. Elementary physical geography . de that it is invertedwhen the wire is reeled in, and anautomatic register of the tempera-ture at the time of inversion is thusmade. Very often several ther-mometers are attached to the lineat different distances, so that weobtain a knowledge of the tempera-ture of the ocean water from thesurface down to the very base ofthe column. Dredging. — In order to obtain aknowledge of the kind of life thatexists in these great ocean depths,another method, that of dredging,must be followed. The dredge, ordeep-sea trawl (Fig. 74), is an ironframe several feet in length,
. Elementary physical geography . de that it is invertedwhen the wire is reeled in, and anautomatic register of the tempera-ture at the time of inversion is thusmade. Very often several ther-mometers are attached to the lineat different distances, so that weobtain a knowledge of the tempera-ture of the ocean water from thesurface down to the very base ofthe column. Dredging. — In order to obtain aknowledge of the kind of life thatexists in these great ocean depths,another method, that of dredging,must be followed. The dredge, ordeep-sea trawl (Fig. 74), is an ironframe several feet in length, towhich is attached a bag net. Thisis lowered to the bottom anddragged over it, usually for severalhours. The sounding apparatus islowered perpendicularly; but thedredge is lowered to the bottom,and then more rope is reeled out,so that it may be kept upon thebottom and dragged over it. This is done partly by attach-ing weights to the dredge, and partly by the natural saggingof the wire rope. After the dredge has been down for a. Fig. trawl. 156 PHYSICAL GEOGRAPHY. sufficient length of time, it is drawn to the surface and itscontents examined. Imagine a balloon sailing through the air at a height ofthree miles or more, and dragging a frame a few feet inlength, over a distance of a few miles. If the operators ofthis apparatus should imagine that, as a result of a few trials,they had obtained a fair knowledge of the life existing onthe surface of the earth, it will readily be seen that theywould be very much mistaken. All swiftly moving animalswould escape, and only those would be taken which weresmall enough to enter the dredge, and so slow that theycould not escape from it. In a measure this is true of ourexplorations of the deep sea. If large animals exist there,our methods of exploration are not calculated to discover them, nor should weexpect to obtainmany animals thatare capable of rapidmovement. Topography of theOcean Bottom: Gen-eral. — There is avery profo
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