SURVEYING INSTRUMENTS. 59 



(where great accuracy is not required) in forming a sketch 

 map, or laying down the details of a country where the 

 relative situation of the principal conspicuous objects have 

 been previously fixed by triangulation. The expedition with 

 which such work may.be performed, by a person who is expert 

 in the use of this instrument, is its chief recommendation. 



The construction and size of the plane-table have been 

 varied at different times, to suit both the convenience and 

 intentions of the surveyor ; but the annexed figure is a re- 

 presentation of that which is now in most general use. It 

 is a board, as A, about sixteen inches square, having its 

 upper edge rabbeted, to receive a boxwood frame, B, which 

 being accurately fitted, can be placed on the board in any 

 position, with either face upwards. This frame is intended 

 both to stretch and retain the drawing paper upon the board, 

 which it does by being simply pressed down into its place 

 upon the paper, which for this purpose must be cut a little 

 larger than the board. 



One face of the frame is divided to 360 degrees, from a 

 centre, 0, fixed in the middle of the board, and these are 

 subdivided as minutely as the size of the table will admit. 

 The divisions are frequently numbered each way, to show 

 at sight both an angle and its complement to 360. There 

 is sometimes a second centre piece, D, fixed on the table, at 

 about a quarter of its width from one of the sides, and at 

 exactly half its length in the other direction. From this cen- 

 tre, and on the other side of the frame, there are graduated 

 180 ; each of these degrees is subdivided to 30 minutes, 

 and numbered 10, 20, 30, &c., both ways, to 180. The 

 object of these graduations is, to make the plane-table sup- 

 ply the place of the theodolite, and an instrument formerly 

 in use called a semicircle. The reverse face of the frame is 

 usually divided into equal parts, as inches and tenths, for 

 the purpose of ruling parallel lines or squares, and for shift- 

 ing the paper, when the work requires more than one sheet. 

 G is a compass-box, let into one side of the table, with a 

 dove-tail joint, and fastened with a milled-headed screw, 

 that it may be applied or removed at pleasure. The com- 

 pass, besides rendering the plane-table capable of answering 

 the purpose of a circumferentor, is principally useful in 

 setting the instrument up at a new station parallel to any 

 position that it may have had at a former station, as well as 

 a check upon the progress of the work. 



