184 BAILEY WILLIS — GRAPHIC FIELD NOTES. 



The traverse table was devised by Mr. GaDnett for the purpose its name 

 indicates. He describes it as follows : 



" The plane-table used for traversing is of the simplest possible form, consisting of 

 a board 15 inches square, into one edge of which is set a narrow box containing a 

 compass needle three inches in length. The table is supported by a tripod of light 

 construction without leveling apparatus, the level of the instrument being effected 

 by the legs of the tripod. The table is adjusted in azimuth or oriented by means of 

 the compass needle, movement in azimuth being provided by simply turning the 

 table on top of the tripod head. There is no clamp to the azimuth movement, the 

 table being held in place simply b}'^ friction. The alidade consists of a brass rule 12 

 inches long, with raised sights hinged to turn down when not in use. Ordinary 

 drawing paper backed with cloth is used for plane-table sheets and is attached to the 

 board by thumb tacks."* 



The operation of traversing with this instrument is very simple. At each 

 station the table is oriented by bringing the compass needle to a mark on 

 its short scale ; the area of the map is usually too small to show any con- 

 vergence of magnetic meridians, and if the magnetic declination be constant 

 it follows that at each station the position of the table is parallel to all those 

 preceding it. Courses sighted and drawn with the alidade, whether succes- 

 sive foresights or alternating foresights and backsights, therefore depart 

 from each other with angles equal to those included by the directions on the 

 ground, and the lengths of the sights being laid off to scale, the plat is a 

 figure mathematically similar to the traverse on the ground. On this plat 

 geologic observations can at any instant be indicated in their proper rela- 

 tions. It is customary to foresight to bend of road, tree, fence-corner or 

 any other distinct object, to wheel or pace to the thing sighted, thence to 

 wheel or pace to a convenient station and set up the table. At this station 

 the operations are: (1) to orient the table, (2) to scale off the first foresight, 

 (3) to sight and draw the backsight and scale it off, (4) to sight and draw 

 the next foresight, (5) to sketch in topography or geology, and then to pro- 

 ceed. Time is economized by occupying alternate stations only, and geologic 

 relations are developed as fast as the traverse line is extended. I believe 

 that this simple instrument will prove to be of great value to geologists and 

 will save time, labor and money in the extensive work of geological map- 

 ping. 



But notwithstanding the simplicity and accuracy of the traverse table, 

 geologists who do not wish to carry a mounted instrument of any kind have 

 tried to accomplish the same object with only hand compass and note-book. 

 To do this is to reduce instrumental impedimenta to a minimum, but the 

 observat^ and recording of the traverse requires more care than on the 

 plane-table. Given an ordinary clinometer compass with square base and 

 sights and a note-book ruled in squares, the operation at any station is as 



* Unpublished MSS. 



