CHAP. VIII. ] THE GULF-STREAM. 403 
undoubtedly a gradual elevation of an intertropical 
belt of the 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 intertropteal 
drift of water 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. Ata 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. 
Di DZ 
