March 12, 1903] 



NA TURE 



439 



The Design of Simple Roof Trusses in Wood and Steel. 



By M. A. Howe, C.E. Pp. viii + 129. (New York : 



John Wiley and Sons; London : Chapman and Hall, 



Ltd., 1902!) Price 2.00 dollars. 

 This little book is intended to serve the purposes of 

 students in mechanical and electrical engineering, who 

 desire to have some knowledge of the methods of design 

 adopted in civil engineering, and hence the examples 

 chosen are two very simple forms of roof trusses. 



The first two chapters give a brief outline of the 

 general principles on which are based the graphical 

 determination of the stresses in the various parts of a 

 roof truss ; then follows a carefully written chapter on 

 the strength of the various materials used in roof work 

 in tension, compression, cross-bending and shear. The 

 author then works out in complete detail the design of 

 a 60-feet-span wooden roof truss, and of a 60-feet-span 

 steel roof truss — one particularly good feature of this 

 part of the book is the extreme care which has been 

 shown in the explanation of the design of the various 

 joints needed in such roof trusses. 



There are three well-drawn plates to illustrate these 

 two roofs and a series of seventeen tables, including 

 a most useful set giving moments of inertia, radii of 

 gyration, &c, for various rolled sections commonly 

 used in the struts of roof trusses. 



We can recommend the book as one likely to be of 

 much use to both teacher and student in classes for 

 the study of civil engineering design. 



Stereotomy. Bv A. W. French, C.E., and H. C. Ives, 

 C.E. Pp. iv'+ 115. (New York: John Wiley and 

 Sons; London: Chapman and Hall, Ltd., 1902.) 

 Price 10s. 6d. 



This is another text-book for the student in civil engin- 

 eering, and treats of masonry work, mainly in arches 

 and domes. 



The two first chapters give a brief account of the 

 various stones used for building purposes, and their 

 physical characteristics, and of the tools used in quarry- 

 ing and cutting the blocks into their finished forms. 

 The third chapter treats of plane-sided structures, such 

 as bridge piers and abutments, with several practical 

 examples illustrated by plates. Chapter iv. deals with 

 structures containing developable surfaces, and in- 

 cludes a detailed treatment of the masonry arch ; the 

 geometry of the arch is explained, and the preparation 

 of the working drawings for use by the stone mason, 

 and also the methods employed in dressing the stones. 

 The oblique or skew arch, difficult both in its geometry 

 and in its constructional details, is worked out in a 

 separate chapter, with several fine illustrative plates. 



As the twenty-two plates which illustrate the text 

 are drawn from actual masonry structures, such as 

 the Worcester City Hall, the Trenton railway bridge, 

 &c. , they will prove extremely useful to the student, 

 more especially as there are few recent text-books which 

 deal at all fully with this branch of the art of the civil 

 engineer. 



Round the Horn before the Mast. By A. Basil Lub- 

 bock. Pp. x + 375. (London : John Murray, 1902.) 

 Price Ss. net. 



These experiences of a public school man, who at San 

 Francisco turned himself into an ordinary seaman and 

 " signed on for two pounds a month for a passage 

 round the Horn, calling at Queenstown for orders, 

 either for the British Isles or Continent," will interest 

 most boys. Probably few adult readers will get to the 

 end of the volume, but Mr. Lubbock can congratu- 

 late himself that most boys will read all he has written 

 and pronounce it " good." 



no. 1 74 1, VOL. 67] 



LETTERS TO THE EDITOR. 



[The 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.} 



Radio-activity of Ordinary Materials. 

 I should like to say a few words in answer to Prof. 

 Armstrong's letter, in which he suggests that the effects 

 observed by Prof. McClennam and myself are not due to 

 radio-activity, but to chemical changes at the surface of the 

 substances experimented upon. In speaking of the radio- 

 activity of ordinary materials, I mean that they show effects 

 differing only in degree from those exhibited by uranium 

 and radium. These effects, as observed experimentally, 

 are as follows : — 



(1) There is a leakage of electricity from a charged body 

 in the neighbourhood. This leakage is proportional to the 

 E.M.F. for small E.M.F.'s, but for large ones independent 

 of it. 



(2) The effect varies with the pressure of the air, being for 

 small pressures proportional to the pressure, and for large 

 pressures independent of it, when the E.M.F. is sufficient. 



(3) The rate of leak is the same for positive electricity as 

 for negative. 



(4) The rate of leak does not depend on the temperature. 



(5) When other gases are substituted for air, the leak 

 is nearly proportional to the density of the gas, except in 

 the case of hydrogen, which gives about one-eighth the 

 effect that air does. 



In every one of these points there is exact agreement of 

 behaviour between uranium and the ordinary materials. 

 On the other hand, I am not aware that any difference 

 has been brought to light, except as to the magnitude of 

 the effects. Until such a difference should appear, I think 

 we may fairly, and without dogmatism, apply the maxim 

 that similar effects are due to similar causes. In other 

 words, we may conclude that the other substances, like 

 uranium, are radio-active. R- J- Strutt. 



A Case of Pseudo-mimicry. 



In Campbell Island, south of New Zealand, the breeze-fly 

 (Helophilus campbellicus), one of the Syrphidse, so closely 

 resembles a blow-fly (Calliphora eudypti) that when, in 

 1901, I captured a specimen of the first, which is rare, I 

 thought it was the blow-fly, which is common ; and it was 

 not until I was transferring my captures to boxes that I 

 found out my mistake. 



C. eudypti has the abdomen metallic bronzy green, with 

 a dark thorax, and black and tawny legs. H. campbellicus 

 has also a metallic bronzy green abdomen, a dark thorax, 

 and black and tawny legs. There is a difference in the 

 stripes on the thorax, but they are obscure. In size the 

 two insects are the same. 



Now in any other locality this resemblance could be put 

 down to mimicry. The blow-fly is common and offensive. 

 The breeze-fly is rare and feeds on flowers. Everything 

 favours this explanation except that in Campbell Island 

 there are no insect-eating birds and no lizards, and con- 

 sequently mimicry would be useless. Evidently, in this case, 

 the resemblance is only a coincidence and has no meaning. 



F. W. Hutton. 



Museum, Christchurch, N.Z., January. 



Accidental resemblances between insects are to be ex- 

 pected. The immense number of species and the necessary 

 limitation in the variety of colours and patterns must lead 

 to coincidences, as, I believe, was first pointed out by Mr. 

 F. E. Beddard in his book on "Animal Coloration." The 

 coincidences would, of course, be relatively more numerous 

 when the patterns are simple. Accidental resemblances 

 being independent of locality and of an origin based upon 

 utility, it follows that a very small proportion of the total 

 number of cases are to be expected to occur under conditions 

 which are the characteristic concomitants of true mimetic 

 resemblance. 



