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; THOMPSON'S COAST PILOT. 135 vations at short intervals, say 'three 'to five minutes of time apart, from about an hour before to an hour after the actual time of low water. "In conclusion, we offer the above observations as solving the problem in question, and as proving the existence of a semi- diurnal lunar tidal wave on Lake Michigan, and consequently on the other great fresh water lakes of North America, whose co-ordinate of altitude is, at its summit, as much as .15 to .25 (7s to 85) of a foot, United States measure." REMARKABLE PHENOMENON. Prof. Mather, who observed the barometer at Fort Wilkins) Copper Harbor, 47° 30' north lat., during the prevalence of one of these remarkable disturbances which are peculiar to all the _ Upper lakes, remarks: "As a general thing, fluctuations in the barometer accompanied the fluctuations in the level of the water, but sometimes the water level varied rapidly in the har- bor, while no such variation occurred in the barometer at the place of observation. The variation in the level of the water. may be caused by varied barometric pressure of the air on the water, either at the place of observation or at some distant point. A'local increased pressure of the atmosphere at the place of observation, would lower the water level where there is a wide expanse of water; 'or a diminished pressure, under the same circumstances, would cause the water to rise above its usual level." | In the summer of 1854, according to the report of Foster and Whitney, made to Congress in 1850, "an extraordinary, retrocession of the waters took place at the Sault Ste. Marie. The river here is nearly a mile in width, and the depth of water over the sandstone is about three feet. 'The phenomenon occurred at noon; the day was calm but cloudy; the water retired suddenly, leaving the bed of the river bare, except for the distance of about twenty rods where the*channel is deep- est, and remained so for the space of an hour. Persons went out and caught fish in the pools formed in the rocky cavities.