Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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raiſeth it, and cannot be eſtimated ſave wthin a ſmall matter.</
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>Moreover, it is neceſſary to obſerve, that it is nothing but the
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redoubling of the Chord, and not the Pulley, that cauſeth this
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Force: for if we faſten yet another Pulley towards A, about
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which we paſs the Chord A B C H, there will be required no leſs
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Force to draw H towards K, and ſo to lift up the Weight E, than
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there was before to draw C towards G. </
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>But if to theſe two Pul
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leys we add yet another towards D, to which we faſten the Weight,
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and in which we make the Chord to run or ſlip, juſt as we did in
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the firſt, then we ſhall need no more Force to lift up this Weight
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of 200 pounds than to lift up 50 pounds without the Pulley: be
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cauſe that in drawing four feet of Chord we lift it up but one
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foot. </
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>And ſo in multiplying of the Pulleys one may raiſe the great
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eſt Weights with the leaſt Forces. </
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>It is requiſite alſo to obſerve,
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that a little more Force is alwaies neceſſary for the raiſing of a
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Weight than for the ſuſtaining of it: which is the reaſon why I
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have ſpoken here diſtinctly of the one and of the other.</
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The Inclined
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PLANE.</
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>If not having more Force than ſufficeth to raiſe 100 pounds, one
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would nevertheleſs raiſe this Body F, that weigheth 200 pounds,
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to the height of the Line B A, there needs no more but to draw
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or rowl it along the Inclined Plane C A, which I ſuppoſe to be
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twice as long as the Line
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A B, for by this means,
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for to make it arrive at
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the point A, we muſt
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there employ the Force
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that is neceſſary for the
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raiſing 100 pounds twice
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as high, and the more inclined this Plane ſhall be made, ſo much
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the leſs Force ſhall there need to raiſe the Weight F. </
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>But yet there
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is to be rebated from this Calculation the difficulty that there is
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in moving the Body F, along the Plane A C, if that Plane were
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laid down upon the Line B C, all the parts of which I ſuppoſe to
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be equidiſtant from the Center of the Earth.</
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>It is true, that this impediment being ſo much leſs as the Plane is
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more united, more hard, more even, and more polite; it cannot
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likewiſe be eſtimated but by gueſs, and it is not very conſide
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rable.</
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>We need not neither much to regard that the Line B C being a
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part of a Circle that hath the ſame Center with the Earth, the
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Plane A C ought to be (though but very little) curved, and to
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have the Figure of part of a Spiral, deſcribed between two Circles, </
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