SECONDARY UNDULATIONS OF OCEANIC TIDES. 



Fil 



completely filled with water, the cock ^ R^ is closed. As the 

 level of the sea changes, the iiiercury in the vessel B moves up 

 or down ; if it is so arranged tliat the np and down motions 

 of the mercury are recorded on a vertical cylinder rotating with 

 a definite rate about its vertical axis, then the tide is traced on 

 the cylinder on a reduced scale. The cyhnder (20 cm. high and 

 9.4 cm. in diameter) usually rotates once per day ; it can also 

 be made to rotate once in two hours. Tlie cock 11^ serves for 

 damping the rapid up and down motions of mercury due to the 

 surface waves. 



The recording arrangement of the tide-gauge was, in our 

 case, modified in, tlic following way (Fig. 2). The fioat made 



of a hollow ebonite box loosely fitted 

 the vessel B, and on the box, a thin 

 aluminium rod was vertically erected. 

 A pen-holder p, which carried two arms 

 perpendicular to the ]jeii, was horizon- 

 tally fixed to the rod. At each end of 

 these arms, a friction wheel was fixed, 

 which rolled between the A^- shaped 

 grooves of two vertical guides gg. 



In this way, the pen was constrain- 

 ed to move in a vertical line ; though 

 the pen was slightly pressed against 

 the recording cylinder E by a weak 

 spring, its friction was (piite insensible. 

 The photographs of the whole apparatus and the recording })en 

 are given in the frontispiece. Photo. Xo. 1 and 2. 



If //j, ]}., and //., be the heights of tlie levels of sea water 

 and of mercury in A and B measured from the bottom of the 



9 g 



