elevations and depressions contributing, as they proceed, but a moderate proportion to each day's tide: whercas, in the atmosphere of these latitudes, where the weekly elevations and depressions go to so large an extent, the diurnal tide is scarce perceptible. This objection was attempted to be met by some reasons, founded on the very different physical constitutions of the air and ocean; the latter being pretty uniform in density and composition, while the former is variable in these respects, and subject moreover, by the rapid changes of temperature which it undergoes, to currents which move in different directions with much greater freedom than in the ocean. It was also stated that at Calcutta, where the weekly variation, as in low latitudes generally, is very small, a daily tide had been distinctly traced by the alternate elevations and depressions of the Barometer. I have also since the date of this essay, met with strong indications of a daily tide in Registers of a much higher latitude. This paper had in substance the following conclusion: "It will be soon enough, however, to enter upon the Theory of the Atmospherical tides, when the facts shall have been examined, aud the influence of the Sun and Moon on the atmosphere established by more extensive observations. For this purpose the subject is now brought forward, and the co-operation of observers in this or other countries is requested. The coincidence, as far as hitherto observed is an important fact, and should it be found to obtain generally, will lead to important consequences, and in the first place to a new and more satisfactory theory of the Barometer. The true reason, likewise, of the weather so frequently agreeing in its changes with those of the Moon (a coincidence which has long served to direct the predictions of the Almanac makers) will be apparent; and the meteorologist will avail himself of this, to form probable conjectures on the changes likely to arise for a certain time, not exceeding that which limits the operation of the known cause or causes." Such was the state of iny information on this subject twenty years ago. The study of the modifications of clouds, and the various phenomena connected with them, afterwards occupying my attention, this particular enquiry was suspended: and when, in 1806, I began a regular Meteorological Journal, it was with more general views. But being still desirous of putting to the test the opinion of a Lunar tide in the Atmosphere, I was induced, as already mentioned, to digest the observations in the form which seemed to afford the greatest facility for this purpose. It remains to show how far the purpose was fulfilled. Plate 4 exhibits, in a system of curves, the variation of the daily mean height of the Barometer through the Solar year 1806-7. These curves are constructed from the Tables, 2 to 14 inclusive, in vol. 1, but with a different arrangement of the periods. For the sake of shewing more evidently the influence of New and Full Moon, the periods are here made to begin with the day of the third quarter; which happens in this instance to be the first day of 1807. After a dotted curve therefore, giving the variation from Dec. 22, 1806 to the end of that year, the several curves a—b, b-c, &c. carry on a series of entire Lunar periods to the 21st of Dec. 1807, at n, where the Solar year closes. To construct these curves, the mean of the period is first ascertained by calculation, and represented by a horizontal line. The relation of the mean of each day to this standard line is next ascertained and marked 1; and a curve carried through the points thus found, represents the variation, at its proper extent, above and below the standard. In doing this, the mean of the day on which a Lunar phase happens, is made always to fall in the intersection of the curve with a perpendicular line appropriated to that phase Some inequalities of time in the intermediate parts of the curve, occasioned by this arrangement, are remedied, where needful, by using an unequal scale of time in those parts. By this method the curves were all obtained of an equal length, and presenting equably the relation of the Lunar points to the Barometrical mean for the period. Their tendency to rise and fall at particular intervals, and their consent or opposition in such movements, was thus also represented independently of the absolute place of the mean of the period, or of that of each day, in the Barometrical scale. Each of the four horizontal lines, on which the curves are made to play, has therefore an elevation peculiar to itself, and relative only to the curve in connexion with which it is viewed: its absolute place in the scale of the Barometer may be gathered from the small curve at the bottom of the diagram; where these monthly means are laid down upon the mean of this Solar year, which is 29.815 inches. All this contrivance was needful in order to exhibit the distinct effect of each Lunar position, unmixed with that kind of variation, from month to month, in the mean of the Barometer, which depends on the season of the year; and of which an account has been given page 150-152, founded, it will be recollected, on a mean of several years. To proceed now to the application-it is difficult not to be struck at first sight with the evident marks of system, which these curves exhibit, from the beginning to the end of the series. Were it possible to obtain, at successive equal intervals of time, the profile of the waves which roll after each other on the surface of the ocean, and were we to reduce these to a scale in like manner, it is not to be doubted that the group would present elevations and depressions indifferently, in all parts of the scale of time; and the intersections of the curves would soon produce confusion in the picture. But it is not so here-the wave occurs too often in the same place; and the intermediate depressions are too regular, for us to admit, that what is called chance has any considerable share in producing them. The most prominent feature of the piece may be said to be, the nearly constant elevation of the curve at the approach of Full Moon—a very contrary result, certainly, to that found in the year 1798, and sufficient, at first view, to invalidate the partial conclusion I then came to, that the true atmospherical tide consisted, in part, of large depressions at this quarter. These ele`vations, however, will be found to have their apex, for the most part about two days before the Full, and to be going off at the time of the phasis. That they are properly connected with its approach, may be fairly inferred, from the manner in which the curve No. 2 rises at this time from a great depression, as if prevented from taking an upward tendency by some unusual cause, and become more elastic in consequence of being thus strongly bent downward. If we now turn to the New Moon, on the left of the plate, we perceive its approach marked, by depressions chiefly in the fore part of the year, and by ele |