tific observations in all parts of the world. Continued improvements in the appliances and instruments have made the results more precise than was possible in the earlier times, and, as the data accumulate, the bathymetric charts of the oceans are becoming more accurate. Not until this work is much further advanced, however, shall we be able to arrive at an estimate of the depths and weights of the oceans at all comparable to our knowledge of the heights and weights of the various great land masses above sea level.
Other important results of these expeditions have been the verification of many reported elevations of the ocean bed formerly considered doubtful, the discovery of new ones, and proof of the non-existence of others, which had been reported as dangers to navigation.
The Geography of the Sea reached a decidedly more advanced stage by the inception of several great scientific expeditions, of which that of the Lightning, in 1868, to the Hebrides and Faroe Islands, under the superintendence of Professors Carpenter and Wyville Thompson, was the forerunner. This was followed by the three years' cruise of the Challenger (Br.) in 1873-75, the Tuscarora (Am.) in 1874, and the Gazelle (Ger.) in 1875, by those despatched under the authority of the U. S. Coast Survey and of the U. S. Fish Commission, and others of lesser importance, sent out under the auspices of European governments, and by private individuals. All of these have contributed in an eminent degree to the progress of the science by giving us a better understanding of the physical and biological conditions of the sea at all depths. Special mention must be made of the splendid work that is being done continually by the expeditions sent out by the U. S. Fish Commission. This branch of the United States service, originally established for the investigation of the causes of the decrease in the supply of useful food fishes and of the various factors entering into that problem, in pursuance of these objects has been prosecuting a detailed inquiry, embracing deep-sea soundings and dredging, observation of temperatures at different depths, transparency, density and chemical composition of sea-water, investigation of surface and under currents, etc.; in other words, making a complete exploration of the physical, natural and economic features of the sea, besides collecting a large number of specimens of natural history. The expeditions sent out by this Commission have brought to light from the deep beds of the ocean an ex-
