Principal Features of Tidal Phenomena 



251 



driven roller carrying a sheet of paper. The paper is driven ahead about one 

 inch per hour, and the device thus traces the marigram automatically. Suit- 

 able reduction is obtained by varying the size of the pulley and the pitch of 

 the threaded rod. An accurate clock makes a special mark every hour, and 

 a fixed pencil traces a reference line. Short-period waves are largely eliminated 



Fig. 106. Construction of a well housing a tide gauge recorder. 



because of the damping in the well, but seiches and disturbances of the sea 

 surface lasting several minutes or more are recorded. From the marigram 

 the hourly heights and the levels and times of high and low water are easily 

 read off. The standard gauge carries enough paper for one month, but the 

 clocks must be wound once a week, and each day the instrument must be 

 checked against a staff or tape gauge to ascertain whether it is functioning 

 properly and to make sure that the holes in the float well are free from seaweed 

 and other detritus. A portable recording unit that can be used by field parties 

 is operated on the same principle. 



The devices so far described can be used only on shore or where some 

 rigid structure extends above the sea surface. Many types of pressure-record- 

 ing devices for use on the sea bottom have been designed. In some the pres- 

 sure element only is placed below the sea surface, with the recording device 

 on land; in others, such as those to be used far from shore, the recording 

 device is an integral part of the^ instrument, which may be placed on the sea 

 floor or anchored and left in position for a week or more. 



A very comprehensive description of high-sea tide recorders can be found 

 in Rauschelbach (1932, pp. 73-129; 1934, p. 177) and Geissler (1939, 

 p. 12; 1941). 



Bartels (1926, pp. 221 and 270) has drawn attention to the fact that 

 in the tropics, it is possible to derive the semi-diurnal lunar tide M 2 of the 



