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THough several of our Experiments sufficiently manifest to the Skilful, that the Pressure of the Air is very considera∣ble; yet because some of them require peculiar Glasses, and o∣ther
Instruments, which are not always at hand, and because there are many that think it surer to estimate the force of Pressure by what they immediately feel, than by any other way; I was invited for the sake of such to imploy an easie Experiment, which usu∣ally proved convincing, because it operated on that Sense, where∣on they chiefly rely'd.
I caus'd then to be made a hollow (but strong) piece of Brass, not above two or three inches high, (that it might be in a trice exhausted,) and open at both ends, whose Orifices were Circular and parallel, but not equal, (the Instrument being made tape∣ring, so that it might be represented by an excavated Conus trun∣catus, or a Gigg, with the lower part cut transversly off.) This piece of Brass being cemented on, as if it were a small Receiver to the Engine, the Person, that would not believe the Pressure of the Air to be near so considerable as was represented, was bidden to lay the Palm of his Hand upon the upper Orifice; and being ordered to lean a little upon it, that so the lower part of his hand might prove a close Cover to the Orifice, one Exuction of the Air was made by the help of the Pump: and then upon the with∣drawing of the greatest part of the Pressure of the internal Air, that before counterballanc'd that of the External, the Hand be∣ing left alone to support the weight of the Ambient Air, would be pressed inwards so forceably, that though the stronger sort of men were able (though not without much adoe) to take off their Hands, yet the weaker sort of Tryers could not do it, (espe∣cially if by a second Suck the litle Receiver were better exhau∣sted,) but were fain to stay for the Return of the Air into the Re∣ceiver to assist them.
This Experiment being design'd rather to convince than to punish those that were to make it, we took care not onely that the Brass should be so thick, and the Orifices so smooth, that no Sharpness nor Roughness of the Metal should offend the Hand; but also that the narrower Orifice (which was the oftenest made use of) should be but about an inch and a quarter in Diameter.
But if any were desirous of a more sensible conviction, 'twas very easie to give it him by making the larger Orifice the uppermost, which was the reason why the Instrument was, as we formerly noted, made tapering. But yet this larger Orifice ought not to exceed 2 Inches, or 2 Inches and 1/2 in wideness, least the great Weight of the Air endanger the breaking or considerably hurting the Hand of the Experimenter. Which Caution I am put in mind of giving, by remembring that I once much endangered my own Hand, through the mistake of him that manag'd the Pump, who unawares to me set it on work, when, for another purpose, I had laid my Hand upon the Orifice of an Instrument of too great a Diameter.
THe famous Experiment of Torricellius, mentioned in the 17th of our already published Tryals, is of that Noblenesse and Im∣portance, that though divers Learned men have (but upon very dif∣fering principles) discours'd of it in Print, which gives me the lesse mind to insist long upon it here, yet I shall not scruple to subjoin some Notes concerning Tryals that I made, (though for want of opportu∣nity I could not repeat them according to my custom,) which I had not met with in Others, and which may serve to confirm the Hypothesis made use of in this Continuation, and the Treatise it belongs to.