 

 the edge of the ruler might assume, ana yet not be shown by the above test ; it is 

 when a part is convex, and a part similarly situated at the other end concave, in 

 exactly the same degree and proportion. In this case, on reversal, a line drawn 

 along the edge of the rule would be coincident with the other, though not a true 

 right line ; this can be tested by an exact straight edge. 



"2. The level attached to the rule. Place the instrument in the middle of the 

 table and bring the bubble to the center by means of the leveling screws of the table ; 

 draw lines along the edge and ends of the rule upon the board to show its exact 

 position, then reverse 180. If the bubble remain central, it is in adjustment ; if not 

 correct it one-half by means of the leveling screws of the table, and the other hall 

 by the adjusting screws attached to the level. This should be repeated until the 

 bubble keeps its central position, whichever way the rule may be placed upon the 

 table. This presupposes the plane of the board to be true. If two levels are on 

 the rule, they are examined and adjusted in a like manner. 



"Great care should be exercised in manipulation, lest the table be disturbed. 



"3. Parallax. Move the eye-glass until the cross-hairs are perfectly distinct, 

 and then direct the telescope to some distant well defined object. If the contact 

 remain perfect when the position of the eve is changed in any way, there is no 

 parallax ; but if it does not. then the focus of the object-glass must be changed until 

 there is no displacement of the contact. When this is the case, the cross-hairs are 

 in the common focus of the object and eye-glasses. It may occur that the true focus 

 of the cross-hairs is not obtained at first, in which case a readjustment is necessary, in 

 order to see both them and the object with equal distinctness and without parallax. 



The Striding Level, its Use and Adjustments. 



In transits reading to minutes and half-minutes, the plate-level in front of the telescope is generally 

 ufficiently sensitive to insure good work. However, an instrument of the class as shown and described under 

 No. 1 d, should always be provided with a striding level, to insure a degree of accuracy in keeping with its 

 greater capability. The sensitiveness of this striding level is equal to that of the long level on the telescope. 

 Thus it will be seen that in a transit of this description the plate-levels serve only the purpose of leveling up 



erally, and that in all cases where the objects vary considerably in height, the striding level only should be 

 depended on at every sight. The striding level of this instrument rests on two cylinders of equal diameters, 

 at points between the standards on the cross-axis of the telescope; and, unlike the method described on page 30, 

 erves also the purpose of adjusting the telescope to revolve in a vertical plane. As shown in the cut, the 

 striding level can be left on the cross-axis when the telescope is revolved in altitude. To verify the 

 adjustment of the striding level -fin other words, to make its axis parallel to the cross-axis) level up the transit 

 and bnng the bubble to the middle of its tube, reverse the striding level on the cylinders and see whether it reads 

 the same; if not, remove half the error by the leveling screws, the other half by the capstan-headed screws at 

 he end, and repeat until corrected. To verify the side adjustment of the level, revolve the telescope 20 or 30. 

 and note whether the reading of the bubble remains the same, if not, correct the error by the capstan-headed 

 crews at the side. To verify the adjustment of the cross-axis of the telescope for right angles to the vertical 

 axis of the transit, revolve the instrument 180 in azimuth, and assuming that both cylinders, on which the 

 jstndtng level rests, are equal in diameter, a change in the reading of the bubble will indicate double the amount 

 To correct it, remove half the error by the leveling screws, the other half by the Wye adjustment of 



standard. Remember, that the pressing of the capstan-headed screws against the level tube acts similar 

 to that of the Wye adjustment of the standard. C. L. B. 



For more information on this subject, see pages 98 andjollowing, of this handbook. 



