Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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I have thought good to omit this Conſideration, and in this manner
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to ſingle out theſe others that I could explain without it: for
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though there be no Motion but hath ſome Velocity, nevertheleſs
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it is onely the Augmentations and Diminutions of this Velocity
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that are conſiderable. </
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>And now that ſpeaking of the Motion of a
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Body, we ſuppoſe that it is made according to the Velocity which
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is moſt naturall to it, which is the ſame as if we did not conſider it
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at all.</
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>The other reaſon that may have hindred men from rightly un
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derſtanding my Principle is, that they have thought that they could
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demonſtrate without it ſome of thoſe things which I demonſtrate
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not without it: As, for example, touching the Pulley A B C, they
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have thought that it was enough to know that the Nail in A did
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ſuſtain the half of the Weight B; to conclude
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that the Hand in C had need but of half ſo much
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Force to ſuſtain or raiſe the Weight, thus wound
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about the Pulley, as it would need for to ſuſtain
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or raiſe it without it. </
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>But howbeit that this ex
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plaineth very well, how the application of the
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Force at C is made unto a Weight double to that
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which it could raiſe without a Pulley, and that I
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my ſelf did make uſe thereof, yet I deny that
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this is ſimply, becauſe that that the Nail A ſu
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ſtaineth one part of the Weight B, that the Force
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in C, which ſuſtaineth it, might be leſs than if it
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had been ſo ſuſtained. </
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>For if that had been true, the Rope C E be
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ing wound about the Pulley D, the Force in E might by the ſame
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reaſon be leſs than the Force in C: for that the Nail A doth not
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ſuſtain the Weight leſs than it did before, and that there is alſo
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another Nail that ſuſtains it, to wit, that to wich the Pulley D is
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faſtned. </
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>Thus therefore, that we may not be miſtaken in this, that
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the Nail A ſuſtaineth the half of the Weight B, we ought to con
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clude no more but this, that by this application the one of the Di
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menſions of the Force that ought to be in C
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to raiſe up this Weight is diminiſhed the one
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half; and that the other, of conſequence, be
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cometh double, in ſuch ſort that if the Line
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F G repreſent the Force that is required for
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the ſuſtaining the Weight B in a point, with
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out the help of any Machine, and the
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Quadrangle G H that which is required for
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the raiſing of it to the height of a foot, the
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ſupport of the Nail A diminiſheth the Di
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menſion which is repreſented by the Line F G the one half, and the
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redoubling of the Rope A B C maketh the other Dimenſion to </
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