When I did, as you may remember, some years agoe, publickly express and desire that some Inquisitive men would
make Baroscopical Observations in several parts of England (if not in forrain Countries * also;) and to assist them, to do so, presented some of my Friends with the necessary Instruments: The declared reason of my desiring this Correspondence was (among other things) that by comparing Notes, the Extent of the Atmospherical Changes, in point of Weight, might be the better estimated. But not having hitherto received some account, that I hoped for, I shall now, without staying for them, intimate thus much to you: That it will be very convenient, that the Observers take notice not only of the day, but as near as they can, of the Houre wherein the height of the Mercurial Cylinder is observ'd: For I have often found, that within less than the compass of one day, or perhaps half a day, the Altitude of it has so considerably vary'd, as to make it in many cases difficult, to conclude any thing certainly from Observations, that agree but in the day.
It will be requisite also, that the Observers give notice of the Scituation of the place, where their Barometers stand, not only, because it will assist men to Judge, whether the Instruments were duely perfected, but principally, because, that though the Baroscope be good (nay, because it is so) the Observations will much disagree, even when the Atmosphere is in the same state, as to Weight, if one of the Instruments stand in a considerably higher part of the Countrey, than the other.
To confirm both the foregoing admonitions, I must now inform you, that, having in these parts two Lodgings, the one at Oxford, which you know stands in a bottom by the Thames side, and the other at a place four miles thence, seated upon a moderate Hill, I found, by comparing two Baroscopes, that I made, the one at Oxford, the other at Stanton St. Johns, that, though the former be very good, and have been noted for such, during some years, and the latter was very carefully fill'd; yet by reason, that in the Higher place, the incumbent part of the Atmosphere must be lighter, than in the Lower, there is almost
always between 2 and 3 Eights of an Inch difference betwixt them: And having sometimes order'd my servants to take notice of the Disparity, and divers times carefully observ'd it my self, when I pass'd to and fro between Oxford and Stanton, I generally found, that the Oxford Barometer and the other, did, as it were by common consent, rise and fall together so, as that in the former the Mercury was usually ⅜ higher, than in the latter.
Which Observations may teach us, that the Subterraneous steams, which ascend into the Air, or the other Causes of the varying Weight of the Atmosphere, do, many times, and at least in some places, uniformly enough affect the Air to a greater height, than, till I had made this tryall, I durst conclude.
But, as most of the Barometricall observations are subject to exception, so I found the formerly mentioned to be. For (to omit lesser variations) riding one evening from Oxford to Stanton, and having, before I took horse, look't on the Baroscope in the former of these 2. places, I was somewhat surprised, to find at my comming to the latter, that in places no farther distant, and notwithstanding the shortness of the time (which was but an hour and a half, if so much) the Barometer at Stanton was short of its usual distance from the other, near a quarter of an Inch, though, the weather being fair and calm, there appear'd nothing of manifest change in the Air, to which I could adscribe so great a Variation; and though also, since that time, the Mercury in the two Instruments hath, for the most part, proceeded to rise and fall as before.
And these being the only Observations, I have yet met with, wherein Baroscopes, at some Distance of Place, and Difference of Height, have been compar'd (though I cannot now send you the Reflexions, I have else where made upon them;) as the opportunity I had to make them my self, rendred them not unpleasant to me, so perhaps the Novelty will keep them from being unwelcome to you. And I confess, I have had some flying suspicions, that the odd Phænomena of the Baroscope, which have hitherto more pos'd, than instructed us, may in time, if a
competent number of Correspondents do diligently prosecute the Inquiries (especially with Baroscopes, accommodated with Mr. Hooks ingenious additions) make men some Luciferous discoveries, that possibly we do not yet dream off.
* This hath been inquired into, and is found, that several Accurate and Curious persons (as the Most Noble President of the Royal Society, the Lord Viscount Brounker, Doctor Beale, Mr. Hook &c.) have observed the same.