April 23, 1896J 



NA TURE 



57 9 



Atj^^ain, on p. 127, the author commences to explain the 

 ■sleeping" of a top, but stops short after briefly indi- 

 cating that the effect is due to friction. It would require 

 considerable mathematical ability to prove the pheno- 

 menon by actually integrating the equations of motion, 

 taking account of friction in the manner suggested. 



Most people find it easier and quite as effectual to 

 explain the observed results from general principles. 



Such difficulties would mostly disappear in the hands 

 of an accomplished teacher. Moreover, the volume is 

 exceedingly rich in examples, both illustrative and other- 

 wise, and, in addition to those contained in the text, there 

 is a collection of 300 problems at the end. As a class- 

 book, or for use in the lecture-room, Mr. Loudon's treatise 

 may therefore be safely recommended. ' G. H. B. 



OUR BOOK SHELF. 



Our Country's Butterflies and Moths, and how to know 

 them. A Guide to the Lepidoptera of Great Britain. 

 By W. J. Gordon, author of " Our Country's Birds," 

 " Our Country's Flowers," &c. With a thousand 

 examples in colour by H. Lynn, and many original 

 diagrams. Crown 8vo, pp. vii + ^SOj plates 32. 

 (London: Day and Son, 1896.) 

 One remarkable circumstance noticeable in the present 

 plethora of works on British butterflies and moths, is that 

 almost every new one is composed on a different plan. 

 The present book reminds us a little of Wood's " Index 

 Entomologicus," except that the figures are not reduced ; 

 and it will be very useful to schoolboys commencing a 

 collection. All the Macro- Lepidoptera are figured, to the 

 Geometridce inclusive, and all the genera of Micro- 

 Lepidoptera, except in the Tinea", where the selection is 

 limited to typical specimens of each family. The execu- 

 tion, though unequal, is fairly good on the whole, and 

 most of the species figured will be easily recognised, 

 though the want of figures of undersides, and of both 

 sexes in the butterflies will be severely felt in many cases. 

 One or two of the figures are, however, so unlike the in- 

 sects they are supposed to represent, that our first 

 impression on openmg the book was that they were 

 intended to represent some foreign species. We may 

 specially instance the figure of Sphinx piftastri on plate 7, 

 while that of Sinerinthus populi is not much better. 

 But this matters less in the case of conspicuous and easily 

 identified species ; and where accuracy is really needed, 

 as in the smaller Geometridce, the execution is much 

 better. The letter-press largely consists of indices and 

 tables, and contains much useful information relating to 

 Lepidoptera, and even to insects in general. The main 

 ciiaracteristics of the families, genera and species are 

 briefly noticed, as well as their sizes and times of appear- 

 ance, but nothing is said about localities or comparative 

 rarity. Notices of the larvie are limited to those of the 

 butterflies ; English as well as Latin names are used 

 throughout. It is only fair to the author to say that we 

 have rarely seen a book in which so much information 

 was compressed into so small a space. 



Handbook for the Bio-Chemical /laboratory. By Prof. 

 John A. Mandel. Pp. lor. (New York : John Wiley 

 and Sons. London : Chapman and Hall, Limited, 

 1896.) 



In this handbook will be found detailed descriptions of 



the methods of preparation of the most important sub- 

 ;;mces which enter into the composition of the fluids 



md tissues of the animal body, and a synopsis of the 



tests for such substances, arranged in alphabetical order. 



Students of physioloj^ical chemistry will find the volume 



a handy laboratory manual. 



NO. 1382 VOL. 53] 



LETTERS TO THE EDITOR. 

 [7'lte Editor does not hold himself responsible for opinions ex- 

 pressed 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.'\ 



Buried Celluloid. 



It has occurred to many people, that perhaps celluloid vd\^\ 

 be useful as an insulator for electric cables. I feared it migh 

 deteriorate. I have made an experiment of 4^ years duration 

 which may be of use in connection with the subject. On August 

 15, 1 891, I took four photographic quarter-plate celluloid plaies, 

 with the gelatine removed, and treated them as follows : — 



No. I was nailed to an outhouse, and became rotten in a year. 



No. 2 was I foot deep in garden soil. 



No. 3, I foot deep in gravel. 



No. 4, I foot deep in a rubbish heap. 



The last three were dug up on April 6, 1896. Nos. 2 and 4 

 were in as perfect condition as ever. No. 3 has some sand 

 rubbed in, but is perfectly sound. 



The experiment was made at Pitlochry, N.B. The specimens 

 can be seen at my office. George Forbes. 



34 Great George Street, Westminster, S.W., April 16. 



Suggested Photography by Transmitted Heat Rays. 



I AM no chemist, and may be mistaken, and what I propo.se 

 would be more curious than useful ; but I believe it would be 

 possible to get a visible shadow of a small object which w as 

 concealed from sight by being enclosed within an opaque 

 material. There are substances opaque to light, but which 

 transmit the rays of heat ; most readily, I suppo.se, tho.se from 

 the sun, and these are substances on which such heat ra)s 

 inpinging would cause some visible change. 



If the heat-transmitting substance allows the rays to pass 

 without dispersion, preserving their rectilinear direction, then 

 these rays falling upon a duly prepared screen would cause a 

 visible change upon a portion of its surface; and any ordinal y 

 opaque object placed within the heat-transmitting substance 

 would cast a shadow, dark, or bright, as the case may be. 



Penzance. Reginald Courtenay. 



Influence of Terrestrial Disturbances on the 

 Growth of Trees. 



As the subject of forestry has recently been much under dis- 

 cussion, and appears to be exciting more interest in this country 

 than it was, I trust I shall not be trespassing upon your sjiace in 

 calling attention to a peculiar case of timber growth which I 

 have noticed, and in soliciting the opinions of tho.se of your 

 readers who are likely to be well-informed upon foreign woods, 

 as to its true cause. 



There is in the British Museum of Natural History the cross- 

 section of a large Douglas fir grown in British Columbia, and 

 stated to be more than 500 years old at the time it was felled, 

 which was, I believe, in 1885. An attractive feature in the section 

 is that the annual rings have been marked off chronologically, 

 and some historical event, contemporaneous with the growth of 

 the ring to which the date is attached, is given. 



A glance, however, at one part of the surface of the wood, 

 which is polished, reveals a very remarkable modification of the 

 annual rings, which appears to have taken place towards the close 

 of the first century of the tree's existence. About twenty of the 

 rings are there crowded so closely together as to present, at a 

 short distance, the appearance of a zone about three-quarters ot 

 an inch wide running round the trunk, and differently coloured 

 from the rest of the wood. It is also to be particularly observed 

 that the change to ordinary growth on either side of the zone is 

 abrupt ; and, further, that no such phenomenon is afterwards 

 presented during the many centuries of the tree's subsequent 

 development. 



The suddenness of the changes puts out of court the idea that 

 the check to growth might have been due to overcrowding in the 

 forest during the period of the .struggle for supremacy over its 

 fellows, which the tree would undergo, because any effect from 

 this cause would only come on gradually, and diminish in the 

 same manner. 



The supposition that twenty bad seasons occurred in succession, 

 is unlikely under any climatic conditions with which we are 



