354 MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT. 



A column of mercury 0.1 inch high weighs as much as a column of water 

 1.3 inches high; and whenever the atmospheric pressure at one point 

 on a lake exceeds the pressure at another point by the tenth of a baro- 

 metric inch, the water level at the first point is, in consequence, 1.3 

 inches lower than the water level at the second point. When a cumulus 

 cloud forms over the water, there is a reaction on the water, disturbing 

 its equilibrium, and the passage of a thunderstorm often produces oscil- 

 lations attracting the attention of even the casual observer. Such sud- 

 den and temporary variations of pressure give rise to waves analogous 

 to those caused by a falling pebble, except that they are broad and low, 

 and these waves not only travel to all parts of a lake, but are continued 

 by reflection, so that a local storm at one point is felt in the water sur- 

 face at all points and for a considerable period. The passage of the 

 great atmospheric waves associated with ordinary cyclonic storms and 

 the impulses given by winds are also able to set the whole body of 

 the lake in motion, so that it sways from side to side or end to end 

 like the swaying water in a tub or basin, and these swaying motions 

 are of indefinite continuance. In the deeper lakes, and probably in all 

 the lakes, they are so enduring as to bridge over the intervals from 

 impulse to impulse. Such oscillations, which appear at any point on 

 the coast as alternate risings and fallings of the water, with periods 

 ranging from a few minutes to several hours, are called seiches. Their 

 amplitude is usually a few inches, but at the ends of lakes is sometimes 

 a foot or more. 



The lakes, like the ocean, are swayed by the attractions of the sun 

 and moon. Their tides are much smaller than those of the ocean, and 

 are even small as compared to the seiches, but they are still measurable. 

 At Milwaukee the lunar tide rises and falls more than an inch and the 

 solar tide a half inch. At Chicago and Duluth each tide amounts to an 

 inch and a half, and their combination at new and full moon to 3 inches. 



Water is continually added to each lake by rivers and creeks, but 

 the rate is not uniform. Usually a few freshets, occurring within two or 

 three weeks, contribute more water than comes during all the remainder 

 of the year. Water is also added in an irregular way by rain and snow 

 falling directly on the lake. It is subtracted by evaporation, the rate 

 of which varies greatly, and by overflow, which varies within moderate 

 limits. The volume of water contained in the lake, being subject to 

 these variable gains and losses, is itself inconstant, and the general 

 height of the water surface therefore oscillates. In average years the 

 range of variation for Lake Superior is 12 inches; for Lakes Michigan 

 and Huron, 12 inches ; for Lake Erie, 14 inches, and for Lake Ontario, 

 17 inches. Low water occurs normally in January or February for 

 all the lakes except Superior, where it occurs in March. High water 

 is reached sooner in the lower lakes, June being the usual month for 

 Ontario, June or July for Erie, July for Michigan and Huron, and 

 August or September for Superior. Figure 4 shows the character of 

 the annual oscillations, as given by averages of long series of years. 



