44 A MANUAL OF TOPOGRAPHIC METHODS. 



of brush and undergrowth, and, if necessary, its sharp inequalities should 

 be leveled. The tape should be supported by a series of low stools, whose 

 legs are pressed into the ground at intervals of not more than 25 feet, while 

 similar stools should sustain each end of the tape. 



The personnel required in the measurement of a base line is, in an 

 ordinary case, as follows: 



First. The chief of the party, who exercises a general supervision over 

 the work, marks the extremities of the tape and provides the necessary pre- 

 cautions against errors in the measurement, as hereafter stated. 



Second. The rear chainman, who adjusts the rear end of the tape to the 

 contact marks and who carries and reads one of the thermometers. 



Third. The head chainman, who adjusts the forward end of the tape' 

 exerts the requisite tension upon it, and carries and reads a second ther- 

 mometer. 



Fourth. A recorder. 



The measurement of a base with the steel tape is a simple matter. 

 Provision must, however, be made, first, for the proper alignment of the 

 base; second, for the proper tension of the tape; and, third, for the measure- 

 ment of temperature. 



The alignment is a simple matter, and is generally marked out upon 

 the ground in advance of the work of measurement. In cases where a 

 railway tangent furnishes the site for the base line, no alignment is needed 

 beyond the provision for keeping the tape always at a uniform distance 

 from one of the rails. 



For insuring a uniform tension of the tape, an ordinary spring balance 

 is used, which is attached to the forward end of the tape, where a tension 

 of twenty pounds is applied. In order to apply this uniformly, and to 

 insure against slip of the tape, an apparatus devised by Mr. H. L. Baldwin, 

 jr., of the Geological Survey, is in use. 



For its use, it is necessary to obtain strips of board about five feet long 

 and four inches in width, in number ecpial to the number of lengths of tape 

 of which the base line consists. Numbered strips of zinc of equal number, 

 each about eight inches long and an inch in width, are tacked to blocks of 

 wood, and these blocks of wood in turn nailed down upon the boards above 



