50 A MANUAL OF TOPOGRAPHIC METHODS. 



to the control of certain sheets. Again, since the primary stations must be 

 occupied by topographers for intersecting on numerous points, they must 

 be selected with reference to this requirement, They should command an 

 extended view, especially of points suitable for cutting in, such as hill and 

 mountain summits, houses, churches, etc. 



The instrument should, wherever possible, be accurately centered under 

 the signal. Whenever it is necessary to set up off center, the direction and 

 distance to the signal should be carefully measured and recorded. 



While signals should be of the simplest and least expensive form which 

 will serve the purpose, their form and material must depend upon the require- 

 ments and the materials at hand. In a mountainous country, where the 

 summits are treeless, simple cairns of stone, 7 to 10 feet in height, are em- 

 ployed. Where the summits are wooded, it is frequently convenient to clear 

 them, leaving a single tree to serve as a signal. In such cases it is advisable 

 to trim the tree of branches, with the exception of a tuft at the top. Where 

 the station is clear, but with green timber easily accessible, it is advisable 

 to make a tripod of small trees, each with a tuft at its top. In undulating 

 and hill country it is often necessary to erect scaffolds. These should be 

 built of sawed lumber and framed in simple fashion. If the lines are short, 

 a pole with a flag may be set in the top. If the lines are long, the tower 

 itself may serve as a signal, in which case its upper part should be clothed 

 in black and white cotton. 



The annexed cut shows a form of framed signals adapted for use on 

 the treeless plains of Kansas and the rolling open hills of New England, 

 and elsewhere, where observing towers are not necessary. (PI iv.) 



It is frequently necessary to raise the instrument to a considerable ele- 

 vation above the ground, in order to overlook surrounding obstacles. In 

 such cases the structures for supporting the instrument should be combined 

 with the signals, and hence they may properly be described and figured 

 here. These observing towers should be in two parts. An interior struc- 

 ture, solidly built of sawed lumber, if available, for the immediate support 

 of the instrument, and a framework surrounding it, supporting a platform 



