82 A MANUAL OF TOPOGEAPHIC METHODS. 



The question of paper for the plane-table sheets, especially in inter- 

 section work, is of great importance, as paper which expands and contracts 

 differently in different directions under varying- conditions of moisture will 

 easily produce errors of magnitude in the work. It matters little if the 

 paper contracts and expands, provided it does so uniformly in all directions, 

 but all paper is made with more or less fiber, and accordingly expands and 

 contracts more in one direction than in another. To counteract this, two 

 thicknesses of paper are used, preferably that known as Paragon paper, 

 mounted with the grain of the two sheets at right angles to one another, 

 and with cloth between the layers. In sheets so prepared it has been found 

 that there is practically no distortion, even under the most severe tests. 



The board is generally made of seasoned white pine, from one-half to 

 five-eighths of an inch thick, with cleats across the ends fastened in such a 

 way as to allow the body of the board to contract and expand freely, and 

 therefore without warping. Into the corners of this board and on the edges 

 at points halfway between the corners are set female screws for holding 

 the paper to the board. At corresponding points in the plane-table sheet 

 are punched holes half an inch in diameter which are lined with eyelets, 

 and through which pass screws with broad heads fitting into the female 

 screws in the board. The holes in the paper, being larger than the screws, 

 allow the paper to expand or contract freely when the screws are loose. 

 When tightened, the broad heads of the screws bind the paper firmly in place. 



THE ALIDADE. 



The alidade used with this plane table consists of a ruler of brass or 

 steel 18 inches to 2 feet in length, graduated upon a chamfered edge to suit 

 the scale of work, and carrying upon a column a telescope having a focal 

 distance of 12 to 15 inches and a power of about 15 diameters. It has a 

 vertical arc reading by vernier to single minutes, and a delicate level upon the 

 telescope. In some alidades there is an adjustment to make the zeros of the 

 vertical arc and the vernier coincide, when the telescope is horizontal, while 

 in others it is necessary to read the index error of the vertical arc and correct 

 for it, there being no such adjustment. The telescope turns in a sleeve, for 

 adjustment of vertical collimation. 



