All marine biological surveys involve physical measurements. The depth is found by hand-lines, deep-sea lines and sounding machines. While a sounding is being made bottom samples are usually collected. Thermometers attached to the sounding lines give the temperatures of the water at any required depth. Water bottles, also attached to the sounding lines, collect samples of the water at various depths and these samples are often examined for dissolved gases, salinity, bacterial organisms, etc. The density of the water at the surface, the bottom or any intermediate depth, is now generally found by making chlorine litrations of samples collected by the water-bottles. The transparency of the water is important and there are several good methods of gauging this. In general, there is no end to the number and variety of physical determinatives that may be made in the course of an ecological survey and the ingenuity of the investigator is sure to be exer cised in such work.
Nowadays, a marine biological investigation requires consider able organization, for the information that can be obtained merely by shore collecting and by trawling and dredging in shallow water is very limited. Deep water at great distances from land must be visited, and so large and well-equipped vessels must be em ployed. The voyage of circumnavigation of the "Challenger," carried out in 1871-73, still remains the model on which all such expeditions are planned. The shallow seas, such as those round the British Isles, or even the seas of the East Indian archipelagoes, have been explored by vessels of the type of the modern steam trawler, or by ships that are not much larger, but deep sea trawl ing, dredging and sounding require the use of powerful vessels capable of keeping the sea in any weather and working in circum stances impossible for the trawlers. The main object in such expe ditions has been to spend as much time as possible merely in collecting, preserving and storing specimens for detailed examina tion ashore after the expedition has returned ; still, much has to be done on board, and so the ship is equipped as a floating laboratory with all the apparatus and materials commonly in use and with everything so arranged that such laboratory work can be done in the somewhat difficult conditions that are experienced on the high seas. The "Challenger" expedition was organized at a time when much less was known about the methods of deep sea investigation than at present. For instance, the modern industry of steam-trawling did not exist; the use of steel wire rope for trawling and sounding, etc., had just been introduced but was quite undeveloped and the fastidious methods of chemical and physical investigation of sea-water that are now employed had not been worked out. The deep sea expeditions of the last 3o
years have had the advantage of these developments but none of them has had the wide scope of the "Challenger" enterprise.
In practice, the methods of collecting animals and plants from the sea and sea bottom that are now employed are those of the professional fisherman. The exploring ships are, in regard to their trawling, dredging and general collecting equipment, modelled on the plan of a steam trawler, using the large trawls, windlasses, and steel wire ropes of the fishing vessels. The latter, however, do not often work in deeper water than about 15o fathoms, whereas the exploring vessels must be able to sound, dredge and trawl in water of any depth down to 5,00o fathoms. The methods, how ever, are only extensions of those employed in the steam trawlers, and though many specialized forms of fishing gear are used, expe rienced skippers and mates of trawlers are able to use such appa ratus and even to devise its form and construction. Added to this, there is the purely scientific side of the expedition, but the basis is that of a powerful and well-equipped steam fishing vessel, worked by professional fishermen.
For some years past the attempt has been made to secure con tinuous records of marine biological data ; thus it is possible to sound continuously; there are theoretically possible methods of obtaining a continuous record of the density of the water through which a ship sails ; thermographs are instruments that give a con tinuous record of sea temperature, and an apparatus for obtain ing a continuous sample of the plankton contained in the water has also been developed. See A. C. Hardy, "A New Method of Plankton Research," in Nature (Oct. 3o, 1926).
BiBuoGRAPHY.—Cambridge Natural History, ed. S. F. Harmer and A. E. Shipley (1895-1909) ; C. Darwin, Journal of Researches during the Voyage of H. M. S. Beagle (184o-42, and many later eds.) ; On the structure and distribution of Coral Reefs (1842, latest ed. in G. T. Bettany's Minerva Library 189o) ; J. A. Johnstone, Conditions of Life in the Sea (5908) ; Challenger Society, Science of the Sea, ed. G. H. Fowler (1912) ; F. W. Flatteley and C. L. Walton, The Biology of the Sea Shore (5922) ; Sir W. A. Herdman, Founders of Oceanography (1923) ; F. M. Davies, "An Account of the Fishing Gear of England and Wales," in Ministry of Agriculture and Fisheries, ser. 2, Sea Fisheries, vol. ix., No. 6 (1927). See also the publications of the International Council for the Exploration of the Sea, par ticularly the series Publications de Circonstance (Copenhagen, 1903), etc.