92 A MANUAL OF TOPOGEAPHIC METHODS. 



assistant who carries on the secondary triangulation, selected as possessing 

 special fitness for that work, and one, two, or more assistants who are 

 engaged in traversing, the number of these assistants depending upon the 

 rapidity with which the country can be sketched relative to the rate at which 

 the traversing progresses. If possible, the different items of work of such a 

 party should follow one another in a certain order. The secondary trian- 

 gulation should be done first in order that the traverse men may be furnished 

 with positions and heights for locating and checking their traverse lines. The 

 traversing should follow, in order that all the control may be furnished to 

 the chief of party for his use in sketching. This order, which is followed as 

 closely as practicable, requires that the members of the party be scattered 

 over a considerable area of country, and if they are living in camp it 

 requires that they remain away from it a considerable part of the time, or 

 else that a large amount of traveling be done in order to reach camp at night. 

 Where they are not living in camp, the most economical disposition is to 

 scatter them at various places within their fields of work. In any case, con- 

 stant communication must be had between the chief of party and his assist- 

 ants, in order that they may work in accord. 



STADIA MEASUREMENT. 



Under certain circumstances it is found advisable to use the stadia 

 method for measuring distances in place of the wheel. This is the case 

 where lines are to be run without reference to roads, and consequently 

 where the wheel cannot be employed with advantage. It has been used, too, 

 in southern Louisiana, where peculiar methods of work imposed by the nature 

 of tl ie topography have made its employment economic. The instrument used 

 for the stadia or telemeter method of measuring distances may be anything 

 carrying a telescope. To the reticule of the telescope are added two or more 

 fixed horizontal wires placed at a certain distance apart. A rod or board 

 subdivided to suit the interval between the wires and painted in glaring 

 colors forms part of the outfit. When this rod is set up at a distance from 

 the telescope, that distance is ascertained from the number of subdivisions 

 of the rod which are included between the wires of the telescope, the value 

 of each division of the rod being known. Upon the Geological Survey cer- 



