. In every marine barometer, the main object it to Five steadiness to the mercurial column, by retarding its motion in the tube ; in short, to imitate the equaliz ing effect of the fly in mechanics. Chieform of construc tion was, instead of the cistern below, to annex a spi ral tube composed of a number of horizontal convolu tions. Passement, an ingenious Parisian artist, about the year 1768, improved on this idea. He twisted the barometer tube near the middle, at least twice round, and joined to its upper end a wide cylinder. But more eftually to prevent all irregular oscine Lions, he teak a tube with a very narrow or capillary bore, and zontracted it below, while he annexed a wide cylindrical piece at its other extremity. The only thing wanted now to the perfection of this in strument, was to devise a mode of suspending it that soften the jerks, and allow it generally to maintain a vertical position. Our English artists have, by repeated trials, at last succeeded in sur mounting all the difficulties. The marine barometer, manufactured by Mr Cary of London, ( See*. 12, Plate XXXII.) is one of the most approved kind. It consists of a capillary tube, about twenty-seven inches long, with a bore scarcely exceeding the thirtieth part of an inch, but terminated by a cylinder four or five inches high, and nearly three-tenths of an inch in diameter. Is is suspended by a spring and jimbols, near the top at a certain point, which in each case is discover ed by actual trial. • By making the suspension lower, it is found, that the agitation of the barometer will same the mercury to rise a little ; while, by ple b clog the suspension higher, the mercurial column suffers always some depression. The reason of this curious observation is not well explained. It pro bably results from the different centrifugal tenden cies communicated to the opposite portions of the columns. The swinging of the instrument would evidently. augment the pressure of the upper portion of the column, while-it diminished that of the under portion. But this lower portion, being longer than the other, its tendency to descend would be propor tionally so much greater. About the point of sus pension, however, the opposite effects of the centri fugal tendencies are balanced, since the superior force being employed to set in motion a narrower co lumn, the reflux and efflux of the mercury in the up per cylinder must be preserved nearly equal.
Marine barometers, thus improved, are now very generally used, and with great benefit to the public service, on board ships of war and Indiamen. To facilitate the keeping of a register of barometrical observations, the meritorious and indefatigable Mr .Horsburgh, hydrographer to the East India Compa ny, has lately published a set of engraved ruled sheets, adapted for the convenience of In these plates, the height of the mercury, from twenty-seven to thirty-one inches, is represented, in inches and, tenth parts, by horizontal lines, while each • successive day has a space apportioned to it by ver tical bars. The state of the barometer at every ob servation is marked with a dot, and these dots being afterwards connected together, exhibit an irregular waved line, stretching across the sheet, and in eat ing the series of the changes of the weather.. At the lowest points, from which the curve again 'returns, a gale generally follows. From the observations made off the Cape of Good Hope, during the month of May by an ingenious and active young officer, Captain Basil Hall, of his Majesty's sloop Victor, it appears that whenever the mercury fell to 29,60 inches, a storm ensued ; the column always rose when the gale abated, and when it reached near thirty inches, the weather became fair. Those gales often came on suddenly, without any visible change in the aspect of the sky, but the marine barometer never failed. to give warning of their approach..
To explain the cause of the variations of the ba rometer, has long perplexed philosophers. Many. hypotheses have at different times been advanced on the subject ; but it would be a mere waste of time, to make any detailed recital of such crude and unsatis factory attempts. The various and often imaginary.
effects of vapours of heat and winds have been eta played in framing an explication of the changes of the atmosphere. The fact that the mercurial co lumn generally falls before rain, seemed at complete variance with the intimation of the senses, it heal a. notion universally prevalent, that the air is. heavier when the sky appears lowering and overcast; ano ther proof, if it were wanted, how fallacious are all current opinions in matters of science..
Leibnitz, fancying he haddiscovered a new principle in hydrostatics, endeavoured, by a sort of metaphysi cal argument, to demonstrate that, though a body . adds its own weight to the pressure of a fluid in which it is suspended, yet it will cease to be ponderous in the act of falling. This alleged principle will not, in the actual state of science, be thought to require any serious refutation ; nor, were it even admitted, would it be found at all adequate to the explication of the phenomenon, since the weight of moisture precipitated from the whole body of atmosphere could never, by the absence of its pressure, occasion a diminution of the tenth part of an inch in the alti tude of the mercurial column.
Dr Halley and Mairan sought to account for the depression of the barometer before a storm, to the withdrawing of the vertical pressure of the atmo sphere, when borne swiftly along the surface of the globe by a horizontal motion. This hypothesis at first sight appears very plausible, and might seem farther confirmed by a noted experiment which most authors have admitted without due examination. Hauksbee, a skilful and ingenious experimental phi losopher, about the year 1704, placed two barome ters, about three feet asunder, with their naked cis term in two close square wooden boxes, connected by a horizontal brass pipe ; die of these boxes had, inserted at right angles, an open pipe on the one side, and a second pipe terminating in a screw, on the other side ; to this end he adapted a strong glo bular receiver of about a foot in diameter, which had been charged, by injection from a syringe, with three or four atmospheres; then suddenly opening the stop-cock, and giving vent for the escape of the air through the box and over the surface of the included cistern, the mercury sunk equally in both the baro meters more than two inches.
This elegant experiment might be deemed entirely conclusive, if a minute circumstance, on which the suc cess really depends, had not unfortunately been over . looked. It will be perceived from the inspection of the figure which Hauksbee has given, that the exit pipe of the box was considerably wider than the pipe which conveyed into it the stream of air. This fluid, escaping from compression, would, therefore, be carried by its elasticity as much beyond the state of equilibrium ; while the width of the orifice, by facilitating its emission, would allow the portion occupying the box and the connected reservoir to preserve its ac quired expansion. If the pipe of discharge from the box had been much narrower than the other, an op posite effect must have taken place ; for the air ac cumulated over the cistern, not finding a ready vent, would remain in a state of condensation. This curious fact is another of the many instances which might be cited to show the great delicacy and circumspection required in performing philosophical experiments. • The same results, however, can be exhibited by a very simple apparatus. Let a small box, or rather a glass ball, have a short narrow tube inserted in the one side, and another wide tube opposite to this, with a cross slider of brass, for contracting the orifice at pleasure ; and, to the under part of the ball, join a long perpendicular tube, bent back like a syphon to more than half its height and containing a double oolumn of water. Now, blow through the narrow tube into the cavity of the ball, while the orifice of