CLASSES OF ICE-FIELDS i i 7 



nearly freezing temperature clown toward the equator. The 

 simpler phenomena of freezing and frozen water, which we 

 shall incidentally have to consider, are attractive parts of the 

 science of physics, for the reason that they are easily com- 

 prehended and are well illustrated by familiar and personal 

 experiences. 



The ice which travels from the Arctic regions toward the 

 equator consists of two very different kinds of masses, the 

 difference being so plain that they are invariably recognized 

 by all observers; in part they are made up of flat or tabular 

 masses, which seldom if ever have a thickness of more than 

 one hundred feet even before they have begun their journey 

 to lower latitudes. Though limited in depth these fields of 

 floe-ice, as they are commonly termed, often have a great 

 horizontal extension. Near the southern extremity of Green- 

 land they sometimes have an area which is to be measured 

 by square miles. Though interesting, these floes have less 

 economic or geographic importance than the group of true 

 icebergs, for the reason that they do not journey so far from 

 their point of origin, and rarely come into the ordinary paths 

 of commerce. The true icebergs differ from the ice-floes in 

 that they are vastly thicker and do not have the same table- 

 like tops, but are usually serrate at their summits. It is 

 impossible to determine by actual measurement the depth of 

 the ice in the true berg, but we know from the specific grav- 

 ity of frozen water that only about one-seventh to one-ninth 

 of their mass rises above the level of the sea, and we can 

 safely infer from the extent of the emerged portion that their 

 bases are often two or three thousand feet below the surface 

 of the ocean. This estimate, though it does not need con- 

 firmation, is fairly proved by the fact that many bergs have 



