July 12, 1900] 



NATURE 



243 



island) and Japan. The land-planarians of the Australian 

 and neotropical regions are alike in one striking feature. 

 The family Geoplanidre is practically divided between 

 them. The neotropical members of the genus Geoplana 

 include most of the flattened primitive ones, and also 

 peculiar forms such as Leimacopsis and Polycladus. Von 

 ■Graff goes so far as to share the opinion that this geo- 

 planid fauna has arisen on a lost Antarctic continent, 

 and has spread on the one hand to New Zealand and 

 Australia, on the other to South America. The distribu- 

 tion of earthworms lends strong support to this view, as 

 Mr. Beddard has shown. 



The concluding section of the work is composed of full 

 systematic descriptions of the families, genera and species. 

 Von Graff makes five families : the Limacopsidiis, with 

 two tentacles ; the Cotyloplanidas (an unnatural family), 

 with suckers ; the Geoplanidas, with scattered eyes ; the 

 Bipaliidie, with the eyes limited to the flattened " head " ; 

 and the Rhynchodemidae, with a pair of large eyes. 

 There are now nineteen genera, many of which are 

 new. 



This monograph will be of inestimable value to all 

 naturalists interested in land-planarians, and the author 

 is to be congratulated on having completed such a labor- 

 ious task with unfailing accuracy. The lithographers and 

 publishers deserve a special word of praise for the beau- 

 tiful plates and printing which adorn this book. 



F. W. Gamble. 



I 



A SCIENTIFIC ENGINEER. 

 Papers on Mechanical and Physical Subjects. By Osborne 

 Reynolds, F.R.S. Vol. i. Pp. xv-l-416. (Cambridge: 

 University Press, 1900.) 



THE Cambridge University Press has during some 

 years past contributed very largely to the progress 

 of physical science by the issue of the collected works of 

 great mathematicians and physicists. The volumes which 

 contain the collected writings of Maxwell, Adams and 

 Cayley form a rich storehouse of knowledge ; and the 

 efforts the Press has made to induce living writers, such 

 as Kelvin, Stokes and Rayleigh, to edit their own papers 

 for issue in a collected form deserve the gratitude of all 

 students. 



Among the latest of such reprints is the volume before 

 us. Its author. Prof. Osborne Reynolds, has passed a 

 busy life as a teacher in a great commercial and manu- 

 facturing city, and his collected papers testify to the 

 breadth of his interests and the wide scope of his work. 



The papers included in the present volume, some forty 

 in number, were published between 1869 and 1882. They 

 range over a great variety of subjects, from the tails of 

 comets and the solar corona to problems connected with 

 the steermg of ships and the bursting of guns. In so 

 varied a collection the relative importance of the different 

 papers differs greatly, and yet all are interesting ; and all 

 have advanced the sum of human knowledge. 



Indeed, on reading them, one cannot help regretting 

 that the author's interests have been so widely diffused, 

 and that he has not had the opportunity of concentrating 

 himself on some one or other of the great engineering 

 problems which await solution, applying to it his practical 

 experience and insight and his mathematical skill. 

 NO. 1602, VOL. 62] 



An extract from the author's preface makes the cause 

 of this clear. He writes : 



"As affording some explanation of the absence of 

 any connection between many of the subjects in this col- 

 lection of papers, it may be pointed out that these sub- 

 jects have not been determined by arbitrary selection, 

 neither have they been the result of following up one line 

 of research. They have for the most part been sug- 

 gested by the discrepancies between the results obtained 

 in definite mechanical arrangements, such as occur in 

 some parts of the large field of practical mechanics, and 

 the conclusions arrived at as to what those results should 

 be for the same circumstances, by means of geometrical 

 and physical analysis, as far as this analysis was developed 

 at the time." 



But to turn to the matter of the papers ; it would take 

 too long to attempt to analyse them al! ; and, indeed, the 

 results of the most important are now classic, e.g. those 

 on the refraction of sound, the action of a screw pro- 

 peller, the steering of screw steamers, and the explanation 

 of the radiometer. 



The two papers on the refraction of sound are num- 

 bered 16 and 22. Stokes had, seventeen years before 

 the date of the first of these papers, suggested the reason 

 why sounds are heard less distinctly against the wind 

 than with it. It is due to the fact that the velocity of the 

 wind rises as we ascend ; hence when a sound-wave is 

 travelling against the wind, the wave-velocity is less in the 

 upper portion of the wave than in the lower ; thus the 

 wave-front is bent upwards, and the sound passes over 

 the head of the observer. The same notion occurred to 

 Reynolds ; he verified it by direct experiment, and 

 pointed out, moreover, that in ordinary conditions of the 

 atmosphere the temperature falls as we ascend ; hence 

 from this cause also the wave-velocity is reduced, and 

 the path of the sound is no longer straight, but curved, 

 with the convexity of the curve turned downwards. If, 

 however, it should happen that the air is warmed above 

 than it is below, the reverse will be the case — the sound- 

 waves will be bent downwards — the sound will thus be 

 audible at a greater distance than previously. 



The papers on the action of the screw propeller form 

 an interesting series. The racing of a screw is proved 

 to be due to the admission of air to the screw ; this, it 

 is shown, interferes with the power of the screw to ob- 

 tain water, and also reduces the resistance which would 

 otherwise be offered by the water the screw would get 

 For consider a vertical plate, totally immersed in water, 

 which is being pushed forward •; its speed may be such 

 that the water behind cannot remain continuously in 

 contact with it. A vacuum will tend to form behind the 

 plate ; the limiting velocity at which this takes place 

 will depend on the pressure in the water behind the 

 plate; if no air can reach the plate, this pressure will 

 be the atmospheric pressure, together with that due to 

 the depth of water above the plate ; if air can reach the 

 space behind the plate, the limiting velocity will depend 

 only on the pressure due to the water, and will be much 

 less than in the first case. The blades of the propeller 

 act like the plate ; a stationary screw will be most effec- 

 tive in propelling water when it is turning so fast that a 

 vacuum is just formed behind its floats, and the rate at 

 which the water is driven past depends on the water 

 pressure just close to the floats ; if air can reach the floats, 



