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

PLATE IV. A description of the Mercurial Gage made use of in the following Experiment.

The Gage A. B. C. D. E. is made up of three Glass Tubes cemented together,* 1.1 so that their Cavi∣ties make a continued Passage, being open to each other; The Tube B. C. D. is designedly crook∣ed, that the lower Orifice of the Tube E. D. as well as the Orifice of Tube A. B. may be ce∣mented to it; which being done, the crooked Tube, whose Diameter is larger than either of the other two, is to be filled with Mercury, and then the Orifice E. of the smaller Tube is to be hermetically Sealed up; but the Orifice A. of the Tube A. B. whose Diameter is larger than that of E. D. is to be left open.

This Gage being put in a Receiver when it is exhausted, the Air contained in the Tube E. D. will expand so powerfully as to descend into the crooked one to C. forcing the Mercury to

rise into the Tube B. A. almost to the Top, be∣fore the included Air hath lost it's Spring.

To discover then the Quantity of Air inclu∣ded in a Receiver, we may make use of the Re∣ceiver F. G. E. which being placed on the Plate L. M. we must try the Torrecellian Experi∣ment, by inverting the Tube H. fill'd with Mercury into the Glass I. and then the Top of the Receiver F. being clos'd with Cement, and the Tube A. B. and E. D. being mark'd with Papers, as the Mercury in the Receiver is sus∣pended at 30, 29, or 28 Inches or less, accord∣ingly as the Receiver is exhausted, the same Marks, viz. 30, 29, &c. being placed upon the Papers, pasted on the Tubes of the Mercurial Gage, at each distinct Surface of the expand∣ed Air and the rising Mercury, for the future, when the Air and Mercury descend and ascend to those Marks, in the exhausted Receiver, it will denote, that the Receiver is so far exhaust∣ed, that the remaining Air is only able to bare up a Cylinder of Mercury in the Torrecellian Experiment, so many Inches as the Mark to which the lower Surface of the Air, and the upper Surface of the Mercury rest at, is more or less in Numbers.

But if the Air contain'd in the Receiver, be more than naturally compress'd, it will like∣wise compress the Air contain'd in the Tube E. D. by squeezing the Mercury somewhat into the lower part of the Tube. How much taller a Cylinder of Mercury Air so compress'd will bear up, than when it is but naturally condens'd, we may learn by the following Computation.

For supposing a determinate Quantity of Air to be contain'd in the Space A. when the Force that compresses it is F. if the Pressure F. be in∣creased by the Pressure G. it will compress the Air contain'd in A. into half the space it took up before;* 1.2 so that the remaining Space B. will be ½ a part of the Whole: And if to the Pressure F. G. we add H. the Air which possessed the Space A. when it was only pres∣sed by F. will be squeez'd into a fourth Part of the Space A. viz. C. And consequently the Air compress'd will possess a Space proportionable to the Whole, as the first Pressure is to a total Pressure.

The Remaining Space: the Total Space.

The First Pressure: the Total Pressure.

So that three of those Quantities being known, the Rule of Proportion will teach us the fourth. For if the Atmosphere as usually condens'd in England, possesses the Tube E. D. and the At∣mosphere is then able to raise a Mercurial Tube of 30 Inches; When that Air is compress'd in∣to the Space N. E. to know the Quantity of it's Pressure, I exactly measure the Space N. E. which if 6 Inches, I constitute that the first Term or Quantity of Pressure; so that the second Term will be the whole Space D. E. suppose 12 Inches; the third Term will be the Height of 30 Inches of Mercury, which was the first Pressure; and so the fourth Term or total Pressure will be found to be 60 Inches of Mercury; From whence I con∣clude, the Air so condens'd in the Receiver, to be able to bear up a Mercurial Cylinder 60 In∣ches tall.

And by the same Principle before laid down, it will be no difficult matter to judge, what the Proportion betwixt the size of the Tube A. B. and E. D. ought to be; for that depending on the Length of the Legs, the higer they are, the more able they are to restrain the Air, when it is but a little dilated in the sealed Part. For supposing the Length of A. B. to be 10 Inches, which Height of the Mercury is ⅓ of the usual Pressure; the Tube A. B. will be large enough if it be as big again as E. D. for when the Mercury hath ascended to A. the expanded Air in the other Leg possessing the Space which it forced the Mercury out of, will take up three times as much more as it did before; and con∣sequently, ⅓ of it's first Pressure will be able to curb it's Spring: But if the Legs were not al∣together so long, then the Mercury being but partly expell'd out of the crooked Tube, the Diameter of the Tube A. B. ought to be pro∣portionably larger in respect of E. D. That the ascending Mercury leaving it more space to expand in, it's Spring may be so far spent, that the Mercury by it's Weight may seasona∣bly resist it's further Expansion: And it would always answer, if the Height of the Gage be in the same proportion to 30 Inches, as the Space first possessed by the Air is to the Whole, which that Air would possess in Vacuo, according to the Principle above delivered.

The Tube is more useful when longer, than shorter, because the expanding Air being more able to drive the Mercury into the Tube A. B. more Degrees of it's Rarefaction will be dis∣covered, tho' less sensibly. But

The D. C. ought to hold Mercury enough to fill the Tube A. B. before the Air in the Tube E. D. can make it's way out; But how much larger, or of what Figure it is, is not so ma∣terial.

Notes

• * 1.1

  See Plate 4 Fig. 1.
• * 1.2

  See Fig. 2.