(my) 
filverjhaving continued in the Tube for fiveye^rsand upwards 5 
hath by its own weight cieanfed it felf better from that little Air 
that was in it 5 and that Air, freed from its entanglement with 
the Quick- filver, being got up into the void part of the Tube 
above the Qiaick- filver, doth ad contrary-wife^ that is, when it 
is by Heat (upon the ftrengthening of its Spring ) expanded^ it 
prtffeth down-ward upon the Quick- filver^ and doth a little de- 
prefs it ^ and, on the contrary, when by Froft or very Cold wea- 
ther this Air (by the abatement of its Spring) iscontrafted, the 
Quick-filver, freed from that preffure, rifeth a little. But the 
Kifingand Sinking on this accompt, Caswell that fomerly^vvhen 
this Air was in the Quick-filver , as that now, whm it is gocrc!! 
above it,) is not very confiderablei hardly exceeding the twelfth 
part of an inch;> or thereabout, 
This Accompt I thought not amifs ta give you, becaufe it 
may pollibly keep your felf, or fome others who make ufe of 
Barofcopes , from being imposed upon by fuch Obfervations as 
if the Sun-(liining or Heat of the weather did make the Air 
heavier than before 5 whereas it feem.s to be but an accidental ope- 
ration upon that un-obferv'd Air latent in the Quick-filver. And 
I thought it the more necefTary to take notice of it, becaufe if is 
not fo eafily difcover'd, unlefs by keeping the Barofcope un- 
altered for a longer time^ than (perhaps) is done by fome of 
thofe, who make ufe of it 5 and, without which, I had not dif- 
cern'd it my felf. 
On thisoccafion. I fliall add another. Accident, which I lately 
took notice of. Obfcrving in the late hard Froft , that the 
Quick-filverdid not rife a little, at fuch time as, byreafonof the 
fitrce freezing, /expeded it might 5 Ididfufped; (as it proved 
tobe^^ that a little drop of water, (which was at firft made 
ufe of for the cleanfiog of the Quick-filver from the Air, and 
\H\-i\d\ hath ever fince remain'd on the top of the Qjiick-fiVer 
within the Tube,) was frozen taft to the glafs, fo as to flop the 
Quick-filver from afcending : And, to try, whether it were fo 
or no , I did a little fliake the Tube by moving it up and down^ 
fo as tomaketheQuick-filver undulate. Whereupon I found 
the frozen drop of Water to keep its place, while the undula- 
ting Quick-filver did feveral times beat againft it, And, 
• Pp ppp 2 (which 
