1 68 The Realm of Nature CHAP. 



goes on just as in the sea, until the lake cools down to 

 39 F., but at that temperature fresh water attains its 

 maximum density, and the similarity to the cooling of the 

 sea ceases. Further cooling of the upper layer makes the 

 water expand, and therefore it remains at the surface until 

 the temperature falls to 32, when it solidifies to form a 

 sheet of ice. Ice is not formed as long as any of the water 

 in the lake is warmer than 39. The heat from the water 

 under the ice is conducted upward very slowly, so that the 

 whole mass of water can only become solid in very shallow 

 lakes when the winter is long and severe. A deep fresh- 

 water lake in a region where the summers are warm is 

 rarely altogether cooled down below 39 during winter, 

 unless the season is very severe, hence the common 

 observation that deep lakes do not freeze. In calm 

 weather the study of the Swiss lakes, carried on by Pro- 

 fessor Forel and others, shows that the upper 5 fathoms 

 of water may be affected by the diurnal range of air tem- 

 perature between day and night, but the annual change of tem- 

 perature between summer and winter exerts some influence 

 to a depth of from 50 to 80 fathoms. Beneath that depth the 

 temperature remains unchanged all the year round at 39. 

 A steady wind blowing in the direction of the length of a 

 long narrow lake ( 240) may, however, mix the water so 

 thoroughly that the temperature is made practically uniform 

 from surface to bottom at any season of the year. 



229. Phenomena of Sea-lochs. Fjords or sea-lochs 

 are miniature enclosed seas of great depth, surrounded on 

 all sides but one by lofty mountains, and barred off from 

 the deep sea outside by a sill rising to within a few fathoms 

 of the surface, as shown in Fig. 55, 339. The sea-lochs 

 of Scotland have been studied in some detail by Dr. John 

 Murray and the author of this book. 2 The lochs are filled 

 with sea-water much freshened on the surface by numerous 

 small mountain torrents, but scarcely less salt at the bottom 

 than the open sea. In summer the surface temperature is 

 greatly raised, but at the bottom, which is cut off from 

 tidal influence, the temperature falls steadily, and comes 

 to a minimum when the surface is warmest. As winter 



