the Old Logic, a term given to the fourth mode of the second figure of syllogisms. A syllogism in baroco has the first proposition a universal affirmative, but the second and third, particular negatives, and the middle term is the predicate in the first two propositions. For example,

Nullus homo non est bipes: Non omne animal est bipes: Non omne animal est homo. Our dictionaries contain such an account of the discovery and construction of this most valuable instrument, as can be drawn from the popular treatises of natural philosophy in the English language. But, unfortunately, the compilers of elementary works have seldom taken the trouble to remount to the original sources of information, and have frequently, by substituting their own fancies, or servilely copying the mistakes of others, contrived to disfigure egregiously the relation of facts, and the history of the progress of invention. We purpose, therefore, as far as our limits will admit, to discuss the subject with more careful research; but, passing rather lightly over the description of the different kinds of barometers and other practical details, to dwell more especially on the successive steps which led to the fine discovery of atmospheric pressure and its application to physical science.

The opinions entertained by the ancients concerning physical subjects appear at best only splendid visions. They speculated boldly in cosmological theories, but were easily satisfied with those conclusions which merely soothe the fancy. Many of the philosophical notions adopted in remote ages have, however, left a durable impression in the structure of language, and still continue to exercise a visible influence in moulding the current sentiments of mankind. The early sages of Greece distinguished matter into the four primary elements, of earth, water, air, and fire, which, by their various combinations, were supposed to produce the animated spectacle of the universe. With these elements were associated corresponding qualities, in a binary conjunction; hot and cold, dry and moist. Earth and water were considered as ponderous and inert; but air and fire, endowed with elastic virtue, were imagined to possess lightness and activity. Fire, though extracted from all bodies by the operations of nature or of art, was yet conceived to be derived, by invisible emanation, from that diffuse lambent fluid which, under the name of Aether, occupied the highest heavens, and furnished the substance and nutriment of the celestial bodies. While the earthly matter would, therefore, naturally settle towards the centre, and the aqueous fluids roll along the surface of the solid globe, the air and fire soared aloft; the former occupying the whole of the region below the moon, and the latter streaming through the boundless extent of space. This sublunary scene is exposed to incessant change, calamity, and decay; but above it was supposed to reign a perpetual calm, the seat of bliss, and of divine and imperishable essence.

Aristotle and some other philosophers, viewing aether as altogether distinct from culinary fire, were disposed to consider it as a fifth element, of a pure, divine, and incorruptible nature; an opinion which afterwards gave occasion to the famous Quinta Essentia, or Quintessence of the schoolmen. The alchemists, who sprung up nearly about the same benighted period, in adopting these notions, modified them to suit their own peculiar views. To the elements commonly received, they joined the active auxiliaries of mercury and sulphur. For quintessence they substituted spirit and elixir; the former, drawn off by the application of fire, being conceived to represent the animating principle of each body; while the latter, extracted by the combined action of heat and moisture, was supposed to exhibit its concentrated and most select qualities.

Some of the ancient cosmologists imagined a vacuum beyond the shining expanse of aether, destined to receive the exhalations from this nether world. Others denied the existence of a separate void, but admitted small vacuities interspersed through bodies. Aristotle, however, maintained the necessity of a plenum, asserting that our tenets of idea of space or extension is inseparable from that of Aristotle's body. To this principle he ascribed the suspension of water in a tube, when the finger is applied to shut the upper end. Yet the very contempt in which that philosopher, from a consciousness of his own superiority, was accustomed to hold the received opinions, might have led him to take juster views. He rejected the notion that air has levity inherent in its nature; nor would he admit the more plausible idea, that a fluid so easily moved must possess the quality of perfect indifference, and be neither light nor heavy. Aristotle not only maintained that air is ponderous, but did not scruple to appeal to direct experiment in support of this assertion. A bladder, he says, will be found to gain some weight, on being blown or filled with air. But this was evidently a mere random assertion, betraying his ignorance of the constitution of fluids. A bag filled with air, and suspended in a like medium, it is obvious, from the laws of hydrostatics, must weigh exactly the same as before. If it be alleged that, in blowing up the bladder, a portion of air would be introduced immediately from the lungs, and containing therefore a small admixture of carbonic acid gas, which is specifically heavier than the common atmospheric fluid; the additional weight, amounting scarcely perhaps to a grain, would be too minute to be detected by any of the jewellers' balances constructed in ancient Greece.

The mutual opposition of the leading philosophical sects of antiquity had, in general, most fatally discouraged the application of mathematical reasoning to the system of the material world. The Academicians, or the disciples of Plato, who cultivated geometry with ardour and brilliant success, were disposed to regard that science as a pure intellectual contemplation; and resigning themselves to the illusion of their lofty dreams, they turned with disdain from the investigation of individual facts and all the vulgar realities of life. The mind of Aristotle was of a more sober and practical cast; acute, profound, and discriminating, it ranged with incredible industry over an immense field of inquiry. That judicious philosopher recommended a careful and constant appeal to external observation, as the only sure ground on which to erect the structure of physics; but unfortunately his scholars neglected too much the study of mathematics, the most powerful instrument for conducting physical research. The precepts of Aristotle, though excellent in some respects, were hence in the sequel unproductive of any genuine fruit. On the contrary, the weight of his opinions, during a long course of ages, repressed the efforts of human genius.

It must be gratefully acknowledged, that the alchemists, styled also philosophers by fire, were the first among the moderns who dared to explore new paths of science, mentalists. Their projects were, indeed, highly chimerical, but they had the merit at least of setting the example of investigating the properties of matter by actual experiment. They likewise formed associations among individuals for the more effectual prosecution of such researches. Hence the origin of that obscure sect known by the fanciful title of Rosicrucians, who sprung up in Germany, and insensibly spread their influence over the Continent. Those principles were afterwards transplanted into the matured