The works of the Honourable Robert Boyle, Esq., epitomiz'd by Richard Boulton ... ; illustrated with copper plates.

CHAP. VIII. Some Observations and Directions about the Barometer, communicated by Mr. Boyle, in the Phil. Transact. of April 1666.

* 1.1THE Design of trying Barometrical Expe∣riments in several Parts, being only that by comparing Notes, The Extent of Atmosphe∣rical Changes, in Point of Weight, might be the better estimated; I shall lay down the following Directions for those, whose Curiosity leads them that way. First, It will be requisite to note the Day and Hour in which Observations are made. Secondly, The Situation of the Place where the Barometer stands as to Height, since by the

Length of the Atmosperical Pillar of Air that presses upon it, the height of the Mercury may vary; tho' not always exactly; for sometimes upon Changes in the Air not otherwise ob∣servable, the Mercury will subside more than u∣sually in that which stands furthest from the Center of the Earth, when at the same time it does not proportionably subside in that which is plac'd in a lower Situation. And

It, perhaps, may be Worth Noting, whe∣ther upon excessive Droughts, when the Ground is parch'd and crack'd, some subterraneal Ef∣fluvia may not rise, which may add a specifick Gravity to the Air.

Nor will it be needless, when other Observa∣tions are making, at the same time to observe the Weather; as also what Winds blow, and whe∣ther violent or more remiss; for sometimes it is observ'd, that when high Winds blow, the Mercury is the lower, tho' not always.

But to favour what hath been intimated, viz. That the Alterations in the Weight of the Air, depend on subterraneal Steams mix'd with it; It hath been observ'd, that after long Droughts, upon a Shower of Rain, so many Steams have been either prevented from rising, or depress'd and precipitated, that the Mercury hath subsi∣ded within 2/16 of an Inch.

Notes

• * 1.1

  Observa∣tions con∣cerning Ba∣rometers.