FRAGMENTS OF SCIENCE. 



139 



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 Cali- 

 fornia and the Sandwich Islands through 

 Albert Kaebele. 



Fields for Exploration in South America. 



Mr. J. Scott Keltie showed in his geograph- 

 ical address at the British Association that 

 there is a wider and richer field for explora- 

 tion in South America than in any other con- 

 tinent 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 ex- 

 ploration 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 tribu- 

 taries 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 north- 

 ern Chaco, which have been only partially 

 described. A survey and detailed geograph- 

 ical 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 Con- 

 gress of Naval Architects and Marine En- 



gineers, 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 inte- 

 rior 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 de- 

 crease 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 un- 

 important character when the blades were 

 widened. Mr. Barnaby thinks that cavita- 

 tion will be a source of much trouble in the 

 future. Already it is becoming difficult to 

 obtain the requisite area in screws of " de- 

 stroyers " without either resorting to an ab- 

 normal 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 there- 

 fore 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 scien- 

 tific 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 eco- 

 nomical ends. There are in agriculture, for- 

 estry, and commerce generally, Prof. Ward 

 continues, " innumerable and important ques- 

 tions for solution, the investigation of which 



