To these examples from Great Britain we may add one or two from the German Ocean. Seven islands have completely disappeared within a very narrow area since the time of Pliny; for he counted twenty-three between Texel and the mouth of the Eider, whereas now there are but sixteen. The island of Heligoland, at the mouth of the Elbe, has been for ages subject to great dilapidation. Within the last five hundred years three-fourths of it have been carried away; and since 1770 the fragment that remains has been divided into two parts by a channel which is at present navigable for large ships. A still more remarkable instance of destruction effected by the waves of the sea occurred in the island of Northstrand, on the coast of Schleswig. Previous to the thirteenth century it was attached to the mainland, forming a part of the continent of Europe, and was a highly cultivated and populous district about ten miles long, and from six to eight broad. In the year 1240 it was cut off from the coast of Schleswig by an inroad of the sea, and it gradually wasted away up to the seventeenth century, when its entire circumference was sixteen geographical miles. Even then the industrious inhabitants,--about nine thousand in number,--endeavored to save what remained of their territory by the erection of lofty dykes; but on the eleventh of October, 1634, the whole island was overwhelmed by another invasion of the sea, in which 6000 people perished, and 50,000 head of cattle. Three small islets are all that now remain of this once fertile district.[25] The breakers of the ocean receive no small aid in their work of destruction from the action of tides and currents which co-operate with the winds to keep the waters of the sea in constant motion. And though the winds may sleep for a time, the tides and currents are always actively at work, and never for a moment cease to wear away the land. But they are even more powerful auxiliaries as agents of transport. If it were not for them, the ruins which fall from the rocks to-day would to-morrow form a barrier against the waves, and the work of destruction would cease. But Nature has ordained it otherwise. When the tide advances, it rolls the broken fragments toward the land, and when it recedes, it carries them back to the deep; and so by unceasing friction these fragments are worn away to pebbles, and then, being more easily transported, they are carried off to sea and deposited in the bed of the ocean: or else, perhaps, they are cast up on the sloping shore, to form what is so familiar to us all under the name of a shingle-beach.