Salusbury, Thomas, Mathematical collections and translations (Tome I), 1667

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1I have thought good to omit this Conſideration, and in this manner
to ſingle out theſe others that I could explain without it: for
though there be no Motion but hath ſome Velocity, nevertheleſs
it is onely the Augmentations and Diminutions of this Velocity
that are conſiderable.
And now that ſpeaking of the Motion of a
Body, we ſuppoſe that it is made according to the Velocity which
is moſt naturall to it, which is the ſame as if we did not conſider it
at all.
The other reaſon that may have hindred men from rightly un­
derſtanding my Principle is, that they have thought that they could
demonſtrate without it ſome of thoſe things which I demonſtrate
not without it: As, for example, touching the Pulley A B C, they
have thought that it was enough to know that the Nail in A did
218[Figure 218]
ſuſtain the half of the Weight B; to conclude
that the Hand in C had need but of half ſo much
Force to ſuſtain or raiſe the Weight, thus wound
about the Pulley, as it would need for to ſuſtain
or raiſe it without it.
But howbeit that this ex­
plaineth very well, how the application of the
Force at C is made unto a Weight double to that
which it could raiſe without a Pulley, and that I
my ſelf did make uſe thereof, yet I deny that
this is ſimply, becauſe that that the Nail A ſu­
ſtaineth one part of the Weight B, that the Force
in C, which ſuſtaineth it, might be leſs than if it
had been ſo ſuſtained.
For if that had been true, the Rope C E be­
ing wound about the Pulley D, the Force in E might by the ſame
reaſon be leſs than the Force in C: for that the Nail A doth not
ſuſtain the Weight leſs than it did before, and that there is alſo
another Nail that ſuſtains it, to wit, that to wich the Pulley D is
Thus therefore, that we may not be miſtaken in this, that
the Nail A ſuſtaineth the half of the Weight B, we ought to con­
clude no more but this, that by this application the one of the Di­
menſions of the Force that ought to be in C
219[Figure 219]
to raiſe up this Weight is diminiſhed the one
half; and that the other, of conſequence, be­
cometh double, in ſuch ſort that if the Line
F G repreſent the Force that is required for
the ſuſtaining the Weight B in a point, with­
out the help of any Machine, and the
Quadrangle G H that which is required for
the raiſing of it to the height of a foot, the
ſupport of the Nail A diminiſheth the Di­
menſion which is repreſented by the Line F G the one half, and the
redoubling of the Rope A B C maketh the other Dimenſion to

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