Page:Popular Science Monthly Volume 52.djvu/151

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tional introductions, until at the present time there is hardly a civilized country which has not firmly established and flourishing within its territory hundreds of species of animals and plants of foreign origin, the time and means of introduction of many of which can not be exactly traced, while of others even the original home can not be ascertained. The paper closed with a suggestion that much may be accomplished by wisely planned and guarded introductions, as in the case of the Australian ladybirds introduced into California and the Sandwich Islands through Albert Kaebele.

Fields for Exploration in South America.—Mr. J. Scott Keltie showed in his geographical address at the British Association that there is a wider and richer field for exploration in South America than in any other continent—even than in central Africa. Along the great river courses our knowledge is fairly satisfactory, but the immense areas, often densely clad with forests, lying between the rivers, are almost unknown. In Patagonia, a great deal has recently been done by the Argentine Government; still, in the country between Punta Arenas and the Rio Negro we have much to learn; while on the west coast range, with its innumerable fiordlike inlets, its islands, and peninsulas, there is a fine field for the geologist and the physical geographer. Indeed, throughout the whole range of the southern Andes, systematic exploration is wanted. There is an enormous area lying to the east of the northern Andes, and comprehending their eastern slopes, embracing the eastern half of Ecuador and Colombia, southern Venezuela, and much of the country lying between that and northern Venezuela, including many of the upper tributaries of the Amazon and Orinoco, of which our knowledge is of the scantiest. Even the country lying between the Rio Negro and the Atlantic is but little known. There are other great areas, in Brazil and in the northern Chaco, which have been only partially described. A survey and detailed geographical and topographical description of the whole basin of Lake Titicaca is a desideratum.

Screw Propellers and Cavitation.—In a paper recently read at the International Congress of Naval Architects and Marine Engineers, by Mr. S. W. Barnaby, we find some interesting data on the above subject. Several years ago the author, in conjunction with Mr. Thornycroft, observed and described this phenomenon of cavitation at high speeds, and suggested that the speed of vessels was approaching a point at which propulsion by screws would become less efficient. If a cavity be formed in any manner in the interior of a mass of water it will tend to become filled with water vapor and with any air which may be in solution, since ebullition takes place at ordinary temperatures in a vacuum. The method used thus far for overcoming this tendency is an increase of propeller-blade surface; in one instance, by increasing the surface forty-five per cent without materially changing the diameter or pitch of the propeller, the same speed (twenty-four knots) was obtained with six hundred and fifty less horse power, and with a decrease of slip to seventeen and a half per cent instead of thirty per cent. The number of revolutions required for twenty-four knots with the screws of small area sufficed to drive the vessel at 28·4 knots when the blade area was increased. The vibration was extreme and dangerous with the narrow blades, but was of a quite normal and unimportant character when the blades were widened. Mr. Barnaby thinks that cavitation will be a source of much trouble in the future. Already it is becoming difficult to obtain the requisite area in screws of "destroyers" without either resorting to an abnormal width of blade or to a larger diameter and pitch ratio than would otherwise be preferable. The one expedient gives undue surface friction, and the other necessitates a reduction in the rate of revolution, and therefore a heavier engine.

Pure and Commercial Science.—It should hardly need saying, as Prof. H. Marshall Ward observes in his British Association sectional address, that the fact that a scientific discovery is found to have a commercial value is no argument against the scientific value of the research; yet some are disposed to depreciate research that may advance economical ends. There are in agriculture, forestry, and commerce generally, Prof. Ward continues, "innumerable and important questions for solution, the investigation of which