CHAP. VIIT.] THE GULF-STREAM. 403 



undoubtedly a gradual elevation of an intertropical 

 belt of tbe underlying cold water, which is being 

 raised by the subsiding of still colder water into its 

 bed to supply the place of the water removed by the 

 equatorial current and by excessive evaporation ; but 

 such a movement must be widely and irregularly 

 diffused and excessively slow, not in any sense com 

 parable with the diaphragm produced in the atmo 

 sphere by the rushing upwards of the north-east and 

 south-east trade-winds in the zone of calms. Perhaps 

 one of the most conclusive proofs of the extreme 

 slowness of the movement of the deep indraught is 

 the nature of the bottom. Over a great part of the 

 floor of the Atlantic a deposit is being formed of 

 microscopic shells. These with their living inha 

 bitants differ little in specific weight from the water 

 itself, and form a creamy flocculent layer, which must 

 be at once removed wherever there is a perceptible 

 movement. In water of moderate depth, in the 

 course of any of the currents, this deposit is entirely 

 absent, and is replaced by coarser or finer gravel. 



It is only on the surface of the sea that a line is 

 drawn between the two hemispheres by the equatorial 

 current, whose effect in shedding a vast intertropical 

 drift of w r ater on either side as it breaks against the 

 eastern shores of equatorial land may be seen at a 

 glance on the most elementary physical chart. 



The Gulf-stream loses an enormous amount of heat 

 in its northern tour. At a point 200 miles west of 

 Ushant, where observations at the greatest depths 

 were made on board the ' Porcupine,' a section of 

 the water of the Atlantic shows three surfaces at 

 which interchange of temperature is taking place. 



D D 2 



