ther-in-law, M. Perier, to try that experiment The Common Barometer. This is represent- cury descend in the tube to some place as E, between 28 and 31 inches above that in the basen at F, as these are the limits between which it always stands in this country on the common surface of the earth. Then measure, from the surface of the quicksilver in the ba son at F, 28 inches to G, and 31 inches to H, dividing the space between them into inches and tenths, which are marked on a scale placed against the side of the tube; and the tenths are subdivided into hundredth parts of an inch by a sliding index carrying a vernier or nonius. These 3 inches, between 28 and 31, so divided, will answer for all the ordinary purposes of a stationary or chamber barometer; but for experiments on altitudes and depths, it is proper to have the divisions carried on a little higher up, and a great deal lower down. In the proper filling and otherwise fitting up of the barometer, several circumstances are to be carefully noted; as, that the bore of the tube be pretty wide, to allow the freer motion of the quicksilver, without being impeded by an adhesion to the sides; that the bason below it be also pretty large, in order that the surface of the mercury at F may not sensibly rise or fall with that in the tube; that the bottom of the tube be cut off rather obliquely, that when it rests on the bottom of the bason there may be a free passage for the quicksilver; and that, to have the quicksilver very pure, it is best to boil it in the tube, which will expel all the air from it. This barometer is coinmonly fitted up in a neat mahogany case, together with a thermometer and hygrometer. As the scale of variation is but small, being included within 3 inches in the common barometer, several contrivances have been devised to enlarge the scale, or to render the mo tion of the quicksilver more sensible. Descartes first suggested a method of increasing the sensibility, which was executed by Huygens. This was effected by making the barometrical tube end in a pretty large cylindrical vessel at top, into which was inserted also the lower or open end of a much finer tube than the former, which was partly filled with water, to give little obstruction by its weight to the motion of the mercury, while it moved through a pretty long space of the very fine tube by a small variation of the mercury below it, and so rendered the small changes in the state of the air very sensible. But the inconvenience was this, that the air contained in the water gradually disengaged itself, and escaped through into the vacuum in the top of the small tube, till it was collected in a body there, and by its elasticity preventing the free rise of the fluids in the tubes, spoiled the instrument as a barometer. And this, it may be observed by the bye, is the reason why a water barometer cannot succeed. This barometer is here represented in fig. 2. where CD is the vessel, in which is united the upper or small water tube AC, with the lower or mercurial one CB. The Horizontal or Rectangular Barometer, fg. 8. was invented by J. Bernoulli and Cassini; whete AB is a pretty wide cylindrical part at the top of the tube, which tube is bent at right angles at C, the lower part of it CD being turned into the horizontal direction, and close above at A, but open at the lower end, where however the mercury cannot run out, being there opposed by the pressure of the atmosphere. This and the foregoing contrivance of Huygens are founded on the theorem of hydrostatics, that fluids of the same base press according to their perpendicular altitude, not according to the quantity of their matter; so that the same pressure of the atmosphere sustains the quicksilver that fills the tube ACD, and the cistern B, as would support the mercury in the tube alone. Hence, having fixed upon the size of the scale, as suppose the extent of 12 inches, instead of the 3, in the common barometer from 28 to 31, that is four times as long; then the area of a section of the cylinder AB must be four times that of the tube, and consequently its diameter double, since the areas of circles are as the squares of their diameters: then for every natural variation of an inch in the cylinder AB, there will be a variation in the tube CD.-But on account of the attrition of the mercury against the sides of the glass, and the great momentum from the quick motion in CD, the quicksilver is apt to break, and the rise and fall is no longer equable; and besides, the mercury is apt to be thrown out of the orifice at D by sudden motions of the machine. The Diagonal Barometer of sir Samuel Moreland, fig. 9. is another method of enlarging the natural scale of three inches perpendicular, or CD, by extending it to any length BC in an oblique direction. This is liable in some degree to the saine inconvenience, from friction and breaking, as the horizontal one; and hence it is found that the diagonal part BC cannot properly be bent from the perpendicular more than in an angle of 45o, which only increases the scale nearly in the proportion of 7 to 5. Doctor Hooke's Wheel Barometer, fig. 7. This was invented about 1668, and is meant to render the alterations in the air more sensible. Here the barometer tube has a large ball AB at top, and is bent up at the lower or open end, where an iron ball G floats on the top of the mercury in the tube, to which is connected another ball H by a cord, hanging freely over a pulley, turning an index KL about its centre. When the mercury rises in the part FG, it raises the ball, and the other ball descends and turns the pulley with the index round a graduated circle from N towards M and P; and the contrary way when the quicksilver and the ball sink in the bent part of the tube. Hence the scale is easily enlarged ten or twelve fold, being increased in proportion of the axis of the pulley to the length of the index KL. But then the friction of the pulley and axis greatly obstructs the free mo of the quicksilver. Contrivances to les, sen the friction are described in Phil. Trans. vols. 52, and 60. Dr. Hooke, who was always fertile in expedients, invented another barometer with an enlarged scale, which was described in No. 185. of the Transactions. Similar inventions were claimed by Huygens and by De la Hire, but Hooke had undoubtedly the priority both in point of invention and publication. His baroineter consists of a compound tube ABCDEFG (fig. 11.), of which the parts AB and DE are equally wide, and EFG as much narrower as we would amplify the scale. The parts AB and EG must also be as perfectly cylindrical as possible. The part HBCDI is filled with mercury having a vacuum above in AB. IF is filled with a light fluid, and FG with another light fluid, which will not mix with that in IF. The cistern G is of the same diameter as AB. It is easy to see that the range of the separating surface at F must be as much greater than that of the surface I, as the area of I is greater than that of E: and this ratio is in our choice. This barometer is free from most of the bad qualities of those above described, being most delicately moveable; and is by far the fittest for a chamber, for observations on the changes of atmospheric pressure: the slightest breeze causes it to rise or fall, and it is continually in motion. But this and all other contrivances of the kind are inferior to the common barometer for all philosophical purposes, and that for a reason which admits of no reply. Their scale must be determined in all its parts by the common barometer; and therefore, notwithstanding their great range, they are susceptible of no greater accuracy than that with which the scale of the conmon barometer can be observed and measured. Except, therefore, for mere chamber amusement (in which case the barometer just described has a decided preference), the common construction of the barometer is to be preferred; and our attention should be entirely directed to its improvement and portability. And in this connection we cannot but point the attention of the reader to the improvement of Mr. Gough, in Phil. Jour. No. 77. N. S. and the ingenious simplifications of Mr. Pugh, described in a note at p. 227. vol. i. Gregory's Translation of Haüy's Philosophy. In Nicholson's Journal, No. 9. N. S. the Rev. J. Wilson has described a method of increasing the sensibility of the barometer ad libitum; which, though ingenious, could not with propriety be inserted here, after the preceding remarks. Other contrivances by Rowning, Amontons, &c. are omitted for a like reason. The Marine Barometer. This was first invented by Dr. Hooke, to be used on board of ship, being contrived so as not to be affected or injured by the motion of the ship. His contrivance consisted of a double thermometer, or a couple of tubes half filled with spirit of wine; the one sealed at both ends, with a quantity of air included; the other sealed at one end only. The former of these is affected only by the warmth of the air; but the other is affect ed both by the external warmth and by the variable pressure of the atmosphere. Hence, considering the spirit thermometer as a standard, the excess of the rise or fall of the other above it will shew the increase or decrease of the pressure of the atmosphere. This instrament is described by Dr. Halley, in the Philos. Trans. No. 269, where he says of it, "I had one of these barometers with me in my late southern voyage, and it never failed to prognosticate and give early notice of all the bad weather we had, so that I depended thereon, and made provision accordingly; and from my own experience I conclude, that a more useful contrivance hath not for this long time been offered for the benefit of navigation." Mr. Nairne, an ingenious artist in London, has lately invented a new kind of marine barometer; which differs from the common barometer by having the lower part of the tube, for about 2 feet long, made very small, to check the vibrations of the mercury, which would otherwise arise from the motions of the ship. This is also assisted by being hung in gimbals, by a part which subjects it to be the least affected by such motions. Mr. Caswell's Baroscope, or Barometer. This is described in the Philos. Trans. vol. 24. Suppose ABCD (fig. 19.) is a bucket of water, in which is the baroscope x rezy osm, which consists of a body arsm, and a tube eye, which are both concave cylinders, made of tin, or rather glass, and communicating with each other. The bottom of the tube 2 y has a leaden weight to sink it, so that the top of the body may just swim even with the surface of the water by the addition of some grain weights on the top. When the instrument is forced with its mouth downwards, the water ascends into the tube to the height yu. To the top is added a small concave cylinder, or pipe, to keep the instrument from sinking down to the bottom: md is a wire: and m S, de are two threads oblique to the surface of the water, which perform the office of diagonals for while the instrument sinks more or less by an alteration in the gravity of the air, where the surface of the water cuts the thread is formed a small bubble, which ascends up the thread while the mercury of the common baroscope ascends, and vice versa. It appears from a calculation which the author makes, that this instrument shows the alterations in the air 1200 times more accurately than the common barometer. He observes, that the bubble is seldom known to stand still even for a minute; that a small blast of wind, which cannot be heard in a chamber, will sensibly make it sink; and that a cloud passing over it always makes it descend, &c. While some have been increasing the sensibility of the barometer by enlarging the variations, others have endeavoured to make it more convenient by reducing the length of the tube. M..Amontons, in 1688, first proposed this alteration in the structure of barometers, by joining several tubes to one another, alter. nately filled with mercury and with air, or some other fluid; and the number of these tubes may be increased at pleasure: but the contrivance is perhaps more ingenious than useful. M. Mairan's reduced Barometer, which is. only 3 inches long, serves the purpose of a manometer, in showing the dilatations of the air in the receiver of an air-pump; and instruments of this kind are now commonly applied to this use. The Portable Barometer is so contrived that it may be carried from one place to another without being disordered. The end of the tube is tied up in a leathern bag not quite full of mercury; which being pressed by the air, forces the mercury into the tube, and keeps it suspended at its proper height. This bag is usually inclosed in a box, through the bottom of which passes a screw, by means of which the mercury may be forced up to the top of the tube, and prevented from breaking it by dashing against the top when the instrument is removed from one station to another. It seems Mr. Patrick first made a contrivance of this kind: but the portable barometer has received various improvements since; and the most complete of this kind has been described by M. De Lue, in his Recherches, vol. 2. p. 5. &c., together with the apparatus belonging to it, the method of construction and use, and the advantages attending it. Improvements have also been suggested by sir George Shuck burgh and colonel Roy, which have been carried into execution, with farther improvements also, by Mr. Ramsden, and other ingenious artists in London. Sir Henry Englefield has likewise much simplified the portable barometer, reducing its weight to less than a pound and a half. See Nich. Journal, No. 55.; Phil. Mag. No. 117.; or Retrospect of Discoveries, No. 13. Humbolt, the celebrated traveller, contrived a portable barometer, composed of several parts which could be easily put together, and which should correct itself at each observation like an astronomical instrument. Tralles of Berne has judiciously observed, that all baro. meters are liable to be broken when the tube is fixed to the scale which belongs to it. In Humbolt's barometer the tube is distinct from the scale; and when the tube breaks, its place can be supplied by another, even on the top of a mountain. The tube, the lower part of which is represented by a b c (fig. 3. pl. 23.) is cemented into an iron one be two inches long, terminating in a nut c, which receives a screw to the depth of 24 lines: the end of the screw is square. When the tube is filled with mercury, and screwed close, it is put into a tube of copper lined with flannel, and covered on the outside with leather. This case may be carried as a cane; the head d of the screw being kept, as far as possible, in a vertical position. If the entrance of air be apprehended, it will lodge itself under d. same to whatever depth it be plunged in the basin, provided care be taken each time to adjust the level by the cock. Muscati has proved, that the barometric height is affected by the size of the vacuum on account of the attraction of the glass in k, and because the minimum of air which the vacuum contains is more or less dilated. In Humbolt's barometer, the same Torricellian vacuum exists on the tops of mountains as in the valleys. The different picces of which this instrument consists are contained in one cylinder, which may be carried on the shoulder like a gun; while several tubes may be contained in a walking-stick. For other portable barometers consult De Luc's Recherches sur les Modifications de l'Atmosphere, and Transactions of the Royal Irish Acad. for 1787. The Phenomena and Observations of the Barometer. The phenomena of the barometer are various; but authors are not yet agreed upon the causes of them; nor is the use of it, as a weather-glass, yet perfectly ascertained, though daily observations and experience lead us still nearer to precision. Mr. Boyle observes, that the phenomena of the barometer are so precarious, that it is exceedingly difficult to form any certain general rules concerning the rise and fall of the mercury. Even that rule fails which seems to hold the most generally, viz. that the mercury is low in high winds. The best rules however that have been deduced by several authors are as follow. Dr. Halley's Rules for judging of the Weather. 1. In calin weather, when the air is inclined to rain, the mercury is commonly low. 2. In serene, good, and settled weather, the mercury is generally high. In that case turn the screw and pour a drop of the Torricellian vacuum depends on the obmercury upon it before it is shut. The whole server: the height of the mercury remains the tube may be examined to see that the mercury is not separated by air bubbles; an advantage wanting in those barometers in which the tubes are half concealed. The mahogany pillar egf (fig 4.) contains the mercury, to be poured into it after it has been screwed into its stand g h (fig. 5.), supported by three feet that fold over each other. The interior part of the column forms a hollow parallelopipedon, the square aperture of which is exactly equal in size to the square of the screw cd. In pushing the tube km a (fig. 6.) into this aperture until cd be above the level of the mercury, you turn the tube to the right, by laying hold of it with one hand between I'm. As the square cd cannot then turn, being held fast in the pillar, the screw is loosened, c d sinks down floating on the mercury, and the atmospheric air has free communication with the mercury in the tube. The Torricellian vacuum is formed under k, and the mass of mercury in the column is increased. The scale no (fig. 6.) is attached to a wooden rod nop, formed of two pieces of timber of a different kind, and screwed at p and q to the mahogany pillar. Turn the three screws rrr until the vertical position of the instrument is announced by the plummet s. If you fix the scale to the tube Ka, by screwing close the screws and m, there will be too much mercury in the pillar. The point of the barometer is placed in the aperture of the cock s, and you will have a constant level by opening that cock and suffering the mercury, which you must collect in a small vase, to flow down. In order that the ivory cock s may not open of itself, there is, at 7, a fork which by a screw can be fixed in each position to prevent it. When the observation is made, you place in ge a small cushion, of covering, held by two screws vv, which prevent the escape of the mercury while you are walking, or you dismount the instrument by again pushing the tube km (unscrewed at m and 7) to the bottom of the pillar f, where you turn the square c d. The Torricellian vacuum is by these means filled, and you turn the tube to the left, holding it with your hand between / and m until you observe that cd has caught, and that the aperture of the tube is closed. You then draw out the tabe with cd well shut in order to inclose it in its case. There is a loss of a few drops of mercury which adhere to the iron. One of these instruments, constructed two years ago, has remained unhurt during a great many journeys over the most rugged roads and the steepest mountains. More time is required to make an observation with it than with a common barometer; but it is the only one perhaps which could with certainty be used, were it to become deranged even in the deserts of Thibet. On coming from a cold to a warm temperature, the screw cd must be loosed before the instrument be brought near the fire. This instrument has an advantage by which it is distinguished from all others: in it the size of 3. Upon very great winis, though they be not accompanied with rain, the mercury sinks lowest of all, according to the point of the compass the wind blows from. 4. The greatest heights of the mercury are found upon easterly or north-easterly winds, other circumstances alike. 5. In calm frosty weather, the mercury commonly stands high. 6. After very great storms of wind, when the mercury has been very low, it generally rises again very fast. 7. The more northerly places have greater alterations of the barometer than the more southerly, near the equator. 8. Within the tropics, and near them, there is little or no variation of the barometer, in all weathers. For instance, at St. Helena it is little or nothing, at Jamaica 3-10ths of an inch, and at Naples the variation hardly ever exceeds an inch; whereas in England it amounts to 2 inches and a half, and at Petersburgh to 34d nearly. Dr. Beal, who followed the opinion of M. Pascal, observes, that, cæteris paribus, the mercury is higher in cold weather than in warın; and in the morning and evening usually higher than at mid-day: that in settled |