t 2I 7 ] 
hilling water, fo far only as they refpedt the con- 
ftrudtion and comparifon of mercurial thermome- 
ters ; and it exhibits an actual comparifon of the 
fcale of M. de luc’s thermometer, with that 
which is likely to continue in general ufe here* In 
the fecond, I date the general principles of mea- 
furing heights by the barometer; or in other words, 
of the modification of the air, with refped to its 
condenfations, at different heights, by the preffure of 
the fuperior parts, exclufive of every other caufe, 
Thefe principles are recapitulated for the fake of 
perfpicuity and order ; but, as they are generally well 
known, I refer to former writers for the demonftra- 
tion of them. This fedlion concludes with the ex- 
pofition of M. de luc’s fundamental rule. The 
third feftion treats of the equation depending on 
the difference of temperature of the quickfilver in 
the barometers, at the different places of fynchro- 
nous obfervation. It explains, whence the neceffity 
of this corre&ion arifes, and confiders a cafe, in 
which the application of it require^ fome particu- 
lar attentions ; namely, the levelling a tradt of flat 
country by the barometer. The fourth feftion ex- 
plains the equation depending on the temperature of 
the air. This is likewise traced to its origin in theory * 
and upon this occafion, I enquire into the conden- 
fations of a fluid, unequally elaftic in its different 
parts, under different degrees of compreflive force* 
The fifth fedtion contains the redudfion of M. de 
luc’s formula. I have found, with pleafure, by 
the perufal of Mr. maskelyne’s paper, that we 
agree in our final conclufions, To this fedtion, I 
have fubjoined two problems ; the one for efti- 
Vq l. LXIW F f mating 
