To the inhabitants of a flat and unprotected country, bordering on river or estuary, the bore is often a thing of terror, for its advent is uncertain and abrupt, and in its upward rush it sweeps everything before it. The entering of such a wave into the Severn is an almost daily event, and it reaches often a height of many feet. Bores are usual, too, in the River St. Lawrence, in the Hoogly, in an estuary of the Bay of Fundy, and in other places innumerable; and they vary in height from two or three feet to over twelve feet. The effect of such a wall of water as this, deluging low lands, carrying away trees and houses and living creatures, may be easily imagined. CHAPTER V. IN OCEAN DEPTHS “My soul is full of longing For the secret of the Sea.”--LONGFELLOW. “Of the old Sea some reverential fear Is with me.”--WORDSWORTH. Through ages of the world’s history, man knew nothing of the Ocean beyond its surface. He could sail on the sea; he could bathe in the shallow waters near land. If a good swimmer, he might go farther out, and might even dive for one or two minutes out of sight. In more recent times, with the help of a diving-bell, he could descend thirty or forty feet; and in a diving-dress he might even, if experienced, get from one to two hundred feet down. But that was all. Of the vast depths beyond he was sublimely ignorant. He did not so much as know of their existence. Until the late scientific expedition made in the vessel _Challenger_, that great world of the Under-Ocean was swathed in mystery. Nothing is known of it now by direct personal observation. No living man may penetrate those depths. The only mode in which we can learn what is, what lives, what happens there, is by means of “soundings,” by sending down and drawing up specially prepared instruments, the reading of which gives us information about that Under-world. Soundings made in earlier times did little more than tell navigators how deep the sea thereabout might be. Such instruments as were then used could neither work beyond a limited depth, nor say their say with accuracy. Of late years immense improvements have taken place, both in the make of the machines employed, and in the methods of working them. Past soundings were few and unsystematic; whereas now they are many and by rule. Thermometers have been made which bring up from the bottom of the sea reports of the prevailing heat or cold, moving unaffected through warm or cold layers between. Specimens of the materials which lie on the ocean-floor have been brought to light, and analysed. Living creatures in great numbers have been hauled from their own domains, examined and classified. All this means progress. And though, no doubt, fifty or a hundred years hence, that which we now know will be looked upon as the barest A B C of the science, still our knowledge is growing year by year. The Under-Ocean is no longer a vague fairy-land of possible mermaids, or a waste region of death.