1 4 THE 1M1'ROV1-:MI-:.\1 OF RIVERS. 



With the same precautions ordinary level work shows most excellent results. The 

 errors due to atmospheric conditions or to setting instruments or turning-points can 

 as well be eliminated in the one as the other method. 



The instrument employed for precise leveling is constructed on the same general 

 principles as the ordinary wye-level, but possesses much more delicate means of ad- 

 justment, and in order to obtain good work it is necessary to keep it in the shade at 

 all times, both in use and in transportation. The adjustments of the telescope and 

 the levels are made in the same way as in the ordinary level. 



On the precise levels of the Mississippi the sights were limited to 150 meters, and 

 the difference between any one fore and back sight to 10 meters, the sums of their dif- 

 ferences between any two bench-marks being made equal. Permanent benches were 

 set every 3 miles, made of vitrified tile 18 inches by 18 inches, by 4 inches thick, placed 

 3 feet below ground, with a 4-inch wrought-iron pipe leading to them. A copper bolt 

 in the tile formed the bench-mark proper. 



The limit of error in closing any polygon was 3 millimeters multiplied by the square 

 root of the distance in kilometers. 



