32 



U. S. COAST AND GEODETIC SURVEY 



computed the following factors which when applied to the difference 

 {R—R') will give the necessary correction to the plane of flotation: 



Steel tape with counterpoise on movable pulley 



Steel tape with counterpoise on end of tape 



Phosphor-bronze tape with counterpoise on movable 

 pulley 



Phosphor-bronze tape with counterpoise on end of tape. 



8>4-inch 

 float 



0. 00042 

 . 00056 



. 00065 

 . 00087 



3>^-inch 

 float 



0. 0029 

 .0039 



.0045 

 .0060 



As the correction varies directly as the weight of the tape and 

 inversely as the square of the diameter of the float, corresponding 

 corrections for floats of other sizes or for tapes of different weight 

 may be readily derived from the above values. For example, the 

 correcting factor for a 9i^-inch float with steel tape and movable 

 pulley is 



0.00042 X (81/2)7(91/2)' = 0.00034. 



INSTALLATION OF STANDARD AUTOMATIC TIDE GAGE 



87. Preparatory to the installation of the tide gage, the instrument 

 should be carefully checked to see that all parts are in satisfactory 

 working order. The scale of the gage and the corresponding float 

 drum will depend upon the extreme range of tide at the locality and 

 will be selected in accordance with the table in paragraph 39. Spe- 

 cial attention must be given to the pencil screw to see that the pencil 

 arm moves freely along its entire length and that the arm is properly 

 released on reaching each end of the screw thread and automatically 

 returned to the thread through the action of the return springs when 

 the direction of rotation is reversed. In the latest type of gage, the 

 pencil screw may be removed from the gage and rotated by hand 

 while the pencil arm is hanging freely by its own weight. Any 

 tendency for the arm to swing upward with the rotation of the screw 

 should be investigated and steps taken to remove the cause of any 

 sticking until all perceptible resistance to a free movement of the 

 arm along the screw has been eliminated. Special attention should 

 also be given to the hour-marking device to see that it is functioning 

 satisfactorily and such adjustments are made as may be necessary. 

 (See pars. 127-128.) 



88. Setting up automatic tide gage. — The standard installation, 

 in which both counterpoise and tension weights are supported by 

 movable pulleys, is illustrated in figure 17. When practicable, the 

 gage is to be placed on a table or shelf over the float well so that the 

 float may be suspended directly from its drum without any interven- 

 ing pulleys. By means of a plumb line or the temporary installation 

 of the float, the position of the gage should then be adjusted to 

 center the float in the well. In doing this, consideration must be 

 given to the possibility that the well may not be exactly plumb and 

 the centering should be done at the water level rather than at the top 

 of the well. 



