366 THE DEPTHS OF THE SEA. [CHAP, VIII. 
such a system of counter-currents, hold at least, in 
some degree, the relation of the supposed water 
and .oil?”* 
«There can be no doubt that Maury concludes 
that the waters in intertropical regions are expanded 
by heat, and those in polar regions are contracted by 
cold, and that this tends to produce a surface-current 
from the equator to the poles, and an under-current 
from the poles to the equator.’’? 
With regard to increased specific gravity produced 
by excess of salt, Captain Maury says,— 
“The brine of the ocean is the ley of the earth. 
From it the sea derives dynamical power, and its cur- 
rents their main strength.’ ‘‘One of the purposes 
which in the grand design it was probably intended 
to accomplish by leaving the sea salt and not fresh, 
was to impart to its waters the forces and powers 
necessary to make their circulation complete.”* ‘In 
the present state of our knowledge concerning this 
wonderful phenomenon (for the Gulf-stream is one 
of the most marvellous things in the ocean), we can 
do little more than conjecture. But we have the 
causes in operation, which we may safely assume 
are among those concerned in producing the Gulf- 
stream. One of these is the increased saltness of 
its water after the trade-winds have been supplied 
with vapour from it, be it much or little; and the 
other is the diminished quantum of salt which the 
1 Captain Maury, op. cit. 
* On Ocean Currents. Part III. On the Physical Cause of Ocean 
Currents. By James Croll, of the Geological Survey of Scotland. 
(Philosophical Magazine, October 1870.) 
3 Captain Maury, op. cit. 4 Thid. 
