These different forms of the instrument have been variously modified, and often brought for ward with claims of novelty. We may notice, bow. ever, the Sectoral Barometer proposed by Magel lan, in which the mercury is always made to rise to the same high point of the tube, by drawing this less or more aside from the vertical position. The arc they described will ipdicate the deviation from the perpendicular, and consequently the actual de scent of the mercury. But the difference between the vertical and the oblique line is not measured by the inclination merely ; it is proportioned to the versed sine of this angle, or nearly to the square of the arc. The advantage of this mode of observing is, therefore, best perceived in small variations of the mercurial column. In the hands of a skilful observer, the best and most accurate barometer, after, all, is that of the original construction, with a tube rather wide, and a broad cistern. To apply minute divisions, is decidedly preferable to any en• largement of the scale. The measuring of such divi sions has been since rendered extremely easy, by the' adaptation of the differential scale—a most va luable contrivance proposed by Vernier, early in the seventeenth century, but strangely neglected long afterwards. This delicate appendage being once adopted, it became the more desirable to im prove the sensibility, and regulate the correctness of the indications of' the barometer.
The first object was carefully to cleanse the mer cury, and to expel any portions of air or moisture ad hering to the inside of the tube. The influence of aque ous vapour in depressing the mercurial column had been observed by Huygens ; but more evapor able fluids were afterwards found to occasion, by their presence, a still greater derangement. Homberg having, about the year 1705, washed a tube with alcohol, to remove the impurities from its internal surface, remarked that the mercury introduced into it stood an inch and half lower than usual ; a depres sion which this ingenious chemist was disposed to at tribute to the elasticity of the spiritous exhalations collected above the mercurial column ; though other academicians, and Amontons among the rest, misled by their Cartesian prejudices, sought to ascribe the el: fect to the different sized pores of the glass. These Imo• malies were removed, by heating or rather boiling the mercury in the tube, till it was completely purged of air and moisture, and brought into close contact with the inside of the tube. But a new fact occurred which long puzzled the mechanical philosophers. The tube . , Of a barometer, which had been filled with more than usual care, was observed to exhibit a luminous ap pearance, when moved or slightly agitated in the dark. This curious phenomenon gave occasion to multiplied and prolonged controversies ; it was attn. buted to the subtile matter of Descartes, or ascribed to a native phosphorescence, or a latent fire inherent in'the mercury. Our countryman, Hauksbee, in the year 1708, gave the first rational explanation of the tact, by referring it to electricity, which he had just begun to cultivate as a distinct science. It resembles
exactly, indeed, the experiment of the exhausted flask, in which an electrical current flashes with a diffuse lambent flame, like the aurora borealis, or the northern streamers. The friction of the mercury against the inside of the tube- excites electricity, while the vacuity, or rather the very attenuated va pour, in which the supposed fluid plays, facilitates its expansion. When the vacuum is rendered very perfect, by the careful and accurate boiling of the mercury, the lambent flashing ceases, for want of a fine medium to conduct and disperse the electri cal influence.
The next point to which experimenters were led to direct their attention, was the effect of the width of the tube on the altitude of the mercurial column. 'Plantade, a lawyer at Montpellier, appears to have been one of the first who remarked that the mercury ' stands always lower in narrow tubei. This fact he communicated about the year 1780 to Cassini, who was then occupied in the south of France, with car rying on the great trigonometrical survey. But the • discrepancies observed by Plantade being unfortu nately confounded with other collateral circumstan ces, were for a time overlooked. In tubes having a narrow bore, the depiession of the mercuq, how ever, is very considerable, as may be readily per. ceived in a small glass syphon, of which the one branch is about half an inch in diameter, and that of the other branch less than the tenth of an inch. Thus, if the narrow tube had a width of only the thirteenth part of an inch, the depression of the mer cury would amount to half an inch, which is about the third part of the elevation to which water in si milar circumstances would be raised by capillary ac • tion. This effbct has not been sufficiently examined; but it appears to result from the attraction of the particles of the mercury to each other exceeding their attraction to the surface of the glass. Mercu ry, in contact with glass, therefore, tends to a sphe rical form, and always assumes a convex surface within a clean tube. Water and other liquids again manifest an opposite character, the mutual attrac tion of their particles being less than their adhesion -to glass. Accordingly, they spread along a vitreous surface, instead of collecting into drops ; and in nar tow tubes they mount above the level, and invariably have a concave termination. If the bore be so small as to be reckoned capillary, the depression of mer cury is, like the elevation of water, inversely as the diameter ; but when the bore has a considerable width, the quantity of depression, depending on the Curvature of the surface of the mercury, diminishes proportionally faster, and follows nearly the inverse duplicate ratio of the diameter. • But on the subject of capillary action, we expect, with no small degree of impatience, to see a paper which was very late ly communicated to the Royal Society of London, by Mr Ivory, of the Military College at Sand hurst, one of the• most original and profound ma thematicians that our island has had the honour to produce.