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The advantages that may be derived from being 
able, by proper inftruments, to afcertain the degrees 
of natural and artificial beat and cold, with refpedt 
to philofopbical inquiries; and alfo to feveral uieful, 
and common purpofes in life; have induced many 
ingenious perfons to apply their thoughts that way ; 
and endeavour to form lome certain ftandard, or 
gage, by which this may be obtained. The three 
principal bodies that have been made ufe of for this 
purpofe ; are air, alcohol, and mercury. _ 
Mr. Boyle, the great promoter of experimental 
philofophy, made a thermometer on the principle of 
air, which, to a certain degree ot heat or cold, an- 
fwered very minutely. Alcohol, or ipirit of wine, 
has been more generally ufed ; but has been foun 
to lofe, in time, much of its expanding quality ; and 
alfo to be frozen by an intenfe degree of cold.. Mer- 
cury, as not deemed fubjedt to thefe mconvemencies, 
has therefore been allowed the moft proper for the 
^ The learned Dr. Halley has obferved, . that mer- 
cury expands very fenfibly at fir ft, attaining very 
nearly the fame height, fome time before thc water 
boils, in which it is placed; that it does on the 
ftrongeft boiling : whereas fpirit of wine expands 
gradually, as the heat increafes; but flower at hilt, 
and falter after it is well warm ; and, at a certain 
degree of heat, which wants much of boiling water, 
being almoft tolerable to the touch, it boils vehe- 
mently ; emitting bubbles, which, coming into t le 
neck of the thermometer, lift the incumbent lpint 
to the top : from whence he concludes, that the 
r cxpaniion 
