now passed away. To the writings of Aristotle, who lived during the fourth century b. c., are credited the first bathymetric data. He states that the Black Sea has whirlpools so deep that the lead has never reached the bottom; that the Black Sea is deeper than the Sea of Azov, that the Ægean is deeper than the Black Sea, and that the Tyrrhenian and Sardinian Seas are deeper than all the others. The first record of a deep-sea sounding should be credited to Posidonius, who stated, about a century b. c., that the sea about Sardinia had been sounded to a depth of one thousand fathoms. No account is given of the manner in which the sounding was taken, and we have no information as to the methods employed by the ancients in these bathymetric measurements.
The opinions of the learned with respect to the greatest depth of the sea, in the first and second centuries a. d., may be gleaned from the writings of Plutarch and Cleomedes, the first of whom says, "The geometers think that no mountain exceeds ten stadia [about one geographic mile] in height, and no sea ten stadia in depth." And the second: "Those who doubt the sphericity of the earth on account of the hollows of the sea and the elevation of the mountains, are mistaken. There does not, in fact, exist a mountain higher than fifteen stadia, and that is also the depth of the ocean."
There was no important addition to our knowledge of the deep sea during the middle ages, and no definite attempt to provide effective means for deep-sea sounding appears to have been made until Nicolaus Causanus, who lived in the first half of the fifteenth century, invented an apparatus consisting of a hollow sphere, to which a weight was attached by means of a hook, intended to carry the sphere down through the water with a certain velocity. On touching the ground the weight became detached and the sphere ascended alone. The depth was calculated from the time the sphere was under water. This apparatus was afterward modified by Plücher and Alberti, and, in the seventeenth century, by Hooke, who substituted a piece of light wood well varnished over for the hollow sphere. Hooke's instrument was no doubt fairly accurate in shallow water, but useless in great depths, where the enormous pressure waterlogged the wood and, by materially increasing its density, greatly diminished the speed with which it rose from the bottom. When used in currents the float was carried away and the record lost.
During the period when the voyages of Columbus, Vasco da Gama, and Magellan added a hemisphere to the chart of the world and forever established the fundamental principles of all scientific geography, navigators had sounding lines of one hundred and two hundred fathoms in length, and, although they eagerly studied the oceanic phenomena revealed at the surface, the deep sea did not