BAROMETRIC PRESSURES ON THE GREAT LAKES 



115 



A frequency table was constructed from the observed periods referred to 

 in the preceding paragraph. It is shown below as table No. 29. 



TABLE No. 29. Frequency distribution of observed periods of oscillation at Buffalo. 



161 periods in first group, mean period = 3. 7 hours. 

 10 periods in third group, mean period = 13.0 hours. 

 Total number of periods 196. 



From a study of this table, combined with a study of the graphs, it ap- 

 peared that probably the 161 periods included in the first part of the table 

 were all disturbed cases of one prevailing seiche period between 3 and 4 

 hours in length. Accordingly, the mean of these 161 peroids, namely, 3.7 

 hours, was taken as a best first approximation to the period of a prevailing 

 seiche at Buffalo. 



The periods involving half hours, such as 2.5, 3.5, 4.5, etc., arose from 

 the fact that where two successive hourly ordinates at a maximum or a 

 minimum on the graph were equal, the time of said maximum or minimum 

 was identified as being at the half hour midway between the two hours. 

 Naturally, there were relatively few of these cases as compared with those 

 in which the observed period was an integral number of hours. The fre- 

 quency-distribution table, No. 29, should therefore be scanned as if these 

 periods involving half hours were distributed equally to the adjacent periods 

 which are in integral hours. The table so scanned shows a maximum fre- 

 quency at 4 hours. 



A study of the graphs showed several cases in which an apparent seiche 

 with a period of about 13 hours stood out clearly with relatively little com- 

 plication by anything else. With this as a clue, it was decided that possibly 

 the 10 periods included in the third part of the table were all disturbed 

 cases of a seiche with a period of about 13 hours. Accordingly, the mean 

 of these 10 periods, namely, 13.0 hours, was taken as a best first approxi- 

 mation to the period of a prevailing seiche at Buffalo. 



Next, the 25 cases in the middle of the frequency-distribution table, No. 



