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.
In these Parts of the World, long Experience has told us, that the rising of the Mercury forebodes fair Weather after foul, and an Easterly or Northerly Wind; and that the falling thereof, on the contrary, signifies Southerly or Westerly Winds, with Rain, or stormy Winds, or both; which latter it is of much more consequence to provide against at Sea than at Land; and in a Storm, the Mercury beginning to rise is a sure sign that it begins to abate, as has been experienced in high Latitudes, both to the Northwards and Southwards of the Æquator.
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 at 38 on the Air-Thermometer, and if the Liquor therein stand at 38 likewise, then is the pressure of the Air the same as at the time of Graduation, viz. 29,5; but if it stand higher, as at 30, at I; then is the pressure of the Air greater; and the division on the sliding Plate against the Liquor, shews the present height of the Mercury to be twenty nine Inches seven Tenths. And this may suffice as to the manner of using it.