MOVEMENTS OF LAKE WATERS. 37 



cooled to 4°, it tends to sink. Any furtlier rise of tlie temperature of tlie surface-water 

 during the warm season, or fall of temperature during the cold season, alike produces 

 expansion, and thus causes it to float on the heavier water below ; so that water at 

 4° C. perpetually remains at the bottom, while the varying temperature of the seasons 

 and the penetration of the solar heat only influence a surface stratum of about '250 

 meters in thickness. It is evident that tlie continual outflow of water from its shallow 

 outlet cannot disturb the mass of liquid occupying the deeper portions of the lake. It 

 thus results that the temperature of the surface-stratum of such bodies of fresh water 

 for a certain depth fluctuates with the climate and with the seasons ; but at the bottom 

 of dee}) lakes it undergoes little or no change throughout the j'ear, and approaches to 

 that wliic'h cori'esponds to the maximum density of fresh water." 



Iiitluoiice of lakes on climate. — Inland water bodies exert an impor- 

 tant inflnence on the climate of their shores, in reference especially to 

 temperature and humidity, and also on the direction and character of the 

 more gentle winds. The surface waters of lakes receive their temperature 

 in a great measure from the air in contact Avith them, and are warmed or 

 cooled at rates having some relation to their depth. The temperature of 

 shallow lakes varies but little from that of the adjacent atmosphere, but 

 changes less- rapidly for the reason, alreadj^ stated, that water surfaces are 

 poor radiators. The differences between the rates of radiation between 

 adjacent land and water surfaces, affect the temiierature of the air above 

 them, and in calm weather give origin to lake and land breezes. 



The tens of thousands of small lakes scattered over the glaciated 

 portions of North America have an important combined influence on the 

 general climate, although their effects may, perhaps, be difficult of direct 

 determination. These lakes cool and moisten the atmos[)here by eva})ora- 

 tion during the hot summer montlis, and when they freeze as winter 

 approaches, a vast amount of "latent heat"' is liberated and moderates 

 the fall in temperature. It is stated by physicists that every ton of water 

 converted into ice gives out as much heat as would be required to raise 

 the same quantity of water from 30° to 174° Fahrenheit. A reverse 

 ju-ocess, not so congenial to the Avelfare of man, takes place, however, 

 when the ice melts iii spring, as tlien an amount of energy equal to that 

 previously lost, must be again absoi-lted in oider that the ice may change 

 to water. The warm southern winds are thus chilled aiul tht' opening of 

 the flowers delayed. 



Deep lakes, as already seen, have a uniform Ijottoni temperature of 30 

 degrees of the Fahrenheit scale, and do not freeze in winter except alxuit 

 their sliores where the water is slinljow. for the reason that tlie low 

 temperature of the air al)ove them does iu)t continue h>ng enough t'oi' the 



