In a paper recently read before the Society of Arts of London, by J. MacGregor, on the paddle wheel and screw propeller, it was observed that in the modes of propulsion employed by aquatic animals may be found almost every plan which has been used by man with machinery. Thus, water is ejected for propulsion by the cuttlefish and " paper nautilus ;" sails are used by the velella and water birds; punting and towing by whelks and some others: a folding paddle by the lobster; feathering paddles by ducks; and oblique surfaces by fish of all kinds. A screw-like appendage is found in the wings of an Australian fly, but it is supposed to be shaped thus only when dried after death. These are well known instances of similarity of natural and artificial means of propulsion; but the author of the above-named paper mentions a remarkable animal which propels itself by a rotary movement, acting on the water by means very similar to those of the paddle wheel and screw propeller combined. This is the infusorial insect " paramecium," which is of an irregular oval or egg-shaped form, with a sulcus or furrowed groove or depression running obliquely round its body. A wave-like protuberance passing along this sulcus (with or without cilia) causes the body to rotate on its longer axis, and thus propels it by the fore and aft stroke of the paddles which the cilia on its surface form, as well as by the screw-like progress induced by the spiral groove.