The Preface.
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 A B, with a small stem about two foot and a half long D C; upon the end of this D I put on a small bended Glass, or brazen syphon D E F (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 A B C D E F into a long Board, or Frame, in such manner, that almost half the head A B may lye buried in a concave Hemisphere cut into the Board R S; then I place it so on the Board R S, 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 X Y on the Frame R T, 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 U T 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 X Y) 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 Air; then inverting it as before, I fill it top full with clear and well strain'd Quicksilver, and having made ready a small ball of pretty hard Cement, by heat made very soft, I press it into the hole E, and thereby stop it very fast; and to secure this Cement from flying out afterward, I bind over it a piece of Leather, that is spread over in the inside with Cement, and wound about it while the Cement is hot: Having thus softned it, I gently erect again the Glass after this manner: I first let the Frame down edge-wayes, till the edge R V touch the Floor, or ly horizontal; and then in that edging posture raise the end RS; this I do, that if there chance to be any Air hidden in the small Pipe E, it may ascend into the Pipe F, and not into the Pipe D C: Having thus erected it, and hung it by the hole Q, or fixt it perpendicularly by any other means, I open the end F,
and
