BARNSTAPLE, a seaport town of England, in.. the county of Devon, situated in a fine vale on the eastern bank of the river Taw, over which there is a.' stone bridge of 16 arches. On account of the shal lowness harbour, which does not admit vessels. of more than 200 tons, a great part of its woollen trade was transferred to Biddeford. Manufactures of baize, however, silk stockings, and waistcoat pieces,, still employ its inhabitants.. Number of houses G1E. ' Population 31.78, of whom 578 were returned as em ployed in trade. See Oldfield's History of the Bo. roughs. (j) _ r• BAROMETER (compounded of gaga; weight, and thirgor measure,) is an instrument for determining the weight of the air, and the variations of its pressure in diffe rent circumstances. As every change in the weight of that fluid is accompanied with corresponding changes of density, and consequently of its disposi tion to absorb or deposit moisture, the barometer is also employed to point out the probable changes of weather ; hence it is not unfrequently called a wea ther-glass. (See METEOROLOGY.) Another purpose, scarcely less important, to which this instrument has lately been much applied, is the measurement of acces sible heights ; and the results obtained by means of it approach so near to perfect accuracy, when all cir cumstances are properly estimated, that this method of determining the heights of mountains is, in many cases, even .preferable to the geometrical methods. (See HEIGHTS.) It also appears from the observations of Captain Flinders, that the barometer may be of the most essential service at sea, not only to foretel changes of weather, but also to indicate the vicinity of land. (See WiNns.) These important properties entitle this instrument to a considerable share of our attention.
Before the discovery of the weight of the air, the barometer was entirely unknown ; and indeed it was the discovery of that fact, which led to the invention of the instrument. The ancients ascribed to the horror of a vacuum all the effects which arise from'atmo spherical pressure. This explanation,absurd as it may now seem, was admitted even by Galileo ; and the in . genuity of a Pascal and a Boyle was scarcely sufficient. to point out its futility, and banish it from the esta blished principles of philosophy. The influence of occult qualities, was, in all difficult cases, assumed to explain the phenomena of nature, and it was less in consistent with their prejudices to ascribe the effects of the weight of the air to an invisible agent, or an .unknown principle, than to the operation of a fluid, which they thought was so obviously destitute of all weight. It was accordingly considered as an indis putable fact, that the -ascent of water in pumps, and other similar effects of atmospherical pressure, were owing to the horror which nature has for a vacuum. This opinion was universally received till the super intendant of the water works of the Grand Duke of Tuscany, wishing to raise water, by means of a pump, to a considerable height, was surprised to find that the water would not rise higher than 32 or 33 feet.
After he had ascertained that this could not be ascri bed to any defect in the construction of the pump, he mentioned the circumstance to Galileo, and requested him to give him an explanation of the cause of this anomaly. Galileo, either not questioning the just ness of the opinion which then prevailed, or being unable to assign any other that was more replied, " That the water was raised to the height of 32 feet on account of the horror which nature has for a vacuum ; but that the horror w:Is limited in its ef fects, and ceased to operate above the height of 32 feet !" This explanation, so unworthy of the name of Galileo, would be totally undeserving of Credit, did we not know the authority that an error which has prevailed for•twenty centuries could have over the hu man mind. It appears, however, that he was by no means satisfied with the explanation which he had given, and that he immediately began to suspect the agency of some external cause ; but his death, which happen ed soon after, prevented him from bringing his thoughts to maturity. His disciple Torricelli, to whom it is supposed lie had mentioned his ideas on the subject, was more successful in his explanation. He suspect ed that the weight of the water was one of the elements which ought to be taken into consideration in inves tigating the cause of the ascent of that fluid in puthps, and that it was probably counterbalanced by the weight of something external pressing upon the sur face below. To put this conjecture to the test of experiment, he took a glass tube about four feet long, (as A B, Fig. 2.) hermetically sealed at one end A, and open at the other. Having filled it with mercu ry, lie shut the open end with his finger ; he then inverted the tube, and introduced the open end of it under the surface of a small quantity of mercury in a hason. Lastly, he placed the tube in a vertical position, and on withdrawing his finger, he observed that a part of the mercury descended in the tube, and that the rest of it was supported at the height of QM inches above the level of the mercury in the bason. By varying the experiment, he found that, in all cases, the mercury was supported at a perpendicular height above its surface in the bason, equal to about the 14th part of the height of the water in the pumps. He therefore inferred, that the mercury in the tube, and the water in the pump, exerted equal pressures on the same base, their altitudes being inversely as their specific gravities, and that the weight of the column in either case was counteracted by some fixed and de terminate force. This force he supposed was the weight of the air.