APPENDIX TO REPORT OF THE SECRETARY. 71 



in diameter and reads to one minute. The instrument also embraces 

 other improvements designed to secure greater stability, with ease and 

 rapidity in manipulation. 



Flane-Tahle. — In the topographic work the gradieutor and sketch-book 

 have been superseded by the plane-table and the orograph. The plane- 

 table in use is of a pattern designed by Professor Thompson especially 

 for work of this character. The drawing-board is made of a series of 

 slats firmly fixed to canvas in such manner that it can be rolled into 

 small compass for transportation ; but when unrolled for work it is so 

 secured by cross-pieces and screws that great stability is attained. 

 When in use it is fastened to the platen of the orograph. The position 

 of important features in the topography is fixed with an alidade by the 

 usual methods of intersection and resection. Details are placed directly 

 upon the map while they are still under the eye of the topographer, and 

 much of the labor and uncertainty of description by notes is avoided, 

 and the experience of five years in its use has demonstrated that the 

 plane-table as modified is equally well-adapted to regions of mountains, 

 hills, plains, or plateaus. The sketches produced are actual maps and 

 not mere map material. They need only to be adjusted in conformity 

 with the triangulation, and but slight adjustment is necessary. And it 

 has been further demonstrated that a topographer in one field-season 

 can extend his work over an area of about 7,000 square miles with all 

 the accuracy necessary for the scale adopted by the Interior Depart- 

 ment for the physical atlas of the Eocky Mountain Eegion, i. e., a scale 

 of four miles to the inch. 



Orograph. — The orograph is an instrument new to topographic survey- 

 ing, adapted to the requirements of this work by Professor Thompson. 

 It consists essentially of a telescope erected above a platen or drawing- 

 board, on which the movements of its oi)tical axis are recorded. The 

 telescope rotates about a vertical and about a horizontal axis, similarly 

 to the telescope of a theodolite, and is connected by simple mechanism, 

 with a pencil which rests on a sheet of paper fixed to the platform. 

 When the topographer moves the telescope so as to carry its optical 

 axis over the profiles of the landscapes the pencil traces a sketch of 

 the same. This sketch, being mechanically produced, is susceptible 

 of measurement, and is a definite and authoritative record of the angular 

 relations of the objects sketched. The instrument is also furnished with 

 graduated circles, on which horizontal and vertical angles may be read 

 to the nearest half minute, and these circles are used for the secondary 

 triangulation. The orograph and plane-table are used conjointly, and 

 their results furnish data for the j^roduction of contour maps. It is 

 believed that by their introduction the quality of topographic work has 

 been much improved without addition to its cost. When a topographer 

 takes the field with these two instruments and plane-table sheets on. 

 which the primary triangulation has been previously plotted, he returns, 

 with a map on which all of the geographic features to be delineated 



