Fifth book of lessons for the use of schools . he hands the power. In levers of the third kind, the fulcrum is also atone of the extremities, the weight or resistance at theother, and the power is applied between the fulcrum and the resistance. Thusthe fulcrum, the weight,and the power, each in itsturn, occupies some partof the lever between itsextremities. But in thisthird kind of lever, theweight being further fromthe centre of motion thanthe power, the difficulty of raising it, instead of beingdiminished is increased. Levers of this description areused when the object is to produce great ve
Fifth book of lessons for the use of schools . he hands the power. In levers of the third kind, the fulcrum is also atone of the extremities, the weight or resistance at theother, and the power is applied between the fulcrum and the resistance. Thusthe fulcrum, the weight,and the power, each in itsturn, occupies some partof the lever between itsextremities. But in thisthird kind of lever, theweight being further fromthe centre of motion thanthe power, the difficulty of raising it, instead of beingdiminished is increased. Levers of this description areused when the object is to produce great velocity. Theaim of mechanics, in general, is to gain force by ex-changing it for time; but it is sometimes desirable toproduce great velocity by an expenditure of force. Thetreadle of the common turning lathe affords an exampleof a lever of the third kind employed in gaining time, orvelocity, at the expense of force. A man, in raising along ladder perpendicularly against a wall, cannot placehis hands on the upper part of the ladder; the power,. MECHANICAL POWERS.—TIIE PULLEY. 281 therefore, is necessarily placed nearer the fulcrum thanthe weight, for the hands are the power, the groundthe fulcrum, and the ladder the weight, which, in this,as well as in the door, may be considered as collectedin vthe centre of gravity of the ladder, about halfway up it, and consequently beyond the point wherethe hands are applied. This kind of lever is em-ployed in the structure of the human frame. Inlifting a weight with the hand, the lower part of thearm becomes a lever of the third kind; the elbow isthe fulcrum; the muscles which move the armv thepower; and as these are nearer to the elbow, than thenand is, it is necessary that their power should exceedthe weight to be raised. It is of more consequencechat we should be able to move our limbs nimbly, thanthat we should be able to overcome great resistance;for it is comparatively seldom that we meet with greatobstacles, and when we do, they ca
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