Monsieur de Robberval
Monsieur de Fermates,
Counsellour of THOULOUSE,
Containing certain Propositions in the


I have, according to my promise, sent you the
Demonstration of the Fundamental Proposi­
tion of our Mechanicks, in which I follow the
common method of explaining, in the first
place, the Definitions and Principles of which
we make use.

We in general call that Quality a Force or
Power, by means of which any thing whatever
doth tend or aspire into another place than that in which it is, be it
downwards, upwards, or side waies, whether this Quality naturally
belongeth to the Body, or be communicated to it from without.
From which definition it followeth, that all Weights are a species
of Force, in regard that it is a Quality, by means whereof Bodies
do tend downwards. We often also assign the name of Force to
that very thing to which the Force belongeth, as a ponderous Bo­
dy is called a Weight, but with this pre-caution, that this is in re­
ference to the true Force, the which augmenting or diminishing
shall be called a greater or lesser Force, albeit that the thing to
which it belongeth do remain alwaies the same.

If a Force be suspended or fastned to a Flexible Line that is
without Gravity, and that is made fast by one end unto some Ful­
ciment or stay, in such sort as that it sustain the Force, drawing