PHYSICS. 461 



face of the whole of Lake Michigan-Huron by 0.20 foot or more for a 

 whole day on eight different occasions during the eight months of 1910 

 and 1911 which have been studied carefully. The extreme case occurred 

 on July 25, 1911, when the disturbance of elevation, in the sense just 

 defined, averaged for the day 0.30 foot, the water surface at Milwaukee 

 being below the mean elevation of the whole lake surface. 



The barometric effects have probably been mistakenly identified as 

 wind effects in frequent cases in the past. 



The constants of the formulae for barometric effects must be derived 

 separately for each gage station. The general method of determining 

 the constants which has been developed is, however, applicable on any 

 body of water in the world. 



The formula for the wind effects, and also the constant in that for- 

 mula as derived from the observations, are both of general application 

 on any free water-surface, on any lake, on the open ocean, or on any 

 gulf, bay, or river. 



The maximum wind effect found at any of the five stations, Buffalo, 

 Cleveland, Milwaukee, Harbor Beach, and Mackinaw, during the 

 period covered by the investigation was 1.00 foot at Buffalo on Octo- 

 ber 27, 1910, for a single hour at 10 a. m., when the wind was 53 miles 

 per hour from the southwest. The maximum wind effect found at 

 Cleveland was 0.19 foot at 7 a. m. of that same day. The wind effects 

 at the three gage stations mentioned on Lake Michigan-Huron were 

 found to be very small — as a rule less than 0.01 foot. The maximum 

 at these three stations within the limits of the investigation was 

 0.03 foot. It is clear that the wind effects are much greater than this 

 in some of the shallow bays tributary to this lake. In general, wind 

 effects in the main portions of Lake Michigan-Huron are very much 

 smaller than on Lake Erie. The contrast arises from the fact, now well 

 demonstrated, that the wind eff"ects are inversely proportional to the 

 cube of the depth of the water. The average depth of Lake Michigan- 

 Huron is much greater than that of Lake Erie. 



It is now clear, from the investigations, that the application of cor- 

 rections for barometric effects and wind effects greatly increases the 

 accuracy with which the elevation of the mean surface of a lake may 

 be determined from the observations at a given gage. For example, 

 the mean elevation of the whole surface of Lake Erie may now be de- 

 termined as accurately from a single day of observation at Buffalo, 

 corrected for barometric and wind effects, as it could formerly be 

 determined from 16 days of uncorrected observations at that gage. 

 Buffalo is a place of very large wind effects and moderately large 

 barometric effects. Even at Milwaukee, where the wind effects are 

 almost negligible, the increase in accuracy gained by correcting for 

 barometric and wind effects is sufficient to insure that the mean ele- 

 vation of the whole surface of Lake Michigan-Huron may be deter- 



