30 U. S. COAST AND GEODETIC SURVEY 



tape attached to an 8i/^-inch float operating in a 12-inch float well. 

 The reading mark should be a well-defined line on a board just 

 back of the tape and from 4I/2 to 5 feet above the floor for convenient 

 reading by the observer. This reading line may be engraved on a 

 metal plate screwed fast to a board or it may be defined by the 

 slot in the head of a brass screw placed in the board close to the 

 tape. The counterpoise of the tape gage should be attached to a 

 movable pulley to increase the range of operation otherwise limited 

 by the height of the tide house ceiling. 



80. To facilitate the determination of the plane of flotation and 

 also to provide greater durability, it is recommended that the bottom 

 portion of the tape which remains below the wharf floor during all 

 stages of the tide be a detachable piece of phosphor-bronze tape. 

 The work of determining the plane of flotation will be further 

 facilitated by installing a secondary reading board about 18 inches 

 above the level of the floor with a horizontal line from the tape to 

 a distance a little greater than the radius of the float. 



81. The float well for the tape gage may be similar to the one used 

 for the automatic gage (page 26). The size of the intake should not 

 be less than that indicated for the automatic gage and must be large 

 enough to show a perceptible motion in the tape when the outside 

 water is moderately rough. When installing the float special care 

 must be taken to arrange the supporting pulley so that the float will 

 swing clear of the sides of the well at all stages of the tide. 



82. Determination of plane of flotation. — To establish directly the 

 relation of the datum of the tape gage to bench marks it is necessary 

 to determine the plane of flotation of the tape gage float under nor- 

 mal operating conditions. This is accomplished by placing the float 

 in a pan or bucket of water of the same density as that in the float 

 well and measuring the distance from the water surface to some 

 graduation of the tape. The pan is placed over the top of the float 

 well and the float must be connected with its counterpoise. In order 

 that the counterpoise may swing clear of the floor, surplus tape 

 may be folded into several loops, avoiding sharp bends, and tied 

 together with a cord. If the tape contains a detachable portion, 

 this may be removed and placed in a coil on top of the float in order 

 that its weight may be included in the determination of the plane 

 of flotation. The length of the removed section must afterwards be 

 taken into account in obtaining the relation of the plane to the 

 scale graduations. 



83. The float must be moved slowly up and down and allowed to 

 come to rest from both the upward and downward movement to 

 ascertain any difference in tape reading due to friction in the sup- 

 porting pulleys. If any difference is noted the float should be 

 placed with the mean of the two tape readings at the reading mark. 

 Careful measurements should now be taken of the vertical distance 

 from the water surface in the pan to some graduation on the tape, 

 which requires that the line of the tape graduation be extended 

 horizontally over the float to a point above the water surface. This 

 may be conveniently done by having a horizontal line drawn on the 

 reading board or on a similar board arranged closer to the floor of 



