March 2, 1922] 



NATURE 



271 



1 



A'OI 



I intf 



Stories of insect behaviour that are recorded in its 

 pages. Fabre is admittedly a difficult writer to 

 translate, and the charm of his diction only too readily 

 escapes if too much freedom be exercised. Mr. de 

 Mattos's task, therefore, has not been an easy one, 

 but he has carried it out conscientiously and with 

 ident care for accuracy. 



First Book of Applied Electricity. By S. R. Roget. 

 (First Books of Science.) Pp. viii + 143. (London : 

 Macmillan and Co., Ltd., 192 1.) 2s. 6d. 

 I'he author has made a very successful attempt to 

 ,ive the elementary principles which underlie the useful 

 applications of electricity and magnetism without 

 )rrying the reader with academical definitions and 

 fficulties. The book has what we think is a great 

 rit — namely, that it is entirely independent of the 

 uirements of examinations. It is therefore more 

 interesting than the ordinary treatise, and covers a 

 much wider field. 



It can be recommended to the general reader anxious 

 get an easily acquired, accurate, and useful know- 

 ge of electrical matters. The ordinary student 

 ding for examinations will also find it a useful 

 troduction to more advanced treatises. 



Perfumes, Essential Oils and Fruit Essences used for 

 Soap and other Toilet Articles. By Dr. G. Martin. 

 [(Manuals of Chemical Technology. — X.) Pp. vii + 

 138. (London : Crosby Lbckwood and Son, 192 1.) 

 i2s. 6d. net. 



. Martin's book is of a severely practical character ; 

 contains much information in a very condensed 

 rm, and should be useful as a work of reference to 

 )se interested in the manufacture of the class of 

 iterials of which it treats. A large number of 

 ictical recipes is given. The section on analysis, 

 :upying only four pages, is too brief to be of real 

 lue. No references to the literature are given beyond 



the mention of a few patents and a list of ten books 



<»n the subjects treated. 



" Power's " Practical Refrigeration. Compiled by the 



Editorial Staff of Power. Pp. viii + 283. (New 



York and London : McGraw-Hill Book Co., Ltd., 



1921.) los. net. 



Fhe practice of ammonia refrigeration, including a 



simple account of the theory and tables of useful 



•onstants, is discussed in this volume. A number 



<i practical hints for users of refrigeration plant, 



vritten in a colloquial style, forms about half the book, 



which should be useful to persons in charge of such 



plant. 



Chemistry of Pulp and Paper Making. By E. Suter- 

 meister. Pp. vii + 479 + 31 plates. (New York: 

 John Wiley and Sons, Inc. ; London : Chapman 

 and Hall, Ltd., 1920.) 365. net. 

 The chemical aspects of paper-making are dealt with 

 in the volume under notice, the mechanical processes 

 'icing described only in so far as they are necessary 

 ior an understanding of the chemistry. Although 

 oncerned chiefly with American practice, and less 

 omplete than the standard English treatises, the 

 olume should be of service to chemists in paper- 

 works laboratories. It is clearly written and well 

 illustrated. 



NO. 2731, VOL. 109] 



Letters to the Editor. 



[TAe 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 refected manuscripts intended for 

 this or any other part of Natuke. No notice is 

 taken of anonymous communications?^ 



The Directive Tendency of Elongated Bodies. 



This letter deals with several topics, perhaps 

 somewhat remotely related to one another, but all 

 suggested by previous letters on the same general 

 subject in Nature of October 20, November 24, 

 December i, and December 22, 192 1. 



In my letter of December 22 it was suggested that 

 Mr. Reeves's results might be explained by the pecul- 

 iarities of the gravity field at the place where the 

 experiments were made. The letter was written 

 before Mr. Reeves's letter appeared and while I was 

 under the impression that his experiments were all 

 made at one place. The suggestion might be plausible 

 as regards any one place, at least until measurements 

 had been made there with an Eotvos balance, but 

 would be highly improbable when applied to every 

 one of the widely scattered places where Mr. Reeves 

 made his tests. 



Col. Grove-Hills in his latter in Nature of Novem- 

 ber 24 directs attention to an important difference 

 between the turning effect of the earth's field on an 

 elongated body supported at its centre of gravity and 

 the turning effect on a similar body when supported 

 by flotation. He attributes quite undeserved credit 

 to me, however, in stating that this matter is fully 

 treated in my article in the September (1921) issue 

 of the American Journal of Science. Only a ver^' 

 special case of the turning effect on a floating body 

 is there treated, and that case is scarcely analogous 

 to the one considered in recent issues of Nature. 



There are two kinds of forces acting on a floating 

 body, namely, the force arising from the earth's 

 gravity field, which is a body force, and the normal 

 pressure of the fluid on the wetted surface. By well- 

 known theorems concerning the transformation of 

 surface integrals into volume integrals it may be 

 proved that the effect of the fluid pressure may be 

 replaced by the body force arising from the earth's 

 field reversed and applied to a solid bounded by the 

 wetted surface and by the free surface of the fluid 

 extended in imagination into the floating body, the 

 density of this solid being the same as that of the 

 fluid. This theorem is proved in very elementary 

 fashion in the ordinary theory, in which gravity is 

 assumed to be constant in intensity and direction, but 

 is equally true when gravity varies in intensity and 

 direction from one part of the region considered to 

 another. 



In dealing with the turning effect on a floating body 

 of the earth's field and of the fluid pressure, it is 

 necessary to make some assumption regarding the 

 depth to which the body is submerged ; a natural 

 assumption is that the body is submerged to such a 

 depth that the downward pull of the earth's field is 

 just balanced by the upward thrust of the fluid press- 

 ure or of the equivalent body force. Let us consider 

 the case of an elongated body symmetrical about a 

 vertical axis through the centre of mass of the body, 

 and let us suppose the earth's field to be also sym- 

 metrical about the same axis ; there is then no 

 moment tending to turn the body about any horizontal 

 axis. Several terms disappear from the general ex- 

 pression for the earth's field on account of the assumed 

 symmetry, but those remaining represent the com- 

 ponent of a force that turns the suspended elongated 

 body about into the prime vertical for a normal field 



