BARO METER.
E Encyclopedia contains such an account of the discovery and construction of this meet valuable in strument, as could be drawn from the popultir trea tises of natural philosophy in the English language. But, unfortunately, our compilers of elementary works have seldom taken the trouble to remount to the original sources of information, 'and have fre quently, by substituting their own fancies, or ser vilely copying the mistakes of others, contrived to disfigure egregiously the relation of facts, and the history of the progress of invention. We now pur pose, therefore, as far as our limits will admit, to remodel the article ; and, passing rather slightly over the description of the different kinds of baro meters, and other practical details already given, to dwell more especially on the successive steps which led to the fine discovery of atmospheric pres sure, and its application to physical science.
The opinions entertained by the ancients concern ing physical subjects, appear at best only splendid vi sions. They speculated boldly in cosmological theo ries, but were easily satisfied with those conclusions which • merely soothe the fancy. Many of the phi losophical notions, however, adopted in remote ages, have left a durable impression in the structure of language, and still continue to exert a visible influ ence 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 sup posed to produce the animated spectacle of the uni verse. With these elements were associated corre sponding qualities, in a binary conjunction : Hot and cold ; dry and moist. Earth and water were con sidered as ponderous and inert ; but air and fire, en dued 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 /Ether, occupied the highest heavens, and constituted the substance and nutriment of the ce lestial bodies. While the earthy matter would, there fore, 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 occu pying the whole of the region below the moon, and the latter streaming through the boundless extent of apace. This sublunary scene is exposed to inces sant 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 ether as altogether distinct from culinary fire, were disposed, however, to consider it as afifth 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 be nighted period, in adopting those notions, modified them to suit their own peculiar views. To the ele ments 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 re present the animating principle of each body ; while the latter, extracted by the combined action of heat and moisture, was supposed to exhibit its concentrat ed and most select qualities.
Some of the ancient cosmologists supposed a va cuum beyond the shining expanse of tether, destin ed 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 idea of space or exten sion is inseparable from that of body. To this prin ciple 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 pater views. He rejected the no tion, 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 in difference, 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 his 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 asser tion, 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 al leged that, in blowing up the bladder, a portion of air would be introduced immediately from the lungs, and containing, therefore, a small admixture of car bonic acid gas, which is specifically lighter 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 jeweller's ba lances constructed in ancient Greece.