The Form of this Instrument is shown in the Cut, by Tab. 4. Fig. 1. wherein,
AB represents the Spirit-Thermometer, graduated from 0, or the freezing Point, through all the possible degrees of the Heat or Cold of the Air, at least in these Climates.
CD, is the Air-Thermometer, graduated after the same manner with the like Degrees.
EF, is a Plate applied to the side of the Thermometer CD, graduated into Spaces answering to Inches and parts of an Inch of Mercury, in the common Barometers.
G, a Hand standing on the Plate at the height of the Mercury thereon, as it was when the Instrument was graduated, as suppose here at 29½ Inches.
LM, a Wire on which the Plate EF, slips up and down, parallel to the Cane of the Thermometer CD.
K, any Point at which the Spirit stands at the time of Observation; suppose at 38 on the Spirit- Thermometer ; Slide the Plate EF till the Hand G stand