36. Sir Is a ac -N e w t 6’N 5 s -Book I.

change the velocity of their motion: therefore this firft 1 awof motion has been illuftrated and confirmed, as much as canbe from the. tranfient.obfervations, which have here been dif-courfed upon; arid in the next chapter all this will be farthereftablilhed by more correct obfervations.

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I y. B u t I fhall now pafs to the fecond law of motion ;wherein, when it is aflerted, that the velocity, with whichany body is moved by the action of a power upon it, is pro-portional to that power; the degree of power is fuppofed tobe meafured by the greatnefs of the body, which it can movewith a given celerity. So that the fenfe of this law is, thatif any body were put into motion with that degree of fwift-nefs, as to pafs in one hour the length of a thoufand yards;the power, which would give the fame degree of velocity toa body twice as great, would give this lefier body twice thevelocity, caufing it to defcribe in the fame fpace of an hourtwo thoufand yards. But by a body twice as great as another*I do not here mean limply of twice the bulk, but one thatcontains a double quantity of folid matter.

1 6 . Why the power, which can move a body twice as greatas another with the fame degree of velocity, fhould be calledtwice as great as the power, which can give the lefier bodythe fame velocity, is evident. For if we fhould fuppofe thegreater body to be divided into two equal parts, each equalto the lefier body, each of thefe halves will require the famedegree of power to move them with the velocity of the lefierbody, as the lefier body it felf requires; and therefore both

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