(51^4) 
thrufl: it downto I , counterballaociog the predure (at C) 
of the purer but not oi the grofler Air (which I take to be 
the fum of the Caufe affign'd by M. Hngens : ) I am yet to 
fcek, why it may not as well penetrate D at firft to begin 
the Defcent, as afterwards to purfueit^ and,whyootas 
well begin the Defcent when the Quickfilver is well clcan- 
fcd of Air , as when it is not fo 5 and why alfo, if the fure 
Air do freely enter at Dj it does not prcfcntly fall ; or^if not 
freely, why, when it docs fall, it falls fuddenly and not lei* 
furely from D to 1 5 efpecially (\nct fo (mall a weight as D H 
of pure Air (for the grofler caiinot enter5)is very inconfide- 
rable, if not at all, or not freely, prcflcd by ^i^nncumbent 
jon D s and the Adhcfion not confiderably lels , by being 
feparated only at the Top, while it yet continues to touch 
the fides. 
I am apt therefore, as heretofore, to adfcribe the Caufe of 
l\vi%?hamnienon to the Spring that is in Air , and the want 
thereof in Quickfilver. For, that in Air there is a Spring or 
Elafticity, is now undoubted 5 but in Waterjclcans'd of Air, 
though many Experiments have been attempted to that 
purpofe, it has not yet been found that there is any : And I 
am apt to thick the like of Quickfilver though I do not 
knowjthat this has been yet fo rigoroufly examin'd* Now 
fuppofingsthatMatterjbeiog atllcft5will fo continue till it be 
put in motion by fome Force ^ this force may be either that 
of PercfiJJion ftom fome Body already in motion ( which is 
the Cafe, when the Quickfilver falls by fliaking o^ ftriking 
the Tubej) or of Fulfion^hom a contiguous Body beginning 
to move J as by the Expanfion of fame adjacent Spring 
(which is the Caie, when the Springy parts of the Air,either 
left in unpurged, or re-adfnitted in the Qnickfilvcr, by ex- 
panding themfelvcs put the Quickfilver in motion, ) Or 
Ibme Conatus or Endevour of its own ; fuch as is that of a 
Spring (from whatever caufe it be, which I do not herein- 
quire,) but has place only in Springy bodies ; and therefore 
if Water and Quickfilver be not fuch, they will not on this 
account put themfelves in motion* 
Gravity 
