252 THE MECHANICAL FUNCTIONS. 



must be endowed. We have already had occasion to notice 

 a remarkable instance of the force and permanence of mus- 

 cular contraction in tliose caterpillars which frequently re- 

 main for hours together in a fixed attitude, with their bodies 

 extended from a twig, to which they cling with their hind 

 feet alone.* Ants will carry loads which are forty or fifty 

 times heavier than their own bodies: and the distance to 

 which many species, such as the Elater, the Locust, the 

 Lepisma, and above all the Pidex, are capable of leaping, 

 compared with the size of the insects themselves, appear 

 still more astonishing. Linnaeus has computed that the 

 Melolontha, or chaffer, is, in proportion to its bulk, more 

 than six times stronger than the horse: and has asserted that 

 if the same proportional strength as is possessed by the Lu- 

 canus, or stag-beetle, had been given to the elephant, that 

 animal would have been capable of tearing up by the roots 

 the largest trees, and of hurling huge rocks against his as- 

 sailants, like the giants of ancient mythology. 



But while we must admit that all these facts indicate a re- 

 markable degree of energy in the contractile power of the 

 miuscular fibres of insects, we should at the same time re- 

 collect that the diminutive size of the beings w^hich display 

 those powers is itself the source of a mechanical advantage 

 not possessed by larger animals. The efficacy of all mechani- 

 cal arrangements must ultimately depend on a due propor- 

 tion between the moving and the resisting forces: hence 

 mechanism of every kind must be adjusted with reference 

 not merely to the relative, but to the absolute dimensions 

 of the structures themselves. This will be evident when 

 we consider that the forces which are called into action are 

 resisted by the cohesion of the particles composing the solid 

 parts of the machine: and this cohesion being not a variable, 

 but a constant and definite force, must necessarily limit the 

 dimensions of every mechanical structure, whether intend- 

 ed for stability or for action. An edifice raised beyond a 

 certain magnitude, will not support itself, because the weight 



* See Fig- 148*, p. 224. 



