SCIENTIFIC NE\VS. 



[Mar. 2 1 5 



uneasy, and buries itself in the mud or sand, as if it 

 disliked the operation. But the mischief is now done, 

 and the parasite has got a lodging within its host. Some 

 weeks appear to be necessary for hatching the embryo. 

 It is unlikely that the young bitterhng preys upon or 

 injures in any way the mussel which gives it shelter. 



This curious history by no means rivals the strange 

 modes of egg-laying already observed among insects, 

 but it is singular among fishes, whose life-history con- 

 tains, as a rule, so little romance. The nest-building 

 sticklebacks and the siluroids, which hatch their eggs in 

 the mouth of the male, are alone worthy to be mentioned 

 side by side with the ingenious bitterling. It may 

 further be remarked that the eggs are few and large, as 

 in nearly all cases where something like parental care is 

 shown. 



«-J>>^>»^i<f-» 



SOME EFFECTS OF DIFFERENCE IN 

 SIZE. 



ENGINEERS have long been familiar with the fact 

 that small objects are better able to support their 

 own weight than large ones of the same materials 

 and the same proportions. A card house may be built 

 strong enough to bear its own weight and a great deal 

 more, but multiplying all its dimensions by loo, and it 

 will collapse by bending or crushing its sides. A large 

 plank, supported only at the ends, will bend to a greater 

 angle than a small one of precisely the same propor- 

 tions. This principle is clearly seen in the skeletons of 

 animals. The bones of small animals appear light and 

 elegant in comparison with those of very large ones, 

 when both are drawn to one scale, and yet they may be 

 subjected without danger to very trying conditions. A 

 dog's hind leg is ordinarily bent at considerable angles, 

 so as to give the possibility of a sudden leap ; the 

 massive limbs of the elephant are only safe when the 

 bones are pretty nearly in the direction of pressure. 

 Strength increases as the square of any linear dimen- 

 sion, weight in the higher ratio of the cube. 



In animals altogether similar except in size what is 

 called the relative muscular force diminishes very fast as 

 the size increases. For contractile force increases as 

 the sectional area, i.e., as the square of any linear dimen- 

 sion, while the weight increases as the cube. Many 

 insects can raise (over a pulley) twenty times their own 

 weight, but a horse can only raise a little more than half 

 his own weight. Nevertheless, the contractile force of 

 the muscles of the horse is, fibre for fibre, far greater 

 than that of any insect. Plateau's experiments show 

 that what he calls the absolute muscular force (contractile 

 force per unit of sectional area) is very low in insects 

 and very high in quadrupeds. 



When the leaping powers of animals are compared, 

 no allowance should be made for size. To bring out a 

 result of any value, we ought simply to divide in each 

 case the work done by the weight. For, in similar 

 animals the work done in contraction is proportional to 

 the volume of the muscles, and the volume of the 

 muscles to the weight. But the work = weight raised 

 X height, and the weight is in each case the weight of 

 the animal itself. Therefore, 



Wt X Ht ^ Wt' X H t' 



Wt Wt' 



The weights, in short, "go out," and Ht = Ht', whenever 



equal efficiency in leaping exists, whatever be the scale 

 of the creatures compared. Hence we may cease \o 

 wonder at such well-known cases as the flea, which 

 leaps a foot high, i.e., 200 times its own length. It has 

 been observed* that this apparently stupendous feat 

 indicates precisely as great efficiency of muscles or other 

 leaping apparatus as would be implied in a man's leap 

 to the same height, viz., one foot. 



Large objects have the advantage of greater stability 

 than small ones, z'.e., they can more easily resist overturn- 

 ing. The overturning forces, where for instance similar 

 structures are exposed to wind of a given velocity, can 

 be shown by very simple reasoning to vary as the cube 

 of any linear dimension, whereas the resisting forces 

 vary as the fourth power. Hence if one obelisk, pillar, 

 chimney, or pinnacle be twice as great in every dimension 

 as another, it will be able to resist twice as much pres- 

 sure of wind per square foot, and the resistance to 

 overturning will be sixteen times as great as in the 

 smaller one.f A tall object may be built more slightly 

 than a short one — where stability only, and not strength, 

 has to be considered — of the very same external propor- 

 tions. Again, a big tree may be equally firm with a re- 

 latively smaller extent of roots than a small tree. An. 

 ostrich can stand or run upon two relatively short toes, 

 while a small bird requires three relatively long ones. For 

 the very same reason, little pebbles are easily rolled along 

 the bed of a stream, while large pebbles of similar pro- 

 portions and of the very same substance stick fast. A 

 yet more familiar, though not exactly comparable case, is 

 that of stones in air. We know how the tiny particles 

 of limestone or granite, rubbed off the road by horse- 

 shoes and wheels, rise into the air with a light summer 

 breeze, while, fortunately for us, the big lumps of the 

 same kind lie still. 



The further investigation of the subject involves 

 considerations of a kind not yet familiar to general 

 readers, but those who have patience to master 

 what may be called the Mechanics of Size may 

 be assured that these studies will give a new interest to 

 many every-day facts, and clear up many long-recognised 

 perplexities. 



MICROBIOLOGY. 



A DISCUSSION is now in progress between MM. 

 Roux and Chamberland on the one hand, and M. 

 Chauveau on the other, concerning the immunity which 

 one attack of a zymotic disease confers against the sub- 

 sequent return of the infection. M. Pasteur and his 

 disciples contend that the micro-organism which occa- 

 sions the disease perishes by exhausting, in the medium 

 where it has been developed, all the elements essential 

 to its life and multiplication. M. Chauveau, on the con- 

 trary, argues that the disease germ perishes by self- 

 poisoning, i.e., that it develops a soluble ferment in the 

 presence of which it cannot exist. He advances as an 

 argument that all lambs born of mothers which had been 

 inoculated have proved non-susceptible even to the 

 most virulent subsequent inoculations. But no micro- 

 bion could have penetrated into their organisms, since 

 the placenta is a filter which does not give passage to 

 any figured ferments. 



* Miall and Denny on " The Cockroach," p. 84. 



f These results are worked out by Professor James Thomson, in his 

 " Comparisons of Similar Structures." Tr. Eng, and Shipbuilders 

 in Scotland, vol. xix., p. 69 (1S75-6). 



