28 ON MAGNITUDE [ch. 



in one of his chapters he deals with the proposition, "Est im- 

 possibile, ut homines propriis viribus artificiose volare possint*." 



But just as it is easier to swim than to fly, so is it obvious 

 that, in a denser atmosphere, the conditions of flight would be 

 altered, and flight facihtated. We know that in the carboniferous 

 epoch there hved giant dragon-flies, with wings of a span far 

 greater than nowadays they ever attain; and the small bodies 

 and huge extended wings of the fossil pterodactyles would seem 

 in like manner to be quite abnormal according to our present 

 standards, and to be beyond the limits of mechanical efficiency 

 under present conditions. But as Harle suggests f, following 

 upon a suggestion of Arrhenius, we have only to suppose that in 

 carboniferous and Jurassic days the terrestrial atmosphere was 

 notably denser than it is at present, by reason, for instance, of 

 its containing a much larger proportion of carbonic acid, and we 

 have at once a means of reconciling the apparent mechanical 

 discrepancy. 



Very similar problems, involving in various ways the principle 

 of dynamical simihtude, occur all through the physiology of 

 locomotion: as, for instance, when we see that a cockchafer can 

 carry a plate, many times his own weight, upon his back, or that 

 a flea can jump many inches high. 



Problems of this latter class have been admirably treated both 

 by Gahleo and by Borelli, but many later writers have remained 

 ignorant of their work. Linnaeus, for instance, remarked that, 

 if an elephant were as strong in proportion as a stag-beetle, it 

 would be able to pull up rocks by the root, and to level mountains. 

 And Kirby and Spence have a well-known passage directed to 

 shew that such powers as have been conferred upon the insect 

 have been withheld from the higher animals, for the reason that 

 had these latter been endued therewith they would have "caused 

 the early desolation of the world J." 



* De Motu Animalium, I, prop, cciv, ed. 1685, p. 243. 



f Harle, On Atmospheric Pressure in past Geological Ages, Bull. Geol. Soc. 

 Fr. XI, pp; 118-121; or Cosmos, p. 30, July 8, 1911. 



X Introduction to Entomology, 1826, ii, p. 190. K. and S., like many less learned 

 authors, are fond of popular illustrations of the "wonders of Nature," to the neglect 

 of dynamical principles. They suggest, for instance, that if the white ant were 

 as big as a man, its tunnels would be "magnificent cylinders of more than three 



