142 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
could not repress exclamations of surprise, for it really appeared 
as if some monster of the deep had hold of the weight below, and 
was walking off with it."* Both officers and men were amazed 
at the sight. 
272. The experiments in deep-sea soundings have also thrown 
much light upon the subject of under currents. There is reason 
to believe that they exist in all, or almost all parts of the deep 
sea, for never in any instance yet has the deep-sea line ceased to 
run out, even after the plummet had reached the bottom. 
If the line be held fast in the boat, it invariably parts, showing, 
when two or three miles of it are out, that the under-currents are 
sweeping against the bight of it with what seamen call a swig- 
ging force, that no sounding twine has yet proved strong enough 
to withstand. 
Lieutenant J. P. Parker, of the United States frigate Congress, 
attempted, in 1852, a deep-sea sounding off the coast of South 
America. He was engaged with the experiment eight or nine 
hours, during which time a line nearly ten miles long was paid 
out. Night coming on, he had to part the line (which he did 
simply by attempting to haul it in) and return on board. Exam- 
ination proved that the ocean there, instead of being over ten 
miles in depth, was not over three, and that the line was swept 
out by the force of one or more under currents. But in what di-' 
rection these currents were running is not known. 
273. It may, therefore, without doing any violence to the rules 
of philosophical investigation, be conjectured, that the equilibrium 
of all the seas is preserved, to a greater or less extent, by this 
system of currents and counter-currents at and below the surface. 
If we except the tides, and the partial currents of the sea, such 
as those that may be created by the wind, we may lay it down as 
a rule 34) that all the currents of the ocean owe their origin to 
dilFerence of specific gravity between sea water at one place and 
sea water at another ; for wherever there is such a difference, 
whether it be owing to difference of temperature or to difference 
of saltness, &c., it is a difference that disturbs equilibrium, and 
currents are the consequence. The heavier water goes toward 
the lighter, and the lighter whence the heavier comes ; for two 
fluids differing in specific gravity (§ 36), and standing at the same 
* Lieutenant Walsh. 
