THE FJ^IMER AT HOME. 263 



In like manner, the common obstacles that present themselves in 

 the public roads, are surmounted by a wheel with peculiar facility 

 As soon as the wheel strikes against a stone or similar hard body, it 

 is converted into a lever for lifting the load over the resisting object. 

 If an obstacle eight or ten inches in height were presented to the body 

 of a carriage unprovided with wheels, it would stop its progress, or 

 subject it to such violence as would endanger its safety. But by the 

 action of a wheel, the load is lifted, and its centre of gravity passes 

 over in the direction of an easy arc, the obstacle furnishing the ful- 

 crum on which the lever acts. Rollers placed under a heavy body 

 diminish the friction in a greater degree than wheels, provided they 

 are true spheres or cylinders, without any axis on which they are 

 constrained to move ; but a cylindrical roller occasions friction, when- 

 ever its path deviates in the least from a straight line. 



The mechanical advantages of a wheel are proportionate to its 

 size, and the larger it is, the more effectually does it diminish the 

 ordinary resistances. A large wheel will surmount stones and similar 

 obstacles better than a small one, since the arm of the lever on which 

 the force acts is longer, and the curve described by the centre of the 

 load is the arc of a larger circle, and of course, the ascent is more 

 gradual and easy. In passing over holes, ruts or excavations, also, a 

 large wheel sinks less than a small one, and consequently occasions 

 less jolting and expenditure of power. The wear also of large wheels 

 is less than that of small ones, for if we suppose a wheel to be three 

 feet in diameter, it will turn round twice, while one of six feet in 

 diameter turns round once ; so that its tire will come twice as often 

 in contact with the ground, and its spokes will twice as often have to 

 support the weight of the load. In practice, however, it is found 

 necessary to confine the size of wheels within certain limits, partly 

 because the materials used would make wheels of great size heavy 

 and cumbersome, since the separate parts would necessarily be of 

 large proportions to have the requisite strength, and partly because 

 they would be disproportioned to the size of the animals employed in 

 draught, and compel them to pull obliquely downwards, and therefore 

 to expend a part of their force in acting against the ground. 



LOCUSTS. Of all animals capable of adding to the calamities 

 of mankind, by destroying the vegetable products of the earth, the 

 migratory locusts would seem to possess the most formidable powers of 

 destruction. In Syria, Egypt, and almost all the south of Asia, these 

 insects make their appearance in legions, and carry desolation with 

 them, in a few hours changing the most fertile provinces into barren 

 deserts, and darkening the air by their numbers. Happily for man- 

 kind, this calamity is not frequently repeated, for it is the inevitable 

 precursor of famine, and its horrible consequences. The annals of 

 the most southern Asiatic climates are filled with accounts of the 

 devastations produced by locusts. They seldom visit Europe in such 



