CHAPTER II. 



MOVEMENTS OF LAKE WATERS AND THE GEOLOGICAL 

 FUNCTIONS OF LAKES. 



Tides. — The waters of lakes are influenced by the attraction of the 

 sun and moon in the same manner as the waters of the ocean. Owing 

 to the comparatively small extent of inland water-bodies, however, the 

 rise of their waters is so small that it is not noticeable, and can only be 

 determined by refined measurements. 



Observations made by the U. S. Lake Survey at Chicago, have shown 

 that Lake iNlichigan has a tide with an amplitude of 1^ inches for the 

 neap tide and 3 inches for the spring tide. 



Waves and currents. — The waters of fresh lakes respond to the 

 influences of the wind more quickly than the heavier waters of the ocean, 

 but the waves produced are smaller and less regular than in the open sea. 

 On the Laurentian lakes, waves from 15 to 18 feet in amplitude have 

 been observed during long continued storms. The heavy ground swell 

 of the ocean is but faintly reproduced by the fresh water "seas." During 

 rough weather on the lakes the waves are more like the short, " chojD 

 seas " than the heavy surges of the open ocean. 



The friction of the wind on the surfaces of lakes produces very decided 

 movements in their waters. In their central portions, especially, there are 

 frequently strong currents due to this cause, in addition to the sIoav 

 movement of the waters toward an outlet. A study of the currents of 

 the Laurentian lakes has been undertaken by the United States Weather 

 Bureau, by means of l)ottles containing a record of the locality where 

 they were set adrift and a request that the finder will note the locality 

 Avhere they are recovered and transmit the record to the Chief of the 

 Weather Bureau. The results of observations made in the sununer season 

 of 1892 and 1893, have been published,^ and the general courses of the 

 currents so far as ascertained, indicated on a chai't wliich is reproduced 

 on Plate 7. The effects of the prevailing westerly winds on the surface 

 movement of certain of tlie Laurentian lakes, is indicated by the trend 



^ U. S. DciKirtinent of Agricultun-, Wcutlicr IUnv;ui, Hullctiii B. 



