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. I had one of these Barometers with me in my late Southern Voyage, and it never failed to prognostick and give early notice of all the bad Weather we had, so that I depended thereon, and made provision accordingly; and from my own Experience I conclude that a more useful Contrivance hath not for this long time been offer for the benefit of Navigation. These Instruments are made according to the Direction of Dr. _Hook_, by Mr. _Henry Hunt_, Operator to the Royal Society, who will furnish any Gentlemen with them, and give them Directions how to use them. _A Discourse concerning the Proportional Heat of the Sun in all Latitudes, with the Method of collecting the same; as it was read before the Royal Society, in one of their late Meetings. By _E. Halley_._