268 On the relation which subsists between a 



How interesting it is to compare the different animals, and to trace 

 the gradual change of form which accompanies each increase of 

 size ! In the smaller animals, the strength is, as it were, redundant, 

 and there is room for the display of the most elaborate ornament. 

 How complex or how beautiful are the myriads of insects which 

 float in the air, or which cluster on the foliage ! Gradually the lar- 

 ger of these become more simple in their structure, their ornaments 

 less profuse. The structure of the birds is simpler and more uniform, 

 that of the quadrupeds still more so. As we approach the larger 

 quadrupeds, ornament, and then elegance, disappear. This is the 

 law in the works of Nature, and this ought to be the law among 

 the works of Art. 



Among one class of animals, indeed, it may be said that this law 

 is reversed. We have by no means a general classification of the 

 fishes ; but, among those with which we are acquainted, we do not 

 perceive such a prodigious change of form. Here, however, the 

 animal has not to support its own weight ; and whatever increase 

 -may take place in the size of the animal, a like increase takes place 

 in the buoyancy of the fluid in which it swims. Many of the smaller 

 aquatic animals exhibit the utmost simplicity of structure ; but we 

 know too little of the nature of their functions to draw any useful 

 conclusions from this fact. 



Having said thus much on the relative strengths of a machine 

 and of its model when at rests, I proceed to compare their strengths 

 and actions when in motion. 



This subject naturally divides itself into two heads j the one relating 

 to the ability of the structure to resist the blows given by the moving 

 parts, either in their ordinary action, or when, by accident, they es- 

 cape from their usual course ; the second treating on the changes 

 which take place on the friction of the parts when these are enlarged 

 or diminished. 



The ability of support to resist the impetus of a moving body, is 

 estimated by combining the pressure which it is able to bear with 

 the distance through which it can yield ere disruption takes place. 

 In the case of a support which acts longitudinally, the strength is pro- 

 portional to the square of the linear dimension, while the distance 

 through which it can yield is as the linear dimension itself. Alto- 

 gether, then, the ability to resist a blow is proportional to the cube 

 of the length ; that is, to the weight of the body which is destined 

 to act upon it. If, then, the linear velocity of the machine is to be 



