midnight, when it fell, and continued to fall, till at four A. M. it had descended as low as it had been at four in the after. noon ; from thitt period till seven or eight it continued rising, and at the latter hour it had attained the highest point of eleva tion. The gentleman who made these observations termed the elevation' and depressions now described equatropical motions, and asserts that they were re gularly performed while the barometers were on the sea, but they were seldom observed on a river, or when the instru ments were on shore. This circumstance leads us to concur with Dr. Thomson, in supposing that the singular fact is to be ascribed to the motion of the ship, "which, by regularly agitating the mer cury, might make its elevations and de pressions more sensible and correct than when thebarometercontinues stationary." The range of the barometer increases gra dually as the latitude advances towards the poles, till in the end it amounts to two or three inches. The following table, composed by the writer just cited, will explain the gradual increase alluded to, which be compiled from the best au thorities.
The range ofthe barometer is consider ably less in North America than in the corresponding latitudes of Europe, par ticularly in Virginia, where it never ex ceeds 1.1. The range is more considera ble at the level of the sea than on moun tains, and in the same degree of latitude it is in the inverse ratio of the height of the place above the level of the sea.
M. Cotte composed a table, which has been published in the Journal de Phy sique, from which it appears extremely probable, that the barometer has an in variable tendency to rise between the morning and the evening, and that this impulse is most considerable from two in the afternoon till nine at night, when the greatest elevation is accomplished; but the elevation at nine differs from that at two by four-twelfths, while that of two varies from the elevation of the morning only by one-twelfth, and that in particular climates the greatest elevation is at two o'clock. The observations of M. Cotte confirm those of Mr. Luke Howard, and from them it is concluded, that the baro meter is influenced by some depressing cause at new, and full moon, and that some other makes it rise at the quarters. This coincidence is most considerable in fair and calm weather; the depression in the interval between the quarters and con junctions amounts to one-tenth of an inch, and the rise from the conjunctions to the quarters is to the same amount.
The range of this instrument is found to be greater in winter than in summer ; for instance, the mean at York, during the months from October to March inclusive, in the year 1774, was 1.42, and in the six
summer months 1.016.
The more serene and settled the wea ther is, the higher the barometer ranges, calm weather with a tendency to rain de presses it, high winds have a similar ef fect on it, and the greatest elevation oc curs with easterly and northerly winds, but the south produces a directly contra ry effect. According to the Asiatic Re searches, it is always observed to be high est with north and north-west winds, and the reverse when the south-east prevails; it falls rapidly previous to violent tem pests, and is greatly agitated while they continue. It has been remarked by Mr. Copland, in the Transactions of the Socie ty of Manchester, that" a high barometer is attended with a temperature above, and a low barometer with one below, the monthly mean." Various but almost al together unsuccessfulattempts have been made to explain the phenomena we 'have enumerated; that of 14. Kirwan carries considerable plausibility, though it is not considered quite satisfactory. In order that his ideas on the subject may be clear ly understood, we shall give what may be considered an abstract of his theory, im. proved by Dr. Thompson. The density of the atmosphere is evidently greatest at the poles, and least at the equator, as the centrifugal force at the latter, the distance from the centre of the earth, and the heat, all contributing to lessen the density of the air, are at their maximum, when at the pole it is exactly the reverse. In every part of the world the mean height of the barometer placed at the level of the sea will be found to be 30 inches, consequent ly the weight of the atmosphere is the same in afl places: its weight depending on its density and height; where the for mer is greatest the height must be the least, and where its density is least the height is the greatest. Arguing from these facts, it will, therefore, appear that the height of the atmosphere must be least at the poles, and greatest at the equator, decreasing ,gradually in the in terval, and thus forming the resemblance of two inclined planes, meeting at the highest part above the equator.
The difference of the mean heat be tween the pole and the equator, when the sun is in our hemisphere in the summer, does not vary so much as in the winter, as the heat at that period in northern coun tries equals that oftbe torrid zone ; hence the thermometer rises to 85° in Russia during the months of July and August; of necessity the rarity of the atmosphere and its height increases ; in consequence, the upper part in the northern hemisphere inclines less, but that of the southern, from different causes, must be much more inclined; during our winter the exact re verse takes place.