MANUAL OF TIDE OBSERVATIONS 



35 



weight and supporting pulley. The extreme limit, however, will sel- 

 dom be required, and a ceiling height of 6^/2 to 7 feet will in general 

 be ample for the operation of the gage. On the open coast the mid- 

 extreme tide may be taken as approximately the same as mean sea 

 level, but in rivers the height above mean river level reached by the 

 extreme high waters is usually much greater than the depth to which 

 the extreme low waters fall. However, in estimating the mid-extreme 

 tide for the adjustment of the gage, no consideration need be given to 

 a high-water height that is above the bottom of the tide gage or to a 

 low water that is below the bottom of the float well, since beyond these 

 limits the gage would cease to function regardless of adjustment. 

 (See par. 119.) 



91. Attaching counterpoise. — The weight of the counterpoise de- 

 pends upon the size of the float drum and is to be selected in accordance 

 with the table in paragraph 89. In the standard installation the 

 counterpoise is used with a movable pulley, but if it is attached directly 

 to the end of the counterpoise wire, its weight should be one-half the 

 tabular value given for the combined counterpoise and pulley. Before 

 installing the counterpoise, the pencil screw should be removed from 

 the gage (par. 29) and the float drum released (par. 34) so that this 

 drum and the counterpoise drum may be turned independently of each 

 other. For the older type of instrument which does not permit the 

 easy removal of the pencil screw, this screw should be rotated until 

 the pencil arm has cleared the screw thread at the end nearest the 

 clock case. Following the scheme of installation illustrated in figure 

 17, the necessary fixed pulleys being secured to the ceiling of the tide 

 house and the counterpoise fastened to a movable pulley, the wire 

 (par. 35) is passed through the fixed and movable pulleys and one end 

 attached to a screw eye provided for the purpose in the ceiling of the 

 tide house. The wire is then cut of such length that the counterpoise 

 will hang just clear of the floor as the free end of the wire is attached 

 to the counterpoise drum. A small hole or clamp near the edge of 

 the drum is provided for this attachment. 



92. Attaching float. — The proper float drum for the range of tide 

 having been selected from the table in paragraph 39, the length of 

 wire to be wound upon this drum at the time of installation will de- 

 pend upon the stage of the tide. Let L equal length of wire to be 

 wound upon the drum at time of installation, G the length of wire 

 necessary to half fill the drum, and H the height of tide at time of 

 installation referred to the mid-extreme tide level, H being negative 

 if water surface is below this level, then L = C+H for the required 

 length. 



The value for G for drums 1% and I34 inches wide may be taken 

 from the following table : 



Circumference of float drum in 

 inches 



Width of drum: 

 l/s------ 



m 



Feet 



4>^ 

 7 



Feet 



10^2 



12 



Feet 

 9 

 14 



16 



Feet 

 12 



18^3 



24 



Feet 



18 

 28 



