C *f.s 1 
this Power of dilating and contracting with Heat una. : 
Cold is as (pecifically in them as their Gravity, Refradti- 
on, &c. But in none is it comparably fo conlpicuous as 
in that rare Eiaftick Fluid the Air ; for by ieveral Expe- 
riments that I have made, I find that the Heat of Summer 
does expand the ordinary Air about a Thirtieth part; 
and that late Honourable Patron of Experimental Philo* 
fophy, Mr. Boyle, in his Hiftory of Cold, Tit. *8. Pa- 
rag. 8. pag<475- alledges his own Trials, proving that 
the force of the ftrongeft Cold in England does not con- 
trad the Air above a twentieth part : So that the Sum of 
a twentieth and thirtieth part being a twelfth part, we 
may conclude that the fame Air which is extream Cold 
occupies twelve parts of /pace, in very hot Summer 
Weather will require thirteen fuch Spaces • which is as 
great an Expanfion as that of Spirit of Wine w ? hen it 
begins to boil : For which reafbn, and for its being fo 
very fenfible of Warmth or Cold, and continuing to ex* 
ert the fame Eiaftick Power after never fo long being in- 
cluded, in my Opinion it is much the moft proper Fluid 
for the purpofe of Thermometers. 
Now the Thermometers hitherto in ufe are of Two 
forts the one fee wing the differing Temper of Heat 
and Cold by the Expanfion of Spirit of Wine, the other 
by the Air ? v but I cannot learn that any of them of either 
fort were ever made or adjufted, fo as it might be con- 
cluded, what the Degrees or Divifions of the laid Inftru- 
ments did mean; neither were they ever other wife gra- 
duated, but by Standards kept by each particular Work— 
- man, without any agreement or reference to one ano- 
ther: So that whatfoever Obfervations any curious Per- 
fon may make by his Thermometer^ to figoifie the degree 
of Heat in the Air or other thing, (which is of conftantr 
Ufe in Philofophical Matters) cannot be unaer-ftood, ur> 
iefsby thofe who have by them Themwmetersoi the force 
Make, and Adjuftment Much , left has tm way 
