BAROMETRICAL MEASUREMENTS.
IT was remarked in the preceding article, that the decisive experiment by which Pascal established the reality of atmospheric pressure, had likewise suggest ed to this ingenious philosopher the method of de termining the elevations points on the sur face of the globe. But the first attempts were very rude, proceeding on the inaccurate supposition that the lower mass of air is a fluid of uniform density. Different authors estimated variously from -eighty to ninety feet as the altitude, which corresponds to a variation of the tenth part Of an inch in the mercurial column. The Torricellian tube or cane, as it was then called, was, on its first introduction to England, carried accordingly to the tops of mountains, or con veyed to the bottom of pits and mines, or even let down to great depths in the sea.
. Among those experimentalists who laboured most assiduously in the study and application of the ba rometer in this part of the island, we should mention George Sinclair. This ingenious person had been Professor of Philosophy in the University of Glas gow, but seems to have conscientiously resigned his office soon after the Restoration, rather than comply with that hated episcopacy which the minions of Charles II. had forced upon the people of Scotland. He then retired to the village of Tranent, _ not far from Edinburgh, and was employed as a practical engineer, in tracing the levels of coal-pits, in di recting le machinery employed in the mines at Leadhllls, and afterwards in the great under taking of conducting water from the heights of the Pentlands to supply the northern metropolis. Though not a profound mathematician, he was skill ed in mechanics and hydrostatics, and possessed no small share of invention. Sinclair is said to be the first who applied to the mercurial tube the name of baroscope, or indicator of 'aright, the more definite appellation of barometer, or measurer of weight, not having been appropriated till many years afterwards. During his excursions in 1668 and 1670, he employ ed that instrument to measure the heights of Arthur's Seat, Leadhills, and Tinto, above the adjacent plains. He followed the original mode of using a tube sealed at the top, with a paper scale pasted against the side, which he carried to the top of the moun tain, where he filled it with mercury ; and, inverting it in a boon, be noted the altitude of the suspended column, and repeated the same experiment be low ; a very rude method certainly,--but no better was practised in England during the succeeding thirty years.
In a small scarce tract, printed in 1688, and bear ing the quaint title of Proteus bound with Chains, Sinclair gives some judicious remarks on the va riations of the barometer, considered as a weather glass, and delivers very sound opinions, on the whole, respecting the causes of the chief meteorological phenomena. In a postscript to that piece, he pro
poses a most efficient and ingenious method of weighing up wrecks from the bottom of the sea. It consisted in employing two large arks, or square wooden boxes, fastened to the sides of the ship, and charged with air carried down to them by a succes sion of inverted casks, open at the lower end. An ark of a cubical shape, and twenty feet in every di mension, the smallest which he mentions, would, as he computes, have a buoyancy equivalent to 448,000 pounds Troy. It is remarkable that the celebrated Mr Watt always employs this very mode, using a large gasometer, floating in a pond dug in the court of his manufactory, and charged gradually by the action of bellows, for raising the ponderous engines constructed at Soho, and lifting them over his walls into the boats, which are stationed to receive them in the adjacent canal.• In all the computations hitherto made from differ eat altitudes of the barometer, the air was considered as an uniform fluid; no regard being had to the gra dual diminution of density which must evidently take place in ascending the atmosphere. To estimate the effect of that gradation, it became requisite previous ly to determine the actual relation subsisting be tween the density of the fluid and its elasticity. This was first ascertained in England by Townley, who inferred from some experiments of Boyle, that the elastic force which the air exerts is exactly proper tional to its density. A similar conclusion was about the same time drawn by Mariotte, a French philoso pher, from a still better series of experiments. Fol lowing out this very simple law, he thought of com puting heights from barometrical observations, by the rules usually employed in constructing tables of lo garithms ; and had, therefore, obtained some glimpse, no doubt by a sort of conjectural process, of the re markable result, that the density of the atmosphere decreases in a geometrical progression, correspond ing to the elevations taken after an arithmetical one. But seemingly not aware of the importance of the principle at which he was pointing, Mariotte Immo. diataly deserted it ; and calculating from a repeated bisection of the column of air between the two sta tions into successive horizontal strata, he contented himself with interpolating the densities according to a harmonic division, which he next abandoned for the simplicity of a series with equal differences. This able experimenter hence only sketched out a mode of investigating the problem of barometrical measurements, without arriving at any very definite or. rule of solution.