July 20, 1905] 



NA TURE 



269 



of these sections is arranged so far as possible 

 alphabetically, so that ready reference is greatly 

 facilitated. We thus have a concise and practical 

 dictionary which should be found of very general 

 utility. 



Murray's Handbook of Travel-Talk. Nineteenth 

 edition. Pp. 688. (London : Edward Stanford, 

 1905.) Price 3s. 6d. 

 That this little pocket-bool: meets the requirements 

 of travellers is shown by the fact that this is the 

 nineteenth edition that has been issued. The success 

 of such a companion depends mainly on the arrange- 

 ment and scope of the material which it contains, and 

 on these points it seems difficult to suggest any 

 improvements. This edition is divided into fourteen 

 distinct but comprehensive groups of subjects, each 

 one containing exclusively those words and phrases 

 which naturally belong to each section. Great pains 

 seem to have been taken to bring the information up 

 to date, motoring, for e.xample, having quite a large 

 part devoted to it. The Britisher is equally helped in 

 either French, German, or Italian, and such a vade 

 niccuin as is here presented should be found of great 

 service to everyone who crosses the Channel. 



LETTERS TO THE EDITOR. 



[Tlie Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can lie undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Pressure of Radiation on a Clear Glass Vane. 



In Nature, June 29, a letter from Mr. G. F. Hull 

 appeared under the above title. In it the writer claims to 

 have verified experimentally that the pressure upon a 

 transparent vane is equal to the difference in the density 

 of energy in front of and behind the vane, and reference is 

 made to a difference of views regarding the theory of the 

 pressure in a non-absorbing medium. 



In regard to the latter point, the same result is obtained 

 for the particular case in question whether the beam of 

 light is considered simply as a carrier of momentum or 

 whether the pressure due to radiation is regarded as arisi-ng 

 from a mechanical bodily force integrated throughout the 

 material medium in which the radiation is being pro- 

 pagated. Consider the latter theory for steady radiation 

 consisting of plane polarised waves of simple harmonic 

 period itiJKC propagated along O-v (see Larmor, Phil. 

 Mag., vol. vii., p. 578, 1904). 



We have 



e 9Y _ _ S^ . _ I 9? _ 3Y . 



7 97 ^ dx' c de a.v ' 



where € is complex if the medium is absorbing. 



The mechanical force per unit volume is directed along 

 O.v and is given by 



F.S.(t,uecurrent,= -^g.^,(|^y]. 



If all the interfaces are perpendicular to O.v, then 7 and Y 

 are continuous throughout, whether the medium vary con- 

 tinuously or abruptly ; consequently the mean value of the 

 mechanical force upon any slice of the medium can be 

 expressed as a pressure per unit area upon each surface 

 equal in amount to the mean value of (7^-|-Y^)/87r at the 

 surface. Thus for any vane suspended in free aether (or 

 air) the resultant mechanical force is equivalent to a 

 pressure per unit area equal to the difference in energy- 

 density in front of and behind the vane. 



The apparent confusion arises from the usual statement 

 that the mean value of 7" + Y- can only vary along O.t 

 in the case of an absorbing medium, but this is true only 

 for progressive waves. For a transparent medium of re- 



fractive index n conveying progressive and regressive waves 

 the mean value of (7^-|-n^Y^)/87r, or the mean value of the 

 energy density, is constant ; but the mean value of 

 {y'-\-\")/8ir varies harmonically along the direction of 

 propagation. For a plate extending from a: = o to x = h, 

 and subjected to a normally incident beam of mean energy- 

 density I, it can easily be verified that the mean value of 

 (7=-f-Y-)/8ir within the plate is equal to 

 I ;(;;■-+ !)--(«-- iV-cos 2i/k{/i -.v)\l\{n-+ i)'- sin- iiK/i + 4ir 



cos- /ik/i]^ 

 consequently the resultant pressure is equal to 



2l{ii'^ - 1)- sin'^ nK/il\(n^+lf sin'' /iKk + ^ir cos- i!Ki\, 



or equal to 2j„I, where J„ is the normal reflecting power 

 of the plate for the radiation used. 



T. H. Havelock. 

 St. John's College, Cambridge, July 14. 



An Omitted Safeguard. 



In two schemes set out in a recent issue of Nature, one 

 dealing with the requirements of Oxford and one with 

 the organisation of applied science in London, there 

 appears a noteworthy omission. 



If the weather is proverbially the first topic of con- 

 versation of Englishmen, it is surely because of the in- 

 fluence it has on the well-being of the community. 



Yet in both the schedules referred to no provision is 

 made for research in meteorology. It is singular how 

 tardy is the recognition of so important a factor in the 

 national welfare. It is to meteorology that we constantly 

 appeal for help. By its daily survey of rainfall it safe- 

 guards our water supply (now a very anxious problem, 

 being outpaced by the ever-increasing demands of popula- 

 tion, sanitation, railways, or manufacturing machinery). 

 We turn to it for the comparison of localities and to study 

 the effects of climate or fog upon health and disease, or 

 to ascertain the relations of temperature, sunshine, or rain- 

 fall to the prosperity of the crops and fruit gardens. We 

 look to the readings of the barometer to protect the safety 

 of those working underground. Meteorology takes 

 cognisance of the force of the wind for the protection of 

 structures, or of storms likely to imperil the mariner on 

 his voyage, and by the extension of, and the improved 

 modes of, forecasting the weather is becoming each year 

 of greater service to all. 



VVithout encroaching further upon the limiCo of your 

 space, sufficient has perhaps been said to show primd facie 

 grounds (while so much is proposed to be devoted to 

 physics, geology, or botany) for the consideration of a 

 possible chair in meteorology, or for in some other way 

 repairing an omission of so serious a kind in the schemes 

 lately propounded. The large amount devoted annually 

 to meteorology in the United .States shows the appreciation 

 of its utilij^y to all classes of the comnmnity by so practical 

 a people as the Americans, and that the outlay is amply 

 recouped by the value of the services rendered by it. 



Richard Bentlev. 



The Hydrometer as a Seismometer. 



In Nature of June 29 Mr. Bennett discusses the motion 

 of a floating hydrometer when vertical motion is imparted 

 to the (rigid) vessel containing the (incompressible) fluid 

 in which the hydrometer floats. The solution offered is 

 that the whole system moves precisely as a rigid body 

 would move, and this solution clearly satisfies the very 

 simple equations of motion in the problem considered. 

 But is such motion stable? In general it is not, and I 

 believe that Faraday studied experimentally the " crisp- 

 afions " of a free surface of liquid when small vertical 

 oscillations were imparted to the containing vessel. 



This hardly affects Mr. Bennett's conclusion that a 

 floating hydrometer is an unsatisfactory form of seismo- 

 meter, but perhaps it may explain the positive results 

 which some observers have obtained ; elastic yielding of 

 vessel or hydrometer, although conceivably an adequate 

 explanation, is not the only one open to us. 



Cambridge. C. V. Burton. 



NO. 1864, VOL. 72] 



