A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT XXVI.

About the making of a Baroscope (but of litle practical use) that serves but at certain times.

TO shew some Ingenious men by a Medium, that has not hither∣to (that I know of) been made use of, That the not subsi∣ding of Quick-silver in an inverted Tube, that is a little shorter than 30 inches, or thereabouts, does not proceed from such a fuga Vacui as the Schools ascribe to Nature, but from the Gravity of the external Air, I devised the following Experiment.

Having made choice of a time, when it appear'd by a good Baroscope, (which I had frequently consulted for that purpose,) that the Atmosphere was considerably heavy, I caus'd Gals∣pipe, Hermetically seal'd at one end, and in length about 2 foot and a half, to be fill'd with Quick-silver, save a very litle wherein some drops of Water were put, that we might the better discern the Bubbles, if any should be left after the inversion of the Tube into an open Glass with stagnant Mercury in it. Having by this means (though not without difficulty) freed the Tube from bub∣bles, we so order'd the matter, that the Quick-silver and the litle water that was about it, fill'd the Tube exactly, without leaving any interval that we could discern at the top, and yet the Mercuri∣al Cylinder was but very little higher than that of our Baroscope was at that time.

This done, the newly fill'd Pipe was left erected in a quiet place, where the Liquors retain'd their former height for divers dayes. But though an ordinary School-Philosopher would con∣fidently

have attributed this sustation of so heavy a Body to Nature's fear of admitting a Vacuum, yet it seems, that either she is not alwayes equally subject to that fear, or some other cause of the Phaenomenon must be assign'd; for when (a pretty while after) I had observ'd by the Baroscope, that the Atmosphere was grown much lighter than before, repairing to my short Tube, I found that according to my expectation the Quick-silver was not incon∣siderably subsided, and had left a Cavity at the top, which after∣wards grew lesser, according as the Atmosphere grew heavier.

NB. 1. The Tube imployed about this Experiment, may be brought to the requisite shortness, either by wearing off a lit∣tle of the Glass at the Orifice of it, or by increasing the height of the stagnant Mercury, into which it hath been inverted.

2. When the Quick silver in our short Tube was much sub∣sided, there appeared in the Water that swam upon it a litle Bub∣ble, about the bigness of a small Pins head, but, considering how careful we had been to free the Tube from bubbles before we set it to rest, it may very well be, that this so small a Bubble was not produc'd till after the subsiding of the Quick-silver, whereupon the Aerial Particles in the Water became less compress'd than be∣fore; not to mention that the Bubble (such as it was) appear'd very much greater than it would have done, if the Pressure of the Atmosphere had not been kept from it by the weight of the sub∣jacent pillar of Mercury.