It is about forty Years since, that the _Thermometers_ of _Robert de Fluctibus_, depending on the Dilatation and Contraction of included Air by Heat and Cold, have been disused, upon discovery that the Airs pressure is unequal; that inequality mixing it self with the Effects of the warmth of the Air in that Instrument. And instead thereof was substituted the seal'd _Thermometer_, including 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 else for some other cause becomes less Elastick, whereby, in process of time, it gives the height of the _Mercury_ somewhat greater than it ought; but this, if it should happen in some of them, hinders not the usefulness thereof, for that it may at any time very easily be corrected by Experiment, and the rising and falling thereof are the things chiefly remarkable in it, the just height being barely a Curiosity.