THE WHEEL. 469 



the ancient habit of the pupa had been remembered by the 

 perfect insect, the long ends of the hinder wings taking the 

 place of the pupal tail, and the legs that of the belt. 



THE WHEEL. 



YET another aid to locomotion is found in the WHEEL, a 

 contrivance for diminishing friction. 



When man first learnt that heavier weights could be dragged 

 than carried, he simply placed them on flat boards to which 

 ropes were attached. The next step was necessarily the 

 invention of the sledge, the burden resting on two parallel 

 runners, the ends of which were slightly curved so as to prevent 

 them from hitching against any small obstruction. In some 

 countries such, for example, as in Esquimaux-land the sledge 

 is the only vehicle practicable, and even Europeans, when 



WHEKL-SPICULK OF CHlRODOfA. CART-WHEEL. 



they visit that country, are foin to adopt the sledge if they 

 would live. 



But, in more temperate zones, the Wheel is paramount. In 

 its earlier stages the wheel was a very simple business. It 

 was simply a section of a tree-trunk, dubbed roughly round, 

 and with a hole in the centre, through which the axle passed. 

 Such wheels are still in existence in many parts of Europe ; 

 and, owing to the want of regularity of outline in the circum- 

 ference, and the utter absence of grease, the wheels keep up 

 a continuous shriek, almost deafening to those who are unused 

 to it, but perfectly unheeded by those who own or drive the 

 vehicle. 



The next improvement was to make the circumference of the 

 wheel as perfectly circular as the art of man could devise, and, 



