TEAVEESE WOEK. 



87 



The alidade consists of a brass ruler, 12 inches long, with folding sights. 

 The edge of the ruler is graduated to facilitate platting of distances. Ordi- 

 nary drawing paper backed with cloth is used for plane-table sheets, and is 

 attached to the board by thumb tacks. 



When traversing is done along roads, as is commonly the case, dis- 

 tances are measured by counting the revolutions of a wheel, usually one 

 of the front wheels of a buggy or buckboard. For counting the revolutions, 

 various automatic devices have been in use. The old form of odometer 

 known as the pendulum was first tried and was unqualifiedly condemned. 

 The form now in general use was devised by Mr. E. M. Douglas of this 

 Survey. See Fig. 9. 



Fig. 9.— Douylas odometer. 



For operating this a cam is placed on the hub of the wheel, which by 

 raising a steel spring as the wheel revolves carries the index forward one 

 division for each revolution. This form is the most trustworthy that has 

 yet been devised, but is not altogether satisfactory, and many topographers 

 prefer to count the revolutions of the wheel directly, using an arrangement 

 by which a bell is rung at each revolution. 



An experience covering many thousands of miles of measurement has 

 shown that as a working method of measuring distances on roads the wheel 

 is superior to the stadia, alike as to accuracy and rapidity 



A traverse man is generally assigned a tract of country within which 

 he is instructed to run traverses of all the public roads and of such of the 

 private roads as appear to be necessary in order to control the entire tract. 

 If practicable, he is furnished with the positions of the points located within 



