Ne’er rock his peaceful pillow”!
What can we hope to find at the bottom of the sea we cannot reach? Yes, but we can reach it. By sounding with Brooke’s lead (a cannon ball, as shown in the illustration), we can arrive at a certain knowledge of the composition of the ocean bed. The right-hand figure of the two is the lead when being lowered, and while it is sinking the cord remains tight. So soon as it touches the bottom the weight of the cannon ball divides the line, and the tube is easily drawn up again. It has been well greased, and so in the cavity of the rod some shells and sand are found adhering. These fragments tell us the composition of the bottom of the sea.
Fig. 703.—Brooke’s Lead.
Here we find tiny shells, just as we find them in chalk, the same formation as that which piled up the cliffs which have risen from, or been discovered, by the sea. By other ingenious contrivances water can be fetched up from the bottom of the ocean, and the temperature can be gauged all along the sounding line. The expedition of the Challenger brought many interesting facts to light. Far down in these solitudes are marine animals,—crustacea, star-fish, seaweeds, and shells,—all of which are carried up by the dredge worked by a steam engine; for the resistance is very great, and the weight supported at the depth of two miles must be considerable, and is equal to four atmospheres. A thermometer has come up crushed even in its iron case, and so the creatures which inhabit and find means to live at the bottom of the sea must be specially fitted by Nature for the locality.
The configuration of the ocean bed has given rise to many different opinions. It has been maintained that there are mountains and valleys, hills and dales underneath the water, all clothed with marine vegetation, equal in height and depth to the terrestrial hills and vales. Again it has been declared that the ocean bed is level; but we find raised portions, which we call islands, which may be the tops of mountains, or portions of the mainland separated from their parent continent by an inroad of the sea, as are our islands of Great Britain.
The sea-bed, however, is very irregular. We find deep and steep valleys, and high hills, but the picturesque peaks caused by the action of air, frost, and water on earth are not, of course, represented under water. Between the Irish coast and Newfoundland we are told the bed is level for nearly four hundred miles. There is a deep declivity before we reach this plain. The centre of the Atlantic is a plain, and on it the most volcanic islands rise, such as Ascension and the Azores. Between England and Greenland there was at one time a land communication, as we have remarked under Geology, and there are submarine terraces now. [An immense river once ran through Western Europe somewhere about where our islands are.]
Fig. 704—The Drag Net.
Under the Atlantic we have remains of foraminifera and other tiny animals, with red clay and volcanic remains which must have been of submarine origin. The Pacific shows us tops of mountains as islands (Hawaii Isles), and an enormous range must be hidden beneath the waters. What a change in the physical geography of the earth a slight sinking of the water of the ocean would make; England and the Continent would be united, and many sea-mountains (islands) discovered. The greatest ocean depth is four miles and a half, but in many places a few hundred feet less depth than at present would reveal many changes in the land.