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BS fp eee Re ONDE sea AGREE a neg aURD IM Be Nan ee eRe ORG ena SE 120 THOMPSON'S COAST PILOT, diurnal lunar spring tide would be shown of as much as one-third of a foot for the periods of highest. tides. "The time of low water and the relative times of duration of the flood and ebb tides are given only approximately. 'The extreme rise of the tide being so little, the precise time of the change from ebb to flood, and hence the duration of the flow of each, can only be accurately determined by numerous observations at short inter vals, say three to five minutes of time apart, from about an hour be- fore to an hour after the actual time of low water. «In conclusion, we offer the above observations as solving the problem in question, aid as proving the existence ofa semi-diurnal lunar tidal wave on Lake Michigan, and consequently on the other great fresh water lakes of North America, whose co-ordinate of alti- tude is, at its summit, as much as .15 to .25 (4 to 98,) of a foot, United States measure."' | Remarkable Phenomena, Prof. Mather, who observed the barometer at Fort Wilkins, Cop- per 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 ac- companied the fluctuations in the level of the water, but sometimes the water level varied rapidly in the harbor, while no such variation occurred in the barometer at the place of observation. 'The varia- tion 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 atmos- phere 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