Febkuakv 3, 1910] 



NA TURE 



397 



Experiments in dynamics succeed those on statics, 

 in wliich velocity, acceleration, inertia, work, kinetic 

 energy, and centrifugal force are measured. The 

 properties of vectors are liere developed, though not 

 -SO fully as might be desired, and occasionally there 

 is a vagueness of language which must give trouble 

 to a student who tries to think clearly. 



Remaining chapters deal with the efficiencies of 

 simple machines ; the application of the link polygon ; 

 the drawing of reciprocal figures for jointed plane 

 frames; and the determination of Young's modulus 

 and of moments of inertia. In addition to the 

 examples appended to each chapter, the authors give, 

 at the end of the book, more than three hundred miscel- 

 laneous examples, and copies of recent examination 

 papers, both theoretical and practical. A good index 

 as provided, and teachers will find this book very 

 helpful indeed. 



Air and Health. By R. C. Macfie. Pp. vii + 34:;. 



(London : Methuen and Co., 1909.) Price ys. 6d. 



net. » 



This is a very readable work, containing much useful 

 information. As the subject is dealt with in a manner 

 which renders it interesting and easy of comprehen- 

 sion, even to people who may have no acquaintance- 

 ship with chemistry and physics, it should appeal to 

 a wide range of readers ; but it will prove of special 

 service to the medical student of hygiene and pre- 

 ventive medicine, and the medical writer doubtless 

 had this end mainly in view. The subject of the 

 CO, respiratory impurity in the atmosphere is treated 

 at some length. In this connection the author states 

 (P- '33) that "Anyone who compares his power of 

 mental work in a pure and in a CO^ laden atmo- 

 sphere, even if the latter be dry and cool, will find in 

 the latter a considerable diminution, showing that at 

 least nerve metabolism is affected." This statement 

 implies that the CO„ in a badly-ventilated room (as 

 ordinarily understood) is capable, per se, of producing 

 results which more recent experimental work demon- 

 strates to be due to the combined effects of the altered 

 physical conditions of the air — the increased moisture 

 and temperature, in a stagnant atmosphere. The 

 important subject of the composition, &c., of sewer 

 air is inadequately discussed in eight lines. 



Mr. Spcnce's suggestion that all the smoke of a 

 city should be conducted by the sewers to a few 

 enormous chimneys, where it could be completely 

 burnt (p. 155), appears to be commended by the author. 

 Certainly Sir B. W. Richardson proposed to adopt 

 this method in his model city of Hygeia, but it is 

 inconceivable that it could be efficiently applied. The 

 various methods of freeing air from trade dusts and 

 gases might have been included with advantage in 

 such a work, and the volume seems incomplete with- 

 out some reference to analytical methods — even if the 

 reference were confined to the more simple practical 

 tests for the more important gaseous impurities of the 

 atmosphere of dwelling and working rooms. 



But notwithstanding these deficiencies, the matter 

 is, generally speaking, so well dealt with that the 

 work may be confidently recommended for study and 

 reference purposes. 



The range of treatment embraces eighteen chapters. 

 These deal with the composition of air ; the discovery 

 of the atmospheric gases ; respiration ; air-pressure in 

 relation to life; the temperature, humidity, and 

 therrnojysis of air, and health ; air and solar radiation ; 

 electricity^ and radio-activity, and their relation to 

 climate ; impure and polluted air ; dust and germs ; 

 ventilation, draughts, and "colds"; artificial atmo- 

 spheres ; the open-air treatment of consumption ; and 

 open-air schools. 



NO. 2IOI, VOL. 82] 



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 0/ Nature. 

 No notice is taken of anonymous communications.] 



The Natural History Museum. 



In reference to the article on the Natural History 

 Museum in Nature of January 20 (p. 343), in which Sir 

 .Archibald Geikie is represented as saying that the questions 

 put to him in the letter from the Speaker of the House 

 of Commons " were entirely in reference to the relations 

 between the Trustees and the Museum," historical accuracy 

 demands that it should be stated that- the questions of Mr. 

 Lowther were two in number (see Nature, December 16. 

 1909, p. 196). They were : — (i) " Whether the Board of 

 Trustees, acting through its Standing Committee, is in 

 your judgment the best authority for the government of 

 such an institution as the Natural History Departments of 

 tlio British Museum? " and (2) " Whether, under the exist- 

 ing statutes and rules, the scientific management of the 

 Natural History Museum suffers any detriment from its 

 association with the Museum at Bloorasbury?" 



It is clear that in his communication to you Sir Archi- 

 bald Geikie has overlooked Mr. Lowther 's second question, 

 with which alone (so far as these two questions are con- 

 cerned) the present agitation has concerned itself. We have 

 always admitted that " a system of control by Trustees is 

 the best " (see Nature, December 30, igog, p. 254). You 

 were therefore quite right in your impression that the 

 main point of contention was the complete separation of 

 the governance and administration of tlie Natural History 

 Museum from the other collections in the British Museum. 

 Mr. Carruthers's letter does not touch tlie point at issue, 

 except in so far as it seems to show that Prof. Huxley 

 was in agreement with our view that " a system of control 

 by Trustees is the best." He says nothing to show that 

 Prof. Huxley came to disagree with the conclusion to 

 which he had twice put his signature, once after a pro- 

 longed inquiry by Royal Commission, that it was of 

 fundamental importance that the governance and adminis- 

 tration of the two Museums should be separate. 



A. Sedgwick. 



2 Sumner Place, S.W., January 30. 



Markings on Mars as seen with Small and Large 

 Telescopes, 



That large telescopes are liable to less accurate definition 

 of planetary markings than small ones can easily be 

 verified in the following manner, and tested for any given 

 occasion. The spurious disc and rings made of a star by 

 a telescope is a real image, due to the interference of the 

 light-waves — as real an image, although produced by a 

 different cause, as that of a planet's disc with its mark- 

 ings. If atmosphere and glass be perfect, the image 

 consists of a round disc, encircled by concentric and con- 

 tinuous rings of light. The only difference in the image 

 with different apertures is that the larger the aperture the 

 smaller the disc, and the closer and thinner the rings. If 

 this image assume any other form, it is either because the 

 objective is poor, which is commoner with objectives than 

 is supposed, as Hartmann's tests have shown,' or because 

 the seeing is defective. In proportion as the seeing is bad 

 the rings of the image begin to waver, then break up into 

 fragments, a sort of mosaic, and finally end in an in- 

 discriminate assemblance of points. In certain kinds of 

 bad seeing the parts may seem quite steady, but that the 

 mosaic exists is proof positive of poor seeing. 



Now this image as made by different apertures may be 

 compared either by observing with different telescopes at 

 the same place and time or by diaphragming down a large 

 objective. When this is done it becomes at once evident 

 that the smaller aperture always gives the more accurate 

 definition of the optical disc and rings in spite of the 

 theoretical greater resolvability of the larger glass. If, 



1 The Lowell Observatory glass has been tested by this method, and proves 

 to yield the best images of any yet examined. (Bulletin in preparation.) 



