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lions. Or every thing may be reduced to a com- 
parifon of lengths, if in comparing the inftruments 
of different countries, the comparifon be imagined 
to be made between two of equal dimenfions, and 
limilar figure, fetting afide that part of each which 
is above the point of melting ice. 
To compare the thermometers, therefore, of 
different countries, the proportions of their funda- 
mental intervals to each other muff be afcertained ; 
or, we muff have fome means of finding, upon one 
lcale, the place of the boiling point of another. For 
this purpofe, a general folution is requifite of the 
following problem : “ The fundamental interval 
“ being given for a given height of the barometer , 
“ to find the fundamental interval , for any other given 
sc height of the barometer .” The folution is furnifhed 
by M. de luc’s laborious refearches. 
M. de luc fixes the boiling point of his ther- 
mometer, when the barometer is at 27 inches 
French that being its mean height at geneva; 
He divides the fundamental interval, after the 
French 'manner, into 80 equal parts; and, by a 
great number of experiments, on the heat of boiling 
water, at different heights above the level of the 
lea, the detail of which is to be found in his EJfai 
fur la Variation, &c: he hath found, that the height 
of his thermometer, plunged in boiling water, may 
be expreffed, in all ftates of the barometer, by th<2 
following formula , viz. log. y — a — T. 
In which, y denotes the height of the barometer, 
in fixteenths of a Paris line : T the height of a 
thermometer, plunged in boiling water, above melt- 
{a) Recherches, fur les Modificat. de 1 ’Atmofphcre, §.451.3. 
ing 
