AT BOTTOMS OF RIVERS AND LAKES. 171 



lowered considerably below 0° before it begins to 

 solidify. Tims, then, at the commencement of 

 the frost, the water of a river being cooled down 

 below 0°, is forced in the course of its motion to 

 the bottom, and here first meeting with some solid 

 substance, gives rise to the first formation of the 

 ice on the ground. The ground-ice, by continued 

 additions, increases its growth, and at last, being 

 lighter than the water, mounts to the top ; bits of 

 earth, stones, and plants to which it was attached, 

 are carried with it, and thus betray its origin. In 

 stagnant water, ground-ice is never found. 



At the bottom of deep lakes the temperature is 

 unchanged from year to year, being in most lakes 

 above 4° C. (39°'2 F.)/but never found to be 

 lower. Since water has its greatest density at 4° C, 

 (that is, starting from this temperature, is expanded 

 and made lighter as well by cooling as by heating), 

 it is impossible that stagnant water can be cooled 

 below tliis degree to any considerable depth. In 

 all deep water-basins, whose surface in the course 

 of the year is cooled to 4°, or below, the bottom 

 must soon be covered with water at 4°, and, as soon 

 as this temperature is there set up, it cannot be 

 changed any more by any influence from without, 

 however great may be the variations at the surface. 

 In the lake of Geneva, the effect of the changes 

 of external temperature, have been traced to a 

 depth of two hundred and forty feet. From this 



