Chap. 4. PHILOSOPHY.

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Chap. IV.

Of the RESISTANCE of FLUIDS.

B EFORE the caule can be dilcovered, which keeps theplanets in motion, it is neceffary firft to know, whe-ther the {pace, wherein they move, is empty and void, or fil-led with any quantity of matter. It has been a prevailingopinion, that all Ipace contains in it matter of lome kind orother * fo that where no fenfible matter is found, there wasyet a fubtle fluid fubftance by which the fpacewas filled up;even fo as to make an abfolute plenitude. In order to exa-mine this opinion, Sir Isaac Newton has largely confi-dered the effe&s of fluids upon bodies moving in them.

z. These effects he has reduced under thefe three heads.In the firft place he {hews how to determine in what mannerthe refiflance, which bodies fuffer, when moving in a fluid,gradually increafes in proportion to the Ipace, they defcribein any fluid; to the velocity, with which they defcribe it jnnd to the time they have been in motion. Under the fc-cond head he confiders what degree of refiflance differentbodies moving in the lame fluid undergo, according to the,different proportion between the denfity of the fluid and thedenfity of the body, The denfities of bodies, whether fluidor folid, are meafured by the quantity of matter, which iscomprehended under the fame magnitude ; s that body being

the