MOVEMENTS OF LAKE WATERS. 35 



stasfe. When the shores of a lake are h)w, broad areas are inundated 

 during storms tliat sweep the waters towards them. New outlets may 

 be established at such times across low divides, and lead to important 

 changes in drainage. 



Seiche. — Lake waters are also sensitive to changes in atmospheric 

 pressure. In some instances variations of level during calm weather, 

 amounting to several feet, have been observed, and are supposed to l)e due 

 to sudden changes in barometric pressure on different portions of the 

 water surface. Besides these larger movements, which can be correlated 

 with atmospheric changes, and are known as seiches,^ there are certain 

 rhythmical pulsations producing a difference of level of as much as four 

 or five inches during calms, when no variation in atmospheric pressure of 

 an analogous character can be detected. These minor movements are 

 not thoroughly understood. 



These and other changes of a similar nature are of great interest in 

 connection with meteorological studies, but have little if any geological 

 significance. 



It is to be expected that earthquakes Avould produce " tidal waves " in 

 lakes similar to those occurring in the ocean; but observations in this 

 connection are wanting. 



Temperature. — Lake waters are warmed by the sun's rays and by 

 contact with the air. It has also been thought by some that very deep 

 lakes may have their bottom temperatures modified by the general internal 

 heat of the earth, but observations do not seem to support this conclu- 

 sion. Water is a poor radiator and an indiffei'ent conductor of heat, and 

 does not respond to atmospheric changes of temperature as quickly as do 

 rock surfaces. Shallow lakes are warmed throughout b}^ the summer's 

 heat and chilled to the bottom by the winter's ct)l(l ; but their tempera- 

 ture is mucli more uniform than that of the adjacent air. The shal- 

 low lakes of the Northern states liave been found to liave a nearly 

 uniform temperature during the summer months of 75° Fahrenheit. In 

 winter their temperature in general is, of course, 32° Fahrenheit. In 

 lakes having a dei)th in excess of about 800 feet, more interesting condi- 

 tions are found. The temperatures of deep lakes are ascertained by means 

 of self-registering tliermometers attached to souiuling lines. In this 



1 E. A. Perkins. "The Seiche in American Lakes," American Meteorological Jcmr., 

 Oct., 1893. 



