Spirit of Wine (first brought into England , out of Italy , by Sir Robert Southwell ) as a proper Standard of the temper of the Air, in relation to Heat and Cold; that Ætherial Spirit being of all the known Liquors the most susceptible of Dilatation and Contraction, especially with a moderate degree of either Heat or Cold. Now this being allow'd as a Standard, and the other Thermometer that includes Air, being graduated with the same Divisions, so as at the time when the Air was included, to agree with the Spirit- Thermometer in all the degrees of Heat and Cold, noting at the same time the precise height of the Mercury in the common Barometers: It will readily be understood, that whensoever these two Thermometers shall agree, the pressure of the Air is the same it was, when the Air was included, and the Instrument graduated: That if in the Air- Thermometer the Liquor stand higher than the Division marked thereon, corresponding with that on the Spirit-glass, it is an indication that there is a greater pressure of the Air at that time, than when the Instrument was graduated. And the contrary is to be concluded, when the Air-glass stands lower than the Spirit, viz. that then the Air is so much lighter, and the Quick silver , in the ordinary Barometer lower than at the said time of Graduation.
And the Spaces answering to an Inch of Mercury, will be more or less, according to the quantity of Air so included, and the smallness of the Glass Cane, in which the Liquor rises and falls, and may be augmented almost in any proportion, under that of the Specifick Gravity of the Liquor of the Thermometer to Mercury. So as to have a Foot or more for an Inch of Mercury, which is another great convenience.
It has been observed by some, that in long keeping this Instrument, the Air included either finds a means to escape, or deposites some Vapours mixt with it, or