Encyclopaedia Americana: a popular dictionary of arts, sciences, literature, history, politics and biography, brought down to the present time : including a copious collection of original articles in American biography : on the basis of the seventh edition of the German Conversations-Lexicon (Volume 8).
- Date:
- 1830-33
Licence: Public Domain Mark
Credit: Encyclopaedia Americana: a popular dictionary of arts, sciences, literature, history, politics and biography, brought down to the present time : including a copious collection of original articles in American biography : on the basis of the seventh edition of the German Conversations-Lexicon (Volume 8). Source: Wellcome Collection.
Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)
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![adapted to their structure and situation ; and it is probable that no animal, man not being excepted, can exert his strength more advantageously by any other than the natural mode, in moving himself over the common surface of the ground* Thus walking ears, velocipedes, &c, al- though they may enable a man to increase his velocity, in favorable situations, for a short time, yet they actually require an in- creased expenditure of power, for the purpose of transporting the machine made use of, in addition to the weight of the body. When, however, a great additional load is to be transported with the body, a man, or animal, may derive much assistance from mechanical arrange- ments. For moving weights over the common ground, with its ordinary asperi- ties and inequalities of substance and structure, no piece of inert mechanism is so favorably adapted as the wheel-car- riage. It was introduced into use in very early ages. Wheels diminish friction, and also surmount obstacles or inequali- ties of the road, with more advantage than bodies of any other form, in their place, could do. The friction is dimin- ished by transferring it from the Surface of the ground to the centre of tjie wheel, or, rather, to the place of contact between the axletree and the box of the wheel; so that it is lessened by the mechanical advantage of the lever, in the proportion which the diameter of the axletree bears to the diameter of the wheel. The rub- bing surfaces, also, being kept polished and smeared with some unctuous sub- stance, are in the best possible condition to resist friction. In like manner, the common obstacles that present themselves i.n the public roads, are surmounted by a wheel with peculiar facility. As soon as rhe wheel strikes against a stone or simi- lar hard body, it is converted into a lever for lifting the load over the resisting ob- ject. If an obstacle eight or ten inches in height were presented to the body of a carriage unprovided with wheels, it would -to]) its progress, or subject it to such vio- lence 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 fulcrum on which the lever acts. Rollers placed under a heavy body di- minish the friction in a greater degree than wheels, provided they are true spheres or cylinders, without any axis on which they * This remark, of course, does not apply to situations in which friction is obviated, as upon water, ice. rail-roads,&c. are constrained to move ; but a cylindrical roller occasions friction, whenever its path deviates in the least from a straight Hue. 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 sim- ilar 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 conse- quently occasions less jolting and expend- iture 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, how- ever, it is found necessary to confine the size of wheels within certain limits, part- ly because the materials used would make wheels of great size heavy and cumber- some, since the separate parts would ne- cessarily 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. Locomotive Engine is that which is calculated to produce locomotion,or motion from place to place. (See Steam-Engine.) Locris was a country of Middle Greece, whose inhabitants, the Locrians, were among the oldest Grecian people. There were four branches of them—the Epicnemidian, the Opuntian, Ozolian, and Epizephyrian Locrians. The last were a colony from the Ozolian stock, and lived in Lower Italy. Their capital, Locri, was one of the most powerful, splendid and wealthy cities of Magna Gr?ecia. Locust. The misapplication of popu- lar appellations, and the mutations of en- tomology, have introduced some confu- sion in regard to the scientific names of many insects. Our American cicada are popularly known here both by the names of harvest-fly and locust ; the latter term, however, is incorrectly applied. Under the generic name locusta is included, by several modern entomologists, the devour-](https://iiif.wellcomecollection.org/image/b21136774_0046.jp2/full/800%2C/0/default.jpg)
