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NATURE 



[Oct. 24, 1889 



metrical development of the two sides of the body is of vital im- 

 portance, and is strictly preserved by natural selection ; but more 

 or less defect of symmetry often occurs as a variation or mon- 

 strosity, and in cases where such asymmetry was useful these 

 variations would be preserved and increased by selection and 

 heredity. An altogether erroneous view is taken of the fact of 

 effort being used in this case, as if it were something unusual. 

 But in all cases selection produces changes which are useful and 

 whose use is often indicated by effort. The giraffe uses effort 

 in stretching its neck to obtain food during a drought ; the ante- 

 lope exerts itself to the utmost to escape from the leopard ; but 

 it is now recognized that it is not the individual change produced 

 by this ejfort that is inherited, but the favourable constitution 

 which renders extreme effort unnecessary, and causes its pos- 

 sessors to survive while those less favourably constituted, and 

 who therefore have to use greater effort, succumb. In the case 

 of the developing flat-fish also, the effort indicated the direction 

 of the useful modification, and any variations tending either to 

 the right kind of asymmetry or to fa mobility of the eye, ad- 

 mitting its being twisted and retained in its new position, 

 during the growth of the individual, would be certainly 

 preserved. 



I wish to take this opportunity of thanking Prof. Ray Lan- 

 kester for his careful and appreciative review of my book. I am 

 too well aware of my own deficiency in training as a naturalist 

 not to admit all the shortcomings which he so tenderly refers to. 

 It is quite refreshing to me to read at last a real criticism from a 

 thoroughly competent writer, after the more or less ignorant 

 praise which the book has hitherto received. I admit also that 

 the term "laboratory naturalist," to which he demurs, was not 

 well chosen. I meant it as the opposite, not so much to ' ' field 

 naturalist" as to " systematic naturalist " ; audit still seems to 

 me that the gentlemen he refers to as not being "laboratory 

 naturalists" are still less "systematic naturalists," in the sense 

 of having specially devoted themselves to the observation, 

 description, and classification of more or less extensive groups of 

 species of living organisms. Alfred R. Wallace. 



A Mechanical Illustration of the Propagation of a 

 Sound-Wave. 



Having to prepare some lectures on sound, I wished, if 

 possible, to illustrate, without any very complicated apparatus, 

 the way in which a sound-wave is propagated. 



The following method suggested itself to me. As I have not 

 met with the method while examining a large number of works 

 on sound and wave motion, I venture to send a description of it 

 to Nature, as it may perhaps be of use to some students of 

 acoustics. 



A row of pendulums of equal length, a, b, c . . . I (Fig. i) 

 are suspended from a rod ab ; in order to start the pe ndulums, 



Fig. I. 



the bobs are held against an angular-shaped board, FCD, the 

 rod being held in a plane slightly behind the plane of the 

 board ; if now the rod and pendulums be raised together vertically, 

 / will first swing, then k, and so on, till all are free : when the 

 pendulums are raised with a uniform velocity, then each pendulum 

 starts at an equal period of time after the one which is next to it ; 

 the result is that a wave-motion is seen to run along the line of 

 bobs as they vibrate to and fro. Such an arrangement has been 

 used to illustrate wave-motion, as each bob moves with harmonic 

 motion. But such an arrangement does not illustrate directly those 

 compressions and rarefactions whereby sound is propagated. A 

 slight movement, however, of the rod at once makes it do so. If, 

 whiie the pendulums are vibrating, the rod from which they are 

 suspended be turned in the horizontal plane through a right 

 nnglp, the direction of the swing of each pendulum is not changed. 



and all the pendulums swing in the same plane. This will become 

 clear from (Fig. 2), where the pendulum bobs viewed along ox 

 appear to trace out wave-motion ; the relative position of the 

 bobs after the rod which supports them is turned through a 

 right angle is shown along ov ; the motion then illustrates 

 mechanically those movements of air-particles which, when in 

 compression and rarefaction, propagate a sound-wave. If the rod 

 be turned back through a right angle, the wave-motion is again 

 restored. The illustration must be taken with the obvious defect, 

 viz. that the bobs move in arcs, and not in straight lines. 



Care should be taken that the amplitude of vibration be not 

 greater than the distance between the points of suspension minus 



Fig. 2. 



the diameter of a bob, otherwise the bobs will hit each other 

 when vibrating in the plane YZ. 



Twelve pendulums made of lead bullets 1*5 centimetre in 

 diameter, suspended from threads 30 centimetres long, with a 

 distance between each of 5 centimetres, were found to answer 

 well by the author. 



If the board used for starting the pendulums be made of the 

 angular shape, fgk, then the movement of the bobs in their 

 second position illustrates the propagation of sound on each side 

 of its origin. Frederick J. Smith. 



Trinity College, Oxford, October i. 



On some Effects of Lightning. 



The twisting of one of the two trees near St. Albans, which 

 were struck in such a remarkable manner by lightning, may well 

 have been caused by the fall of the top of the tree, as Mr. 

 Griffith suggests, and not directly by the lightning. 



I have been unsuccessful in ascertaining whether the core of 

 the tree is situated nearer that side where the explosion seems 

 to have been most violent; but a more detailed examination only 

 enforces the conclusion which Mr. Griffith and I arrived at, 

 that the explosion must have occurred inside the stem, if not 

 actually at the core of the tree. 



The effects in this case can meet with no explanation from the 

 supposition that the lightning passed between the bark and the 

 tree, generating thereby sufficient steam to blow off the bark 

 and shatter the stem — an explanation which Mr. Maclear sug- 

 gests in his letter of September 25. I doubt if any source of 

 heat would ever convert water so quickly into steam as to endow 

 it with the power which dynamite has of shattering a hard object 

 lying in contact with it, while the gases formed are restrained 

 by the Comparatively feeble resistance of the bark and outer 

 air ; nor can we suppose that sufficient heat could paS into the_ 

 stem to generate steam there adequate for such an explosiot ~ 

 even if the uncharred condition of the wood did not prove it 

 contestably that the temperature had not been raised very higl 

 It seems more probable to me that such explosions must 

 caused by the lightning electrolysing the liquids in the stem, an 



