8 PHYSICAL GEOGRAPHY OF THE SEA. 
there is only one very marked mass of mountains, the voleanic 
group of the Agores.” 
Accurate soundings are as yet much too distant to justify 
a detailed description of the bed of the Atlantic. I will merely 
state that after sloping gradually to a depth of 500 fathoms to 
the westward of the coast of Ireland, in lat. 52° N., the bottom 
suddenly dips to 1700 fathoms, at the rate of from about 15 to 
19 feet in the 100. From this point to within about 200 
miles of the coast of Newfoundland, where it begins to shoal 
again, there is a vast undulating plain averaging about 2000 
fathoms in depth below the surface—the “telegraph plateau” 
on which now rest the cables through which the electric power 
transmits its marvellous messages from one world to another. 
Our information about the beds of the Indian, the Antarctic, 
and the Pacific Oceans is still more incomplete, but the few 
trustworthy observations which have hitherto been made seem 
to indicate that neither the depth nor the nature of the bottom 
of these seas differs greatly from what we find nearer home. 
The inclosed and landlocked European seas are very shallow 
when compared with the high ocean: the Mediterranean, how- 
ever, has in some parts a depth of more than 6000 feet; and 
even in the Black Sea, the plummet sometimes descends to 
more than 3000 feet; while the waters of the Adriatic every- 
where roll over a shallow bed. 
The researches of Mr. Russell on the swiftness of the tide-wave, 
showing that the rapidity of its progress increases with the 
depth of the waters over which it passes, afford us another means, 
besides the sounding line, of determining approximately the 
distance of the sea-bottom from its surface. According to this 
method, the depth of the Channel between Plymouth and 
Boulogne has been calculated at 180 feet; and the enormous 
rapidity of the flood wave over the great open seas (300 miles 
an hour and more) gives us for the mean depth of the Atlantic 
14,400 feet, and for that of the Pacifie 19,500. 
Natural philosophers have endeavoured to calculate the 
quantity of the waters contained within the vast bosom of the 
ocean ; but as we are still very far from accurately knowing the 
mean depth of the sea, such estimates are evidently based upon 
a very unsubstantial foundation. 
So much at least is certain, that the volume of the waters of 
