174 MECHANICS. 



weight will be eight times greater ; if it be tripled, 

 the weight will be twenty-seven times greater. Hence, 

 the larger any part or machine is made, the less able 

 it becomes to support the still greater increasing 

 weight. If a model is made one tenth the real size 

 intended, then its different parts, when enlarged to full 

 size, become one hundred times stronger, but they are 

 a thousand times heavier, and so are all the weights 

 or parts it has to sustain. All its parts would move 

 ten times faster, which, added to their thousand-fold 

 weight, would increase their inertia and momentum 

 ten thousand times greater. For this reason, a model 

 will often work beautifully when made on a small 

 scale ; but when enlarged, the parts become so much 

 heavier, and their momentum so vastly greater, from 

 the longer sweep of motion, as to fail entirely of suc- 

 cess, or to become soon racked to pieces. 



This same principle is illustrated in every part of 

 the works of creation. The large species of spiders 

 spin thicker webs, in comparison with their own diam- 

 eter, than those spmi by the smaller ones. Enlarge a 

 gnat until its whole weight be equal to that of the 

 eagle, and, great as that enlargement would be, its 

 wing will scarcely have attained the thickness of writ- 

 ing-paper, and, instead of supporting the weight of the 

 animal, would bend down from its own weight. The 

 larger spiders rarely have legs so slender in form as 

 the smaller ones ; the form of the Shetland pony is quite 

 different from that of the large cart-horse; and the 

 cart-horse has a slenderer form than the elephant. 



The common flea will leap two hundred times the 

 length of its own body, and the remark has been some- 



