BASE LINE MEASUREMENT. 45 



mentioned, while the boards are, in case measurement is made along the 

 railway tangent, nailed down to the railway ties. These boards are 

 designed to support the devices for maintaining the tension, and the con- 

 tacts are marked upon the strips of zinc Mr. Baldwin's apparatus consists 

 essentially of a wheel worked by a lever and held by ratchets in any 

 desired position. This wheel is attached to the spring balance in such a 

 way that by turning it the strain is put upon the spring balance, which is 

 held at the desired tension by the ratchets. A small mechanism at the rear 

 end of the tape is employed to hold the zero of the tape at the opposite 

 mark. The great length of the tape, 300 feet, allows considerable friction 

 or drag when the supports are frequent, and in order to insure a reasonably 

 uniform distribution of the strain upon the tape, it should be raised and 

 allowed to fall with the strain on. 



The measurements should be made at night, or during cloudy days, 

 in order that the temperature of the air, which is that indicated by the 

 thermometers, and that of the tape be as nearly as possible the same. The 

 temperature must be carefully observed by at least two thermometers at 

 each tape length, in order that the best possible data for temperature cor- 

 rection may be obtained. 



The base should be measured at least twice, and the two results com- 

 pared by sections of 1,200 feet, or four tape lengths. The ends of the 

 base must, if possible, be permanently marked by means of stone monu- 

 ments set into the ground so that their surfaces are but a few inches above 

 its level and the exact position of the ends should be indicated by a cross 

 cut in a copper bolt embedded in the head of a stone, in order that the 

 base may be preserved for future references. 



A line of levels must be run over the site or over the stools which 

 support the tape for the purpose of obtaining its profile and thereby the 

 means for deducing its horizontal length. 



REDUCTION OF BASE LINE MEASUREMENT. 



The first correction to be applied is that <>t' reduction to a standard. 

 The correction for this is obtained by comparison with the standard of the 

 U. S. Coast and Geodetic Survey. The correction for the entire line is in 



