(inO 
So fan Dr^ Fetik. What Dr. Wallis was f leafed to import 
en this Subje^ ^ tvas written Jmmry ij\l-y at Oxford^ 
as follows. 
C concerning my TlieriTiOfcope and Barofcope (theforniierof 
-^' which is known to give acconapt of the Temper of the 
Air,^ as TO Heat and Cold 5 tlie other, of its vi eight 5 ) I have fame 
few particulars to add to the Obfervations, I fent you fome years 
agoe. 
The frflls, that, whereas I did then obferve^ that in Hot 
weather rhe Qu^ick-filver in the Barofcope did ufeto rifcobfer- 
vably, efpecally inSun-fliine and the Heat of the day; which 
might feem to ai'ouQ the Air to be thereby made heavitr3(which/ 
find was the cafe of fome other Barofcopes as well as of mine^and 
put us to fome thoughts concerning the reafon of it j 1 do 
now find (having kept the fame Barometer^ for the fpace of five 
years, unalter'd,) the cafe, for thefe two years laft paft, to be 
fomewhat otherwife: And that in hot Sun-fhiny weather the 
Quick-filver doth rather fubfide a little 5 and in extreme Cold 
and Frofty weather it rifech. 
Which maketh me Judge the Gaufe of thefe c:ntrary Ob* 
fcrvations to be this^i^/'-c. That the Qjiick-filver , at its firft 
putting into the Tube or Barofcope^ was not fo perfedlv cleansM 
from Air, but that fome fmall quantity of it did remain, undif- 
cern'd, in the Quick-filver: Which latent particles of Air, 
though fo fmall as not to be at all difcernable to the eye by bub- 
bles, yet by the external heat (adding newftrength, as it ufeth 
to do, to its Elaftickor Springy power) were fo much expanded 
;is to give fomewhat a greater bulk to the fame quantity of 
Quick-filver, with which it was mingled, and confequently to 
make it rife fomewhat higher, as bting fpecifically lights: (that 
js, having the fame weight in a larger dimenfion,:or, in the fame 
dimenfions, a lefTer weight; ) and, upon the recefsof the exter- 
nal heat, the Spring again flackeningj the Air, being more com- 
prefs'd, fufTer'd the Quick-filver to be again contraded into its 
form t r leflfer Dimenfions, and fo to become heavier, and not to 
r.fe fo high as before, when it was hotter. Bat now, the Quick- 
filver 
