I say then the Cause why some Sollid Bodyes descend to the Bottom of Water, is the excesse of their Gravity, above the Gravity of the Water; and on the contrary, the excess of the Waters Gravity above the Gravity of those, is the Cause that others do not descend, rather that they rise from the Bottom, and ascend to the Surface. This was subtilly demonstrated by _Archimedes_ in his Book Of the NATATION of BODIES: Conferred afterwards by a very grave Author, but, if I erre not invisibly, as below for defence of him, I shall endeavour to prove. I, with a different Method, and by other meanes, will endeavour to demonstrate the same, reducing the Causes of such Effects to more intrinsecall and immediate Principles, in which also are discovered the Causes of some admirable and almost incredible Accidents, as that would be, that a very little quantity of Water, should be able, with its small weight, to raise and sustain a Solid Body, an hundred or a thousand times heavier than it. And because demonstrative Order so requires, I shall define certain Termes, and afterwards explain some Propositions, of which, as of things true and obvious, I may make use of to my present purpose. DEFINITION I. _I then call equally Grave_ in specie, _those Matters of which equall Masses weigh equally._ As if for example, two Balls, one of Wax, and the other of some Wood of equall Masse, were also equall in Weight, we say, that such Wood, and the Wax are _in specie_ equally grave. DEFINITION II. _But equally grave in Absolute Gravity, we call two Sollids, weighing equally, though of Mass they be unequall._ As for example, a Mass of Lead, and another of Wood, that weigh each ten pounds, I call equall in Absolute Gravity, though the Mass of the Wood be much greater then that of the Lead. _And, consequently, less Grave_ in specie. DEFINITION III. _I call a Matter more Grave_ in specie _than another, of which a Mass, equall to a Mass of the other, shall weigh more._ And so I say, that Lead is more grave _in specie_ than Tinn, because if you take of them two equall Masses, that of the Lead weigheth more. DEFINITION IV. _But I call that Body more grave absolutely than this, if that weigh more than this, without any respect had to the Masses._ And thus a great piece of Wood is said to weigh more than a little lump of Lead, though the Lead be _in specie_ more heavy than the Wood. And the same is to be understood of the less grave _in specie_, and the less grave absolutely. These Termes defined, I take from the Mechanicks two Principles: the first is, that AXIOME. I. _Weights absolutely equall, moved with equall Velocity, are of equall Force and Moment in their operations._ _DEFINITION V._ Moment, amongst Mechanicians, signifieth that Vertue, that Force, or that Efficacy, with which the Mover moves, and the Moveable resists.