Page:Polar Exploration - Bruce - 1911.djvu/164

160 net 15 or 16 feet in diameter; the largest net the Scotia used was eight feet in diameter. The most generally useful size and that most frequently employed was, however, one of four inches diameter, and three feet in length, made of the finest Miller's silk, which catches almost all the minutest forms except possibly cocospheres and rhabdospheres. (The finest Miller's silk, known as No. 20, has 5,926 meshes to a square centimetre: each side of the mesh is 0.05 mm. long.) The larger nets are made of coarse muslin. Among the various designs of these plankton nets some are devised to open and close at definite depths, so that a definite stratum of the sea may be explored to see what animals live there; others are so constructed as to enable an approximate estimate to be made of the number, as well as of the species, of animals that live in a certain volume of water. All these different kinds of nets were extensively used on board the Belgica, Gauss, Scotia, Français, and Pourquoi-pas? and less extensively on other recent Antarctic exploring ships. The Discovery and Nimrod did not use these nets or other marine biological apparatus so extensively, because their explorations were more specially on the land rather than the sea. The Scotia used an 8-foot vertical net as far south as 71° 50′ S., 23° 10′ E., lowering it there to 1,000 fathoms below the surface. The