MODIFICA TION F THE (j R K. | T /. . i kES 239 



water, (listurl)ing its equililiriuni, and the i)assage of a thunder- 

 storm often produces oscillatit)ns attracting the attention of even 

 the casual oliserver. Sucli sudden and temporary variations of 

 l)ressure give rise to waves analogous to those caused l)y a fall- 

 ing pebble, except that they are broad and low. and these waves 

 not only travel to all parts of alakel)ut are continued l)y rellee- 

 tion, so that a local storm at one point is felt in the water surface 

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

 greater 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 an}' 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. 



'J'he lakes, like tlie 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, l)Ut they 

 are still measural)le. At Milwaukee the lunar tide rises and falls 

 more than an inch and the solar tide a half inch. At Ciiicago 

 and Duluth each tide amounts to an inch and a half, and their 

 combination at nmv and full moon to tliree inches. 



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

 but the rate is not uniform. Usually a few freshets, occurring 

 witiiin two or tiiree 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 overfloAV, 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 Su{)erior 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 .lamiary or 

 February for all the lakes except Superior, where it occurs in 

 March. Ili'di water is reacbt sooner in the lower lakes. .luno 



