134 SEA AND LAND 



frostwork of winter and to fit the earth for the uses of vegeta- 

 tion. In the case of the icebergs the process of their melting 

 recjuires that some cubic miles of the relativel)' warm ocean 

 waters, hax'ing a temperature of, say, 50" 1' ., shall come in con- 

 tact with the ice before an ordinary berg will be dissolved. 



As long as the icebergs are rapidly pushed forward by 

 the action of the undercurrent which urges them through the 

 superficial layer of warm water, the portions of their masses 

 which are exposed to the northward-setting stream rapidly 

 melt; because they are by their motion ever brought into 

 contact with the fresh fields of warm water, the cold portion 

 of the ocean which tlu;y have chilled being left behind. But, 

 as we have already noticed, the rate of this motion steadfastly 

 decreases until they finally cease-to travel over the surface, 

 and meet only such water as the winds or the gentle north- 

 ward - scttin^f current sends against them. In a certain 

 measure they provide by their owm action for a slight current 

 movement wdiich promotes their melting. The water next 

 their sides, being chilled by contact with the ice, becomes a 

 little heavier than that of the surrounding sea, and so tends 

 to sink into the depths, its place being taken by the warmer 

 tluid whicli Hows in over the surface. If w^ater became con- 

 tinuously heavier as it cooled, all the way down to the freez- 

 inc^-point. this action w'ould be much more effective than it is; 

 but among the many extraordinary features of this substance 

 we hav(! to note that it is the densest at about 39'' F., expand- 

 ing a little if the heat is decreased below that temperature. 

 Consequently this circulatory movement just described is 

 not strong. 



The protection of the bergs from melting is also pro- 

 moted by the fleet-like manner in which they occupy large 



