C '9i 1 
city's of bodice arc to one another as the fqtiarc roots of th 
heigths to which they may afcend; and so in this occafio» 
they are alfb as the wots of the fpecifick Grauity's, 
If therefore we would know what is the Velocity of Air 
being driven by any degree of preffurc whatfbcuer, we 
ougiit but to find what would be the velocity of water vn- 
der tl)e fame effort: and tl en take the Iquare roots of 
the fpecifick grauitys of thefe t^o hq!iar\; becauk; as much 
as the fquiirc root of the fpCifick Gravity of Water, doth 
exceed thefquare Root of the Ipecifick Gravity of Air y r>^ 
much in Proportion will the vel jcity of A r exceed che veloci- 
ty of water. Forcxample,when I would compute what fliouid 
be tli^ fwiftnefs of a bullet fhott by the Pneumacick Engine^ 
ts hath been deftribed in PhUofo^jicd TrmfA^tkn^ Num. ? 79.. 
X fhould firft compute what was the velocity of the Air 
it fclf that droue the Bullet: I did therefore take notice that 
in this occafion the Air bares a prelTu^ e much about the lame 
as that of water when it^s fpring is J3 foot high:^ now fiich 
water would fpout out with a lufficient velocity to afcend 
3 a foot perpendicular, and therefore,, according to the rule*, 
and obferuation of GaliUus^ Halley and others, fuch water 
hath the velocity of 45 foot in a fecond. remains there- 
fore but to know the proportion of the grauity of Air to 
that of Water .*and we ha ue found it not to be always the 
iamej becauiethe hcigth, the heat, and the moifture of th« 
Atmofphere are variable : neucrthclels we may lay in gene- 
ral that the rcalbn between the fpecifick grauitys of watev 
and Air is much about 840 to 1. Taking then their (quar« 
roots, as I haue iaydaboue, which roots are 29 and t, wc 
may conclude that the velocity of Air muft exceed 
that of water by 29 limes: and fo multiplying 45^ the veloci- 
ty of vvater, by 29, we ihall find that the velocity of the Air 
driven by the whole prcflfvir^ of ihe AtjDolphcrc, vs al^wt 
13^5 ia a fccoiiJ* 
