Page:Philosophical Transactions - Volume 001.djvu/192

 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 season, that in the Higher place, the incumbent part of the Atmosphere must be lighter; than in the Lower, there is almost Rh