A Description of An Invention, Whereby the Divisions of the Barometer May be Enlarged in Any Given Proportions; Produced before the Royall Society By Mr. Robert Hook R. S. Soc. and Profess. Geom. Gresham

A Description of an Invention, whereby the Divisions of the Barometer may be enlarged in any given proportions; produced before the Royall Society by Mr. Robert Hook R. S. Soc. and Profess. Geom. Gresham. Since the discovery of the Alterations, that are in these parts of the World, in the weight of the Atmosphere, by the means of the Torricellian Tube, there has been several contrivances thought on, to make the more minute variations in the Airs pressure sensible. And first, the Wheel-Barometer was invented and Published by Mr. Hook, Anno 1665, in his Micrography: (where it is described at large) but this did not answer fully the designed exactness, both for that the Mercury being apt to stick against the sides of the Glass, would rise and fall per saltum, all at once, and because it is very difficult to adjust the Ball and Thread and other apparatus of this Instrument, as also that it is exceeding apt to be out of order, for which reason it is at present almost wholly laid aside. Upon this in June 1668 (as appears on the Journal of the Royal Society) he bethought himself of an other device to do the same thing, which was to encrease the divisions by putting coloured Spirit of Wine, or some other Liquor not capable of Freezing, on the Mercury, which Liquor was made to rise as the Mercury fell, and fall as it rose, in a narrow Cane, so as to make the utmost limits about two foot asunder. This invention was afterwards in the Year 1673 Published in France by Mr. Hubin, who neatly performed the Glass-work; but the Cane being necessarily small and apt to be obstructed with bubbles, (whereby the intercourse of the outward Air would be intercepted) and besides the utmost limits of rise and fall scarce reaching two foot and a half, Mr. Hook was not yet satisfied, till he had found out the means of increasing the Divisions of the Barometer ad libitum, by away free from such Objections, which finally he produced before the R. Society at their meeting on Feb. 3d. 1685 f. vet. The contrivance whereof is this. Figure I. Tab. II. Represents the Glass of this Baroscope; the Cylinder A may be of what Diameter you please, the bigger the better, but it need not be above 2 inches long, the Cane AD must be so long, that the upper part of the Cylinder B may be 29 inches + such a part of the height of the other Cane BC, as the weight or specific Gravity of the Liquor that is to fill that Cane is to the specific Gravity of Mercury below the line ab in the Cylinder A. The third Cylinder C may be as high as you please above the Cylinder B, but is most conveniently made, so as the square of the Diameter of the Cane BC be to the square of the Diameter of the Cylinders B or C, (which must be exactly equal) as the rise of the Mercury in the Cylinder B, is to the whole Length of the Cane BC: for in this case there will be nothing Superfluous, but the divisions enlarged to the utmost advantage. As to the method of filling this Baroscope, though the Inventor hath not yet declared his own contrivance for the doing it, yet it will not be unnecessary to shew here how it may be done. One way, (and the best that occurs at present), is to leave a small hole at the top of the Cylinder A, and another near the top of the Cylinder B: this latter being well slopt, pour in as much Mercury, at the other hole in A, as shall fill both Canes as high as the Level of the said hole; which done, stop either by Hermetically sealing it, or else by a drop of seal-wax (the glass being first ground rough to make it stick) the hole in A; then opening the hole in B, draw off as much of the Mercury of the Cane B C till it will runn no longer: which done, stopp firmly the hole in B (which may be done as you please, there being no pressure against you) and you will have the Cylinder A evacuated of Air for your purpose; and the height of the Mercury will be as is usual in the ordinary plaine and Wheel-Barometers. Then Then pour into the Cane B C as much Spirit of Wine tinged with Cochineale, and Oyle of Turpentine, equal parts of each, as shall stand above the surface of the Mercury so many feet as you make the enlarged scale of your Barometer, or as is between the middle of the Cylinders B and C, and you will find the Mercury sink in the Cane B C, and rise in the other Cane A D, in such proportion, that each 13 foot of Oyle and Spirit, will raise the Mercury ten Inches: This done, you must pour on, by the Cane B C, so much Mercury as may fill up the Cylinders A & B to such heights, considering the present weight of the Atmosphere, that the surface of the Mercury in both, may, at the utmost limits, (which have not in England been found to exceed 30, 6 and 28, 6. Inches) always fall within the bodies of the Cylinders, and never enter into the Canes. Here note that these Liquors are chosen upon two accounts, First they are exceeding near of a weight, and Spirit of Wine highly rectified is somewhat lighter than Oyle of Turpentine, but by a very small addition of Phlegme or Water, the Spirit will preponderate and be undermost; so that you may make them as near of a weight as you please, and consequently a Cylinder of the Oyle insensibly differing from an equal Cylinder of the Spirit of Wine. Secondly they are Liquors that will not mix; so that the Oyle of Turpentine swining on the top, will be divided by a line only from the tinged Spirit of Wine, which the Oyle will keep from Evaporating. The effect of this Baroscope will be, that when the Atmosphere is heavy, and the Mercury raised high in the Cylinder A, and retired out of B, the spirit of Wine will descend into the Cylinder B, and the Oyle of Turpentine will fill the Cane, so as to make the partition of the two Liquors near the Cylinder B. But on the contrary when the Air is light, the Mercury will sink in A and rise in B, so as to drive the Spirit of Wine into the Cane, and the Oyle of Turpentine into the Cylinder C, so that the section of the 2 Liquors will be near C, and the Variation of the height of the Mercury will be ena larged into almost the length of the Cane, without that the Counter-pressure from the Liquors will be in the least altered, the height and weight of the incumbent Cylinders being always the same. That little alteration that may happen by the dilatation and contraction of the Spirit of Wine by heat and cold, which ought to be accounted for, may be best discovered by a Thermometer hanging by it (containing the same quantity of Spirit of Wine, and whose Cane is, as near as may be, of the same Diameter with the Cane BC in the Barometer) whose descent and ascent must be added and subtracted to reduce it to a rigorous exactness; but it is still worth while to enquire if the Mercury itself do not shrink and swell with cold and heat, so as not to need this correction. Thus is a remedy found out for the defects and inconveniences of the Barometers hitherto produced, and an Instrument discovered which like a new Sense, will most nicely shew those alterations in the Air, which without it would by no means be perceptible, and of which undoubtedly very great uses might be made in order towards a perfect Meteorologie, which, without some such help as this, can hardly arrive at any great point of certainty. But I forbear to say more about it, least (by omission of some material circumstance) I should prejudice the Ingenious Author of this discovery, who has promised to publish a more particular Account thereof; what is here said being only intended to assert the Right of the first Invention of this Useful and Subtile Instrument to its proper Author, from the pretensions of all others.