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of the mercury again>t the sides of the glass, and the great momen-tum from the quick motion in FG, the quicksilver is apt to break,and the ri s e and fall is no longer equable; and besides, lire mer-cury is apt to be thrown out of the orifice at G by sudd-h motionsotthe machine.
Barometer, Marine. This was first invented by Dr. Hook,to be used on board of ships, being contrived sons not to be af-fected or injured by the motion of the ship. His contrivance iconsisted of a double thermometer, or a couple of tubes bait jfilled with spirit of wine; the one sealed at both ends, with |a quantity of air included ; the other sealed at one end only. Theformer of these is a tied eel only by the warmth of the air; but theother is affected both bv the external warmth and by the variablepressure of the atmosphere. Hence, considering the spirit-ther-mometer as a standard, the excess of the rise or fall of the otherabove it will shew the increase or dec-ease of the pressure of theatmosphere. This instrument is described by Dr. Halley, in thePhilos. Trans. No. 20!), where he says of it, “ l had one of thesebarometers with me in mv late southern voyage, and it never failedto prognosticate and give early notice of all the bad weather wehad, so that I depended thereon, and made provision accordingly; jand from my own experience I conclude, that a more useful eon- Itrivance hath not for this long time been offered for the benefit ofnavigation.” Mr. Nairne, an ingenious artist in London , liaslately invented a new kind of Marine Barometer: which differsfrom the common barometer by having the lower part of the tube,for about two feet dong, made very small, to check the vibrationsof the mercury, which would otherwise arise from the motions ofthe ship.
Barometer, Bentiant, invented by M. Amontons, in 1695,consists of a single conical tube living up by a thread, the largeror opeu end downwards, and having no vessel or cistern, becausethe conical figure supplies that, and the column of mercury sus-tained is always equal to that in the common barometer-tabe;which is eifected thus: when the pressure of the air is les--, themercury sinks down to a lower and wider part of the tube, and eon-sequentlv the altitude of its column will be less; and on the con-trary, by, a greater pressure of the atmosphere the mercury is forcedup to a higher ami .narrower part, till the length of the column beequal to that in the tube of the common barometer. The incon-venience of.this barometer is, that as the bore must be made verysmall, to prevent the mercury from falling out by an accidentalshake, the friction and adhesion to the sides of the tube prevent thefree motion of the mercury.
Barometer, Portable, is so contrived that it may be carriedfrom one place to another without being disordered. The end ofthe tube is tied up in a leathern bag not quite full of mercury ;which being pressed bv the air, forces the mercury into the lube,and keeps it suspended at its proper height. This bag is usuallyinclosed in a box, through the bottom of which passes a screw, by ,means of which the mercury may he forced up to the top ol the |tube, and prevented from breaking it by dashing against flic topwhen the instrument is removed from one station to another. Itseems Mr. Patrick first made a contrivance of this kind : but theportalile barometer has received various improvement: since; andthe most complete of this kind has been described by M. De Luc,in his Rt eherches, vol. 2, page 5, fee. together with the apparatusbelonging to it, the method of construction and use, and the ad-vantages attending it. Experience has furnished us with a sulfi-cient number of facts, from which we may,'with tolerable precision,ascertain the present, and predict the future state of the surround-ing element, by the practical use of the barometer. Thus, 1. Therising of the quicksilver generally presages fair weather, as itsfalling does the contrary, or rain, snow, high winds and storms; 2.In very hot weather, the sudden falling of the mercury portendsthunder ; 3. In winter, the rising iiuVicates frost; and in frosty
weather, if the mercury falls three or four divisions, there will cer-tainly follow a thaw; but if it rise in a continued frost, it will al-ways be accompanied with snow ; 4. When- foul weather quicklysucceeds after the tailing of the mercury, it will not be of long du-ration; nor are we to expect a continuance of fair weather, whenit soon succeeds the rise of the quicksilver,; 5. If, in foul weather,the mercury happens to ascend considerably, and continues in anadvancing state for two or three days successively, then we mayexpect also a continuance of fair weather: 6. If in clear weather,the mercury kills remarkably for two or three days together, be-
fore the rain sets in, it is then highly probable that it will be suc-ceeded by much rain, and perhaps high winds; 7. The unsettledmotion of the mercury indicates changeable weather; 8. Respect-ing the words engraved on the register-plate they cannot lie strictlyrelied upon to correspond exact!}' with the state of the weather;though it will in general agree with them as to the mercury risingand falling. These words deserve to be particularly noticedwhen the mercury removes from “ Changeable” upwards; as thoseon the lower part should be adverted to, when the quicksilver fallsfrom “Changeable” downwards. In other eases, they are of nouse: for ns its rising in any part forebodes a tendency to fair, and| its falling to foul weather, it fellows, that though it descends in thej tube frmn Settled to Lair, it may nevertheless lie attended with aj little rain: and when it rises from the words “Much Bain” to“ Rain ” it shews only an inclination to become fair, though the wetweather may still continue in a less considerable degree than it waswhen the mercury began to rise. But if the mercury, afterhaving fallen to “ Much Rain, ” should ascend to “ Changeable,”it predicts fair weather, though of a shorter continuance than if thequicksilver had risen still higher ; atul so on the contrary, if the mer-cury stood at “ Fair,” and descends to “Changeable,” it pre-sages foul weather, though not of such duration, as if it had fallenlower. These observations are founded on experience, and we are in-debted for them chiefly to Mr. Patrick, who lias investigated thissubject with considerable precision. It appears from the result ofthese facts, that the height of the mercury is not the principal cri-terion for ascertaining the probable changes of the weather, butrather the relative motion of that fluid m the tube. Hence, toenable us to judge rightly of the impending variations, we oughtto possess a correct knowledge, whether the mercury is actuallyrising or falling. For this purpose, the following rules, slated byMr. Rovtimg, may be of advantage : 1. If the surface,of the mer-cury be com ex, suutding higher in the middle of the tube than atthe’sidcs, it generally indicates the rishig of this fluid metal.; -2. Ifiis surface he concave, it is then sinking; 3. If it appear level, itis stationary ; 4. If, after shaking the tube of a small glass, themercury rises about half a tenth of an inch higher than it stood lie-fore, it’is a proof that the air has become heavier ; but if it sinksas much, it follows, that the atmosphere is lighter. Hence, inmaking observations on the weather, such a glass should alwavsbe previously shaken; because the metal which adheres to th«sides of :lie tube, prevents its free motion, til! disengaged by aslight agitation of the instrument. These phieimmena are peculiarto places situated at a distance from the equator, and thereljore de-serve to be attended to in our climate; on the contrary, at St.Helena , they would be of little or no service ; for, according tothe accurate observations of Dr. Halley, made in that island, themercury remained stationary in all weathers. ’) his judicious na-turalist has furnished us with! he following curious account of thesepha-nomena ; -of which we shall give a concise alwtracl: 1. In calmweather, when there is a prospect of rain, the mercury is commonlylow ; 2. In serene and settled weather it is generally high ; 3. Oivdie approach of tempestuous winds, though, unaccompanied withrain, the mercury sinks lower than on any other occasion ; 4. Thegreatest height of the mercury is observable during the prevalenceof easterly or north-easterly winds; 5. In calm, frosty weather, thequicksilver generally stands high ; 6. Alter very great tem-
pests, when the mercury has been very low.it generally againquickly ascends; 7. The barometer in the northern regionsexhibits greater variations than in those lying more towards thesouth; 8. Within and near the tropics, there is little alteration inthe mercurv. Hence Dr. Halley is of opinion, that the principal'cause of the rise and the fall of the quicksilver must be attributedpartly to the variable winds in the temperate/one, the great in-constancy of which, in Britain , is well-known ; ami partly to theuncertain exhalation'and precipitation of the vapours floating in theatmosphere, which is at one time more saturated than at another,and consequently heavier ; though the precipitation of aqueous va-pours chiefly depends on the previous degree of evaporation. Thechanges in the barometer depend on the heat in the atmosphere,not as producing commotions there, but as altering the specificgravity of the air by the changes of heat and cold. The princi-ples of this theory are, 1st, That vapour is formed by an intimateunion between the elements of tire and water, by which the tireor heat is so totally enveloped, and its action so perfectly suspend-ed by flia aqueou* particles, that it not only loses its pro) ernes of
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