April 13, 1905] 



NA TURE 



559 



generation, and the stimulus they gave to the progress 

 of agriculture were out of all proportion to the value of 

 the knowledge' or even of the ideas they contributed to 

 the subject. Davy gave dignity to the study of agri- 

 cultural science ; where Davy had laboured no man in 

 future need be ashamed to work. Two articles follow- 

 on fruit farming, by Mr. Charles Whitehead, and on 

 vegetable farming, by Mr. James Udale. Both are 

 sound enough, but they are rather jejune perform- 

 ances for the Journal of the Royal .'\gricultural 

 Society, since from the inevitable limitations of space 

 they are too lacking in detail to be of service to any- 

 one but the amateur. When it comes to reproducing 

 pictures of the wireworm from the Society's text-book 

 of agriculture, instructions for making Bordeaux mix- 

 ture, and similar elementary matters, the farmer 

 reader may well wonder where the editor's blue pencil 

 has been lying. Mr. Dudley Clarke writes on a burn- 

 ing question of the day, the cost of labourers' cottages, 

 and gives a number of sensible plans, bringing out the 

 cost of a brick and tile cottage with three bedrooms 

 at about 150?., including the land and the cost of a 

 well. 



Mr. A. D. Hall writes on the agricultural experi- 

 ments of Mr. James Mason, the well-knovirn founder 

 of the firm of Mason and Barry, who spent his later 

 leisure in attempting to apply science to agriculture 

 with some success, while the rest of the volume is 

 occupied with the last Park Royal show, with reports 

 of the experiments in progress at the VVoburn Farm, 

 and with other society matters. 

 Mediaeval Lore from Bartholomew Anglicus. By 



Robert Steele ; with preface by William Morris. 



Pp. XV -I- 195. (London: Alexander Moring, Ltd., 



1905.) Price IS. 6d. net. 

 This beautiful addition to the "King's Classics," 

 of which Prof. Gollancz is the general editor, is 

 likely to prove of interest to students of science. 

 Written by an English Franciscan, probably before 

 1260, to explain the allusions to natural objects met 

 with in the Scriptures and elsewhere, it is really an 

 account of the properties of things in general so far 

 as they were understood by an educated writer of the 

 Middle Ages. After studying the quaint and pleasant 

 accounts of medijeval science, medicine, geography, 

 and natural history which the book contains, the 

 student will begin to realise that during the Middle 

 Ages science was not stagnant, but, by gradual de- 

 velopment, was making possible the rapid growth of 

 scientific knowledge characteristic of the nineteenth 

 centur)'. The reprint deserves to be read widely. 

 Ergebnisse iind Prohleme der Zcugungs- iind 



V ererhungs-lehre. By Prof. Oscar Hertwig. Pp. 



31. (Jena : G. Fischer, 1905.) Price i mark. 

 Prof. Oscar Hertwig is well known as a pioneer 

 in the researches on fertilisation. In 1875 he made 

 the important discovery that the essential fact in the 

 process lay in the fusion of a single male with a 

 female cell, and he also s^w and recognised the fusion 

 of the nuclei. It was ntting that at the congress 

 held at St. Louis last year he should choose this 

 subject as the text of his lecture. The reprint forms 

 a clear statement of the chief details of fertilisation, 

 and also indicates some of the theoretical conclusions 

 towards which modern cytology is tending. The 

 sketch of the so-called " reduction divisions " is 

 specially good, and the author shows how clear a 

 light they throw on the modern experimental results 

 obtained from the study of heredity. The lecture will 

 be welcomed by all who are interested in these and 

 kindred questions, and those who know Prof. Hert- 

 wig's writings will not be surprised to find that if the 

 treatment is of necessity brief, it is masterly of its 

 kind. 



NO. 1850, VOL. 71] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he 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 Dynamical Theory of Gases. 



In Mr. Jean's valuable work upon this subject he attacks 

 the celebrated difficulty of reconciling' the " law of equi- 

 partition of energy " with what is' known respecting the 

 specific heats of gases. Considering a gas the molecules 

 of which radiate into empty space, he shows that in an 

 appro.ximately steady state the energy of vibrational 

 modes may bear a negligible ratio to that of translational 

 and rotational modes. 



I have myself speculated in this direction ; but it seems 

 that the difficulty revives when we consider a gas, not 

 radiating into empty space, but bounded by a perfectly 

 reflecting enclosure. There is then nothing of the nature 

 of dissipation ; and, indeed, the only effect of the appeal 

 to the jether is to bring in an infinitude of new modes of 

 vibration, each of which, according to the law, should 

 have its full share of the total energy. I cannot give the 

 reference, but I believe that this view of the matter was 

 somewhere expressed, or hinted, by Maxwell. 



We know that the energy of aetherial vibrations, corre- 

 sponding to a given volume and temperature, is not in- 

 finite or even proportional to the temperature. For some 

 reason the higher modes fail to assert themselves.' A full 

 comprehension here would probably carry with it a solu- 

 tion of the specific heat difficulty. Ravleigh. 



The Physical Cause of the Earth's Rigidity. 



Since publishing the paper in the Astronomische Nach- 

 richten (No. 3992), the investigations there outlined have 

 been considerably extended, and lead to some remarkable 

 results. My only purpose in this letter is to direct atten- 

 tion more particularly to the physical cause of the earth's 

 rigidity. This seems to have remained rather obscure, and 

 I am not aware that any definite theory has been adopted 

 to account for the remarkable fact established by the re- 

 searches of Lord Kelvin and Prof. G. H. Darwin. 



It was pointed out in the .Astronomische Nachrichten 

 (3992) that the physical cause of the earth's high effective 

 rigidity is to be' found in the great pressure existing 

 throughout the interior of our globe. This may be made 

 somewhat more obvious by remembering that in any con- 

 centric spherical surface the resistance of the enclosed 

 nucleus must be just equal to the pressure of the surround- 

 ing shells resting upon it, and thus the strain upon the 

 matter of the globe increases towards the centre according 

 to the same law as the curve of pressure given in the 

 .istronomische Nachrichten (3992). This pressure is 

 sustained by the increasing density and rising temperature 

 of the matter in the earth's interior, which is thus under 

 an inconceivable strain, far surpassing the strength of any 

 known substance. .\s the matter is above the critical 

 temperature of every element, it is essentially a gas re- 

 duced by pressure to a hardness greater than that of steel, 

 and with an elasticity and rigidity infinitely near to per- 

 fection.. The result is that the explosive strain upon the 

 matter of our globe from within, which is everywhere 

 just equal to the pressure sustained by the enclosed nucleus, 

 renders the interior matter more rigid than any known 

 substance ; and even the outer layers, which are but 

 slightlv compressed, yield so little under the action of 

 external forces that the globe as a whole is more rigid 

 than steel, as Lord Kelvin and Prof. G. H. Darwin found 

 from their profound researches on the long-period tides of 

 the ocean. 



It was these considerations which led to the conclusion 

 that all the heavenly bodies of considerable mass when con- 

 densed to moderate bulk have nuclei of great effective 

 rigidity, and experience no sensible circulation at great 

 depths'. T. J. J. See. 



U.S. Naval Observatory, Mare Island, Cal., March 20. 



I Compare "Remarks upon the Law of Complete Radiation" (.Pkil 

 .llag., xlix. p. 539, 1900). 



