April 1, 1896.] 



KNOWLEDGE 



81 



swing with very little vertical raotion. The disturbance of 

 the water in the case of an oscillating wave reaches to a 

 depth of, say, one wave-length. It is true that the surface 

 particles do not descend in thr-ir swing to a depth of more 

 than, say, one-tenth of this distance ; but each swinging 

 surface particle puts in motion a particle beneath it, so 

 that below any point of the surface there is a vertical 

 column of swinging particles, the amplitude of swing 

 diminishing rapidly with the depth. A certain depth, 

 about equal to the length from crest to crest, is therefore 

 necessary for the free motion of a typical deep-water wave, 

 in which the swing is circular and in which the energy 

 advances at half the velocity of the wave. When the 

 depth of water is much less than the wave-length corre- 

 sponding to a certain velocity, a boat moving at that 

 velocity will no longer generate a circular swing in the 

 water ; the bottom of the canal interferes with the vertical 

 component of the swing, much as a 'vertical breakwater 

 annuls the horizontal component when it converts a 

 travelling wave into a stationary wave. (See Knowledge for 

 February.) Under these conditions almost the whole of 

 the swing is horizontally backwards and forwards, very 

 little vertical motion remaining. The moving hillock of 

 water, which is the visible thing about the transmission of a 

 solitary wave, travels along the surface of the canal, the 

 actual particles of water in the hillock constantly changing 

 the while, for the march of the hiUock is a true wave 

 progression effected by transmission rather than trans- 

 lation. The hillock will have vanished from over section 

 A (Fig. 4), and will surmount the section B, when a 

 quantity of water equal to that con- 

 tained in the hillock has passed from 

 section A to section B. The rate at 

 which this will be effected depends 

 upon two things — the difference of 

 pressure and the cross section. 

 Now, as the hillock extends across 

 the canal we need not concern our- 

 selves about the width of section, 

 but only about its depth. The given 

 difference of pressure will effect the 

 transference of a given bulk of water 

 across each unit section in each unit 

 of time, and, therefore, as the depth 

 of the section is increased the given bulk of water will be 

 more quickly transferred from section to section. Hence 

 the relation between the depth of the canal and the velocity 

 of the solitary wave. Following this principle, a short 

 mathematical investigation shows that the (horizontal) 

 velocity of a free solitary wave in a canal is equal to the 

 (vertical) velocity which a body would acquire when falling 

 freely in air through a distance equal to half the depth of 

 the canal. Thus, for example, if the depth be eight feet, 

 the velocity of the solitary wave is eleven miles per hour. 

 This is the velocity of free propagation of any loiif/ uavi', 

 such as the tide wave, in which the motion of the water 

 particle is mainly horizontal, and in which the water is 

 equally disturbed throughout its whole depth. To the 

 subject of the Tide Wave we return in our nest article. 



B 



. FlO. 4.— The Propa- 

 gation of tlie Solitary 

 Ware. 



Noticfs of JSoolts. 



An Exercise Book of FAementary Practical Physics. By 

 R. A. Gregory, F.R.A.S., Oxford University Extension 

 Lecturer. This is quite an elementary book, and must be 

 intended for very small boys. It is, moreover, said to be 

 arranged according to the syllabus of the Headmasters' 

 Association. The plan may appeal to some, but it seems 



rather suggestive of cramming. Why is it thought neces- 

 sary to perpetuate the obsolete phraseology of antiquated 

 test-books on mechanics, which treated " power " and 

 " weight " as things of the same character, and confounded 

 " force " and " pressure "' ? 



The Cambridge Natural History. Peripatus, Myrinpods, 

 Insects. By A. Sedgwick, M.A., F.R.S. ; F. G. Sinclair, 

 M.A. ; and Dr. David Sharp, M.A., F.R.S. (MacmiUan.) 

 Illustrated. 17s. 



Of the five hundred and eighty-four pages in this new 

 volume of the " Cambridge Natural History," only twenty- 

 four are devoted to an account of Peripatus by Mr. Sedgwick, 

 and fifty-one to Mr. Sinclair's essay on Myriapods, the 

 remainder being taken up with the first part of an original 

 and strikingly attractive description of insects. 



Peripatus is interesting because it is a kind of half-way 

 animal between the Arthropoda and Annelida, though it 

 really belongs to the former group, and only possesses 

 Annelidan aflinities. Mr. Sedgwick is a distinguished 

 authority upon the genus he describes, and his brief essay 

 on the features, habits, anatomy, development, and distri- 

 bution of it is clear though necessarily special. 



Mr. Sinclair's article on the Mi/riapoda is a valuable 

 summary of the natural history of this group. The 

 classification adopted is that of Koch, with two orders 

 added, viz., Symphyla and Pauropoda. After describing the 

 general characteristics of the whole group the distinctive 

 features of each of the orders are studied, the account con- 

 cluding with sections on the embryology and palreontology 

 of Myriapods. 



Dr. Sharp's work on insects is a masterpiece, as valuable 

 to zoologists as it is to entomologists. The part now 

 given includes the Aptera, Orthoptera, Xeumptera, and a 

 portion of Hymenoptera. It is full so far as it goes, trust- 

 worthy, interesting, and in every respect satisfactory, and 

 ranks high among the best literature of the subject ever 

 published. Every naturalist, and everyone interested m 

 natural history, should strive to add Dr. Sharp's work to 

 their libraries. 



The volume is well printed and excellently illustrated, 

 and is a worthy addition to what will undoubtedly become 

 a standard series. 



The Oriyin of Plant Structures by Self-adaptation to the 

 Environment. By the Rev. George Henslow, M.A., etc. 

 (Kegan Paul.) 5s. This seventy -seventh volume of the 

 "International Scientific Series" carries on the argument set 

 forth by the author Ln a work published in the same series 

 in 1888. The Darwinian hypothesis assumes (erroneously, 

 j according to Prof. Henslow) that plants, when they vary, 

 j do so indefinitely in nature, and that the variations which 

 i are of service under any particular environment survive 

 and are perpetuated, so that natural selection and heredity 

 may eventually lead to the production of new species. 

 Prof. Henslow denies that natural selection plays any part 

 in the origin of species. He holds that a change of 

 environment induces a plant to form definite and not 

 indefinite variations in nature, which are, therefore, always 

 in the direction of adaptation to the surrounding circum- 

 stances of life. Environment in its widest sense, and 

 " the responsive power of protoplasm." are thus held to be 

 the causes of adaptive variations in plants, and natural 

 selection is put out of court. In the present volume this 

 view is applied to account for the origin of vegetative 

 structures, as it was used in the former one to that of 

 floral structures. The cases studied by the author to show 

 that definite variations are produced by the responsive 

 action of the protoplasm in various species of plants, called 

 into play by the external forces of the environment, are 



