360 THE MECHANICAL FUNCTIONS. 



often has it been found that a machine which 

 works admirably in a small model, will totally 

 fail in its performance when constructed on a 

 larger scale ? Any lever, of whatever form, may 

 be increased in its dimensions until the force of 

 gravity becomes superior to the cohesion of its 

 own particles : and consequently any structure, 

 like a vegetable or animal body, composed of a 

 combination of levers, would, if its size were to 

 exceed a certain limit, fall to pieces merely by 

 its own weight. This can be prevented either 

 by employing materials of greater cohesive 

 strength, or by increasing, at the points where 

 the strains are greatest, the thickness of the 

 parts compared with their length : but the 

 choice of materials is necessarily restricted within 

 narrow limits, and the latter expedient would 

 entirely alter the relative proportions of the 

 parts, and would require a complete change 

 in the plan of their construction. In passing 

 from the smaller to the larger animals, we find, 

 accordingly, that new models are adopted, a 

 new order of architecture introduced, and new 

 laws of developement observed. We have, 

 next, then, to direct our attention to the pro- 

 cedure of nature in the execution of this more 

 enlarged and comprehensive scheme of animal 

 organization. 



