120 EFFECTS OF WINDS AND OF 



a continuous and unusually large oscillation at the Mackinaw gage, of which 

 the apparent period between successive maxima or minima was either 

 6, 7, or 8 hours in more than one-third of the cases. The mean period of 

 oscillation for these 6 days was found to be 6.8 hours. 



The computation of the theoretical period of oscillation through the 

 Strait of Mackinac needs confirmation, and especially needs a careful study 

 of the possible errors due to certain necessary assumptions made in the 

 computations. So, too, the evidence from the necessarily cursory examina- 

 tion of the graph of hourly elevations at Mackinaw is weak. Nevertheless, 

 the investigator reached the conclusion, subject to the reservations implied 

 in the preceding two sentences, that there is probably a seiche through the 

 Strait of Mackinac of the character indicated, with a nodal line near the 

 mid-length of the narrowest part of the Strait, with a period of about 7 hours. 



It should be noted, in closing this general statement in regard to seiches, 

 that as this investigation was based on observed hourly elevations of the 

 water surface it is incapable of detecting seiches having periods less than 

 1 hour. 



The present investigator feels that there is a very interesting field open 

 for further study of the seiches in the Great Lakes, and that rapid progress 

 is possible in this field, which will lead to a much better knowledge than is 

 now available in regard to seiches and tidal oscillations. But he finds, with 

 regret, that he must turn aside, after his little excursion into the field, to 

 carry forward other lines of investigation and of work to which he has 

 already committed himself. 



EXAMPLES OF SEICHES. 



Plates 14, 15, and 16 show some examples of seiches. All of these plates 

 are drawn to the same scale. On each of them four graphs plotted from 

 hourly ordinates are shown. 



The observed hourly elevations of the water surface are shown by the dot- 

 and-dash graph. The elevation of the water surface at each hour referred 

 to mean sea-level may be read in feet from the scale of numbers at the right 

 margin of the plate. 



The barometric effect at each hour is shown by the dotted graph. Its 

 amount in feet may be read from the scale of numbers at the left margin of 

 the plate. 



The wind effect at each hour is shown by the dashed graph. Its amount 

 in feet may be read from the scale of numbers at the left margin of the plate. 



The elevation of the water surface at each hour after correction for baro- 

 metric effect and for wind effect is shown by the continuous graph. The 

 corrected elevations referred to mean sea-level may be read from the scale 

 of numbers at the right margin of the plate. Each ordinate on this graph 

 is that of the first graph (observed elevations) diminished algebraically by 

 the ordinates of the second (barometric) and third (wind) graphs. This 



