I\ TIIK NORTH AMERICAN LAKES. 



11 



roiitimicd giilr from the southeast during the entire doy. At 7^ A. M. tlie water 

 was low. At ,1'. P. M. it was still lower, being at nine feet nine inches, rising 

 in the space of three hours to thirteen feet ten inches, a change of four feet one 

 inch. 



The highest monthly average is that of September, 1856, when the mean depth 

 in the canal was thirteen feet four and ^Vo inches. During the season of navi- 

 gation the water of Lake Superior is higher than during the winter months, but a 

 fall of four feet in the general surface of the Lake below the highest known state 

 would reduce the canal depth in September to nine feet four inches, and might 

 interfere with the passage of large craft. 



At present the shallowest parts of the St. Mary's river are less than nine feet, 

 but the canal was intended to have a depth of water of never less than twelve feet! 



Tins is an instance of the importance of Lake registers to engineers and those 

 engaged in improving navigation. 



