4 U. S. COAST AND GEODETIC SURVEY 



represented in figure 2, there is a fixed reading mark on a board and the 

 height of the tide is read on the tape graduations as they pass the 

 fixed mark. In the latter case, the tape graduations, expressed in feet 

 and hundredths, should increase towards the float so that the readings 

 will increase with a rising tide. The tape itself may be either stainless 

 steel or a bronze alloy — the advantages of the former being flexibility 

 and legibility, and of the latter durability. Some of the advantages 

 of both may be secured by using the bronze alloy for the lower section 

 of the tape which is normally in the float well and the stainless steel 

 for the upper section from which the readings must be taken. 



13. The float for the tape gage must be cylindrical in shape in order 

 that small changes in the plane of flotation will not affect the cross- 

 section area in this plane. The top and bottom, however, may be 

 tapered to permit easy passage over any roughness which may be 

 accidentally contacted inside the float well. Either the 8i/2-inch 

 float of the standard automatic gage or the 314-inch float of the portable 

 gage may be used for the purpose, but the larger float is to be preferred 

 as its plane of flotation is more stable and less sensitive to change from 

 frictional resistance in the operation of the gage. 



14. When a tape gage is used, a distinction must be made between 

 a visible scale zero and the true datum of the gage to which the ob- 

 served heights are referred. The tape gage datum may be defined 

 as the level of the water surface at the time the gage reading is zero. 

 This will obviously be a number of feet lower than any visible scale 

 zero inside the tide house. For the installation of a tape gage and 

 the determination of its datum see page 29. 



PIPE GAGE 



15. The pipe gage is a type of float gage which has been used on 

 offshore shoals in a depth of 30 feet of water. This gage consists of 

 a white-pine rod staff, cross section 1 by 1 inch, with rounded edges, 

 graduated on each of the four sides in feet and two-tenths with the 

 zero (0) at the top, and set in a hollow cylindrical white-pine float 

 iy2 inches outside diameter and y^-mch inside diameter. The float 

 should be thoroughly covered with shellac and liquid paraffin. The 

 length of the rod will depend upon the range of tide in the locality 

 where it is to be used and the length of the float should be about 

 four-tenths that of the rod. The float well consists of a 2-inch iron 

 pipe, the bottom of which is set in a 1,000-pound concrete block to 

 serve as an anchor. The pipe should be long enough to reach above 

 the ordinary waves at high tide and a 14-inch hole should be drilled 

 in the pipe several feet above the concrete anchor. A cap with a 

 square hole through which the staff passes, is screwed on top of the 

 pipe after the. float staff has been placed inside. Just below the cap 

 a 2-inch flange for the attachment of guy wires is screwed on the pipe 

 and four small sheaves, one for each guy wire, secured to this flange 

 by wire loops. The top of the float pipe is secured in a vertical posi- 

 tion by four guy wires of No. 6 wire with leads making an angle of 60° 

 or more with the vertical. The end of each guy wire is anchored to 



