There may be also some other mechanical wayes found out, of sensibly perceiving the effluvia of Bodies; several Instances of which, were it here proper, I could give of Mineral steams and exhalations; and it seems not impossible, but that by some such wayes improved, may be discovered, what Minerals lye buried under the Earth, without the trouble to dig for them; some things to confirm this Conjecture may be found in Agricola, and other Writers of Minerals, speaking of the Vegetables that are apt to thrive, or pine, in those steams.

Whether also those steams, which seem to issue out of the Earth, and mix with the Air (and so to precipitate some aqueous Exhalations, wherewith ’tis impregnated) may not be by some way detected before they produce the effect, seems hard to determine; yet something of this kind I am able to discover, by an Instrument I contriv’d to shew all the minute variations in the pressure of the Air; by which I constantly find, that before, and during the time of rainy weather, the pressure of the Air is less, and in dry weather, but especially when an Eastern Wind (which having past over vast tracts of Land is heavy with Earthy Particles) blows, it is much more, though these changes are varied according to very odd Laws.

The Instrument is this.  I prepare a pretty capaceous Bolt-head AB, with a small stem about two foot and a half long DC; upon the end of this D I put on a small bended Glass, or brazen syphon DEF (open at D, E and F, but to be closed with cement at F and E, as occasion serves) whose stem F should be about six or eight inches long, but the bore of it not above half an inch diameter, and very even; these I fix very strongly together by the help of very hard Cement, and then fit the whole Glass ABCDEF into a long Board, or Frame, in such manner, that almost half the head AB may lye buried in a concave Hemisphere cut into the Board RS; then I place it so on the Board RS, as is exprest in the first figure of the first Scheme; and fix it very firm and steady in that posture, so as that the weight of the Mercury that is afterwards to be put into it, may not in the least shake or stir it; then drawing a line XY on the Frame RT, so that it may divide the ball into two equal parts, or that it may pass, as ’twere, through the center of the ball.  I begin from that, and divide all the rest of the Board towards UT into inches, and the inches between the 25 and the end E (which need not be above two or three and thirty inches distant from the line XY) I subdivide into Decimals; then stopping the end F with soft Cement, or soft Wax, I invert the Frame, placing the head downwards, and the Orifice E upwards; and by it, with a small Funnel, I fill the whole Glass with Quicksilver; then by stopping the small Orifice E with my finger, I oftentimes erect and invert the whole Glass and Frame, and thereby free the Quicksilver and Glass from all the bubbles or parcels of lurking