In the year 1872 the good ship _Challenger_ quitted British shores for her long tour of discovery. During nearly three years and a half she cruised about the world, dipping her instruments into the water at frequent intervals, measuring the depths, studying the temperatures, making note of other conditions, and bringing up from the ocean-bed materials whereby to judge of the state of that dark nether-world, hidden from our eyes by intervening miles of water. Three and a half years of steady work; hundreds upon hundreds of soundings; thousands upon thousands of miles traversed; tens of thousands of specimens hauled up; days and weeks and months devoted to unremitting study of those specimens! This is what the _Challenger_ Expedition meant. No wonder our knowledge of the Under-Ocean has grown by leaps. And yet no wonder it is still confined within narrow limits. So enormous is the extent of the Ocean,--so few comparatively are the parts which have been under close examination! Imagine a monster giant striding with vast steps over the sea, and at intervals of a mile or two dipping a long arm into the depth, to bring away a handful of whatever might lie upon the bed below. No doubt he might and would thus learn a good deal that he had not known before. Still, at the best, tens of thousands of square miles would lie around in all directions, untouched by his searching fingers. This is very much what the Expedition was able to do. A handful here and a handful there was brought up, as a specimen of what might be found below. But tens of thousands of square miles, to north and south and east and west, remained untouched. None the less, from these occasional “dips,” however few by comparison with reaches not examined, we know far more than our forefathers could have dreamt of as within the bounds of possibility. Instruments, many and complicated, are used in deep-sea researches--far too many and too complicated to be described here. Two or three may be mentioned. Sounding-machines proper are largely employed, sometimes carrying water-bottles, and always brass tubes, weighed down by “sinkers,” which remain behind when the tubes are drawn up full of mud or ooze. Small dredges are dragged along the sea-bottom, gathering whatever may lie there loosely, and bringing it to the surface. Trawls also are necessary, with beams from ten to seventeen feet in length. Such trawls, held by strong steel cables thousands of fathoms long, can often lift seven tons of material. Into the bottom of a dredge-bag and of a trawl-net fine cloth is frequently sewn, so that mud and small animals may not slip through and be lost. [Illustration: DRAGGING THE TOW-NET TO CATCH MINUTE SURFACE ANIMALS Note sailors with small nets] [Illustration] [Illustration: BEAM-TRAWL AND TOW-NET EMPLOYED IN DEEP-SEA RESEARCH _Face page 188_ ]