MANUAL OF TIDE OBSERVATIONS 5 



concrete blocks, giving a total weight of about 2,000 pounds to each 

 anchor. For convenience in handling each concrete block may be 

 cast with wire rope loops projecting. After the anchors have been 

 set the guy wires are led through the sheaves at the top of the float 

 pipe and drawn taut, a fence-wire stretcher being convenient for this 

 purpose. 



PRESSURE GAGE 



16. The pressure gage operates by measuring the variation in pres- 

 sure at the bottom of a body of water due to the rise and fall of the 

 tide. Although not as satisfactory as the types of gages usually 

 employed for tide observations close to shore, pressure gages have 

 been used with some success for observations taken on shoals at some 

 distance from land where it has been impracticable to install the usual 

 type of gage. 



FATHOMETER 



17. The fathometer is primarily an instrument for measuring the 

 depth of water but, since this depth varies with the rise and fall of the 

 tide, the instrument also has been used with some success for observing 

 tides from a ship at anchor offshore. The measurements depend 

 upon the interval of time required for a sound wave to travel from 

 the ship to the bottom of the ocean and for its echo to return. A 

 full description of the instrument will be found in the Coast and 

 Geodetic Survey Hydrographic Manual. When using the fathometer 

 as a tide gage, the ship should be anchored where the ocean bottom 

 is near level as possible, so that there will be no material change in 

 depth due to the swinging of the vessel. The anchorage should be 

 buoyed and the observations repeated on different days to detect irreg- 

 ularities due to causes other than the tide. The instrument might also 

 be operated with the transceiver anchored to the bottom of the ocean, 

 in which case the sound wave first travels upward and is then reflected 

 from the water surface back tq the transceiver. By this method the 

 tide record would not be affected by irregularities in the ocean bottom 

 at the ship's anchorage. 



STANDARD AUTOMATIC TIDE GAGE 



18. The present standard automatic tide gage used by the Coast 

 and Geodetic Survey is a development of the Stierle gage adopted by 

 this Survey many years ago. A float operates in a vertical box or 

 pipe to which the slow moving tide has free access while the more 

 rapid moving waves resulting from winds are largely damped out by 

 the relatively small size of the inlet to the box. The rising and falling 

 of the float operates a worm screw on the gage which moves a pencil 

 to and fro across a wide strip of paper which is moved forward by 

 clockwork. The combined motion of pencil and paper gives a con- 

 tinuous graph showinfi: the rise and fall of the tide. 



