114 EFFECTS OF WINDS AND OF 



SEICHES. 



A seiche is an oscillation in the waters of the lake under the influence of 

 inertia. It is a free oscillation as distinguished from a forced oscillation. 

 It is a wave motion involving both horizontal transfer of water back and 

 forth and a vertical oscillation of the water surface. 



The seiches of Lake Erie, and to a lesser extent of Lake Michigan-Huron, 

 were studied in this investigation because it became evident that they are 

 one of the principal sources of error. It seemed probable that the more 

 clearly the seiches were understood the more effectually could one guard 

 against the occasional abnormally large errors arising from this source. 



The seiches were studied principally in three ways : 



They were studied by graphs such as are shown on plates 7 to 16. These 

 graphs aided especially in finding the relation of a given seiche to the impulse 

 which started it and in indicating how rapidly the oscillation died out and 

 under what conditions. 



The seiches were studied by statistical methods, mainly for the purpose of 

 determining the prevailing periods of oscillation. 



The seiches were studied by computing the theoretical period of oscil- 

 lation of the lake, or of certain parts of it, from the known dimensions of the 

 lake, horizontal distances and depths. This served to establish the probable 

 manner of oscillation by identifying an already determined period as per- 

 taining to a particular manner of oscillation. 



SEICHES AT BUFFALO. 



At Buffalo 22 days of hourly elevations of the water surface observed 

 at the gage were studied in detail for seiches. These particular 22 days had 

 been especially selected to include periods of unusually large barometric 

 effects and wind effects, especially the latter. It was to be presumed, there- 

 fore, that these days included the impulses which started many new seiches 

 of unusual size. 



From the observed hourly elevations corrected for wind effects and baro- 

 metric effects, according to best evidence then available, and the correspond- 

 ing graphs which had been drawn for all 22 days, a table was made showing 

 every maximum point and every minimum point. The intervals of time 

 from each maximum to the next following maximum and from each mini- 

 mum to the next following minimum were also tabulated. Each of these 

 intervals is the period of one of the complete oscillations which actually took 

 place about the equilibrium points instantaneously fixed by the wind and 

 barometric effects. Each of these oscillations is probably in general a 

 composite of two or more seiches of different periods and of the forced 

 oscillation produced by the changing winds and changing barometric con- 

 ditions during that time interval. The immediate problem which confronted 

 the investigator was to detect from a study of 22 days of these composites 

 the seiche periods which prevail at Buffalo. 



