Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

674 A FEW REMARKS burning equally well ; a bituminous oil ; a substance like coal- tar ; and a residuum, similar to coke ; result from its distil- lation. Electricity may have been a powerful agent in crystalli- zation ; in the rapid deposition of strata ;* in the formation of mineral veins ;-f" in earthquakes and volcanoes ; in the forma- tion or decomposition of water ; and in other ways of which we are yet, and perhaps ever shall be, totally ignorant. Successive strata may have been rapidly deposited by tidal oscillations and currents, aiding chemical or mechanical com- binations. The depth to which bodies would sink in an ocean several miles deep has not been proved, and there is reason to think that it is much less than people generally imagine. An eminent man has said that a knowledge of " the depression of the bed of the ocean below the surface, overall its extent, is attainable (with whatever difficulty and however slowly) by direct sound- ing ;''':|: and, in consequence of a conversation on this subject with him in 1836, he wrote to me, suggesting a mode which might be tried. I consulted with a friend as to the possibility of success, and his letter,§ taken in connection with the facts * Crosse. + Fox. ' J Treatise on Astronomy, by Sir John Herschel — Cabinet Cyclopae- dia — pag-e 154. § " I return Sir John Hersehel's letter on deep-sea sounding. Any- thing from him is sure to be interesting and instructive; but there is a circumstance unnoticed in his communication which might obstruct the descent of a sounding apparatus to very great depth. " Mr. Perkins found that at a depth of only 3,000 feet, sea-water was compressed 1 -27th of its bulk at the surface. (Lib^ of Useful Knowledge, vol. 1. Art. Hydrostatics.) Hydrostatic pressure has usually been esti- mated from depth alone, assuming that the density of the fluid was uni- form ; such, however, cannot be the case in an elastic fluid like water, for at great depths, being in a compressed state, it is more dense than at the surface. "In estimating the amount of hydrostatic pressure at great depths, we should know the vertical height of the column and mean density of the fluid ; and since density increases with depth, by reason of superincum- bent pressure, the water at great depths must be enormeusly compressed, and.