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The corresponding oscillations of a deep lake or arm of the sea are 

 more complicated, but those observed at any locality often have one 

 or more fairly well defined periods. In a deep narrow lake with a 

 regular shore line, the observed period of the oscillations usually 

 agrees with the computed period for a canal of equivalent dimensions, 

 closed at both ends. Even when the conformation of the bed and 

 shores of the lake is not so regular, the observed periods may agree 

 with those computed for an equivalent canal extending to a selected 

 point on an opposite shore; but as the selection of the point must be 

 based on the known period, the analogy does not serve much useful 

 purpose. 



Seiches are damped out by the frictional resistance of the currents 

 which they produce. For an oscillation of a given amplitude these 

 currents decrease as the depth of the lake increases. Seiches there- 

 fore are most marked in deep bodies of water. At many localities on 

 the Great Lakes, seiches of a foot or more occur with such frequency 

 that an allowance is made for them in the design of navigation channels. 



Seiches in tidal waters are superimposed upon, and more or less 

 obscured by, the fluctuations of the tide; but the tide gage records at 

 some stations occasionally are marked by saw-toothed irregularities 

 produced by seiches. Their characteristics may be ascertained by 

 taking off their departures from a smooth tide curve. In San Fran- 

 cisco Bay, seiches produced by variations in the barometric pressure 

 and winds have a period of about 45 minutes, and may have a range 

 of as much as a third of a foot. It has been observed that earthquake 

 waves reaching the bay from distant points set up oscillations of the 

 same period. 



At some of the ports on the Pacific coast in California, the currents 

 set up by seiches often cause a surging of large vessels at wharves, 

 sometimes with sufficient violence to break the mooring lines. The 

 surge is experienced chiefly at the wharves in the less enclosed parts 

 of the harbors. The reason for the prevalence of a troublesome surge 

 in this region is not clear. It is possible that the characteristic 

 periods of the seiches may agree with the period in which a vessel, as 

 customarily moored to a wharf, comes and goes with the stretching 

 and slackening of its lines. The usual remedy is to make fast to the 

 wharf with short, taut lines. 



