10 



BULLETIN 194, U. S. DEPARTMENT OF AGEICULTTJEE. 



mining the exact surface of the water at the ends of the reach in 

 order to measure tlie fall as was the following arrangement. 



Bench marks. — In his experiments in Utah, S. Fortier determined 

 the hydraulic slope of the water surface by leveling between the tops 

 of nails, sharpened at both ends and driven into stakes whose tops 

 were below the surface of the water.^ After considerable experimen- 

 tation the writer decided that this general arrangement was better 

 than that involving a line of levels between the ends of the reach 

 and in addition another chance of error in measuring down from the 



established bench marks to the surface of 

 the water. However, the writer modified 

 Dr. Fortier's plan in the following manner: 

 It was found that driving the nail, sharp- 

 ened at the top as well as at the bottom, 

 dulled the point at the top so that it did 

 not make a good point in order to get a 

 ''bead" on the surface of the water in a 

 stilling box, and it also did not make a 

 sohd bench mark from which to run levels. 

 A better bench mark would be the ordinary 

 top of a well-made wire nail, but this is 

 not directly adaptable, as it can not be 

 driven to a nicety as a surface gauge. To 

 obviate this feature the device shown in fig- 

 ure 1 was constructed. It consists of a 

 motor-cycle spoke soldered into a hole near 

 the edge of the surface exposed by cutting off 

 a three-eighths inch punch. The spoke was 

 bent in the form of a buttonhook and care- 

 fully sharpened. The end of the spoke 

 forming the pointed hook was originally 

 threaded for the nipple, which was retained 

 and served as a protection to the point 

 in carrying the device. The final sharp 

 point is in the same plane as the clean 

 Therefore, if the wire nail is driven by 

 in much the same manner as a carpen- 

 ter sets a nail below the surface of wood with a carpenter's punch, 

 then the top of the wire nail is just flush with the surface of the 

 water when the sharp point of the hook shows at the surface of 

 the water as an ordinary hook gauge. This presupposes that the 

 punch has been held truly vertical in setting the nail. This was 

 assured in the following manner: Near the top of the punch two 

 holes one-sixteenth inch in diameter were bored at right angles to 



Fig. 1. — Hook device for setting 

 nail head flush with water sur- 

 face, two-thirds size. 



cut end of the punch 

 means of the punch. 



U. S. Geol. Survey, Water-Supply and Irrig. Paper 43 (1901). 



