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In May 1722, Vauvenargues having been superseded, the dispossessed proprietors pierced, under ground, the works which had been constructed the year before, and immediately the warm springs of the town were seen to diminish, and even entirely dry up.

In July 1722 the breaches were filled up by the vigilance of the authorities, and the inhabitants of Aix saw the waters reappear. Things continued in this state for five years; but in 1727 the tenants of the mills of Barret clandestinely made a new opening in the dam constructed in 1722. The knowledge of this misdeed was only acquired by a falling off in the quantity of water. In order to terminate this obstinate contest between private interest and the general benefit, the town passed an act defining the property, and caused a stone pyramid to be erected upon it, in 1729.

To these details, which we have entered into in order to establish the fact, that the waters of the pyramid of Barret feed the warm springs of the town of Aix, we shall add, that M. Dauphin, locksmith, assured M. Robert, a physician of Marseilles, in 1812, that he witnessed an experiment which places the matter beyond a doubt: he stated, that lime having been mixed with the water in the basin of the pyramid, the springs of Cours and of Mennes became milky.

Under the pyramid of Barret, the basin which the water occupies is also of stone; it is about 113 feet long and upwards of 63 feet broad. In June 1812, M. Robert sent down two men to ascertain the temperature of the water; they found it 62-6° Fahr. At the same period, the baths of Sextius were at 84.2° Fahr.

It appears, therefore, established, that the cold waters of Barret become, at least the greater part, the warm waters of Aix, by traversing the short space which separates these two points,—that is to say, a horizontal distance, estimated in the official memoirs from which we have given an extract, at about a thousand geometrical paces*.

It will be observed that we have employed the words the greater part, and they, in fact, indicate precisely the question which remains to be answered. If it could be proved that all the warm water of the baths of Sextius originated from the cold water of the basin of Barret,—that the phenomenon does not consist merely of an intermixture which may take place near the surface, between the water of Barret and that of an ordinary thermal spring nearer Aix,—and that in its passage the fluid does not become chemically charged with any foreign substance,-the theory of thermal springs would have made a decided step in its progress. Every one would then be satisfied of their similarity to the sources of Artesian wells, the high temperature of which is evidently owing to the great depth from which they issue.

Without pretending to devise the best means of investigation which the survey of the places might suggest, I conceive that if permission were obtained to withdraw the waters of Barret, for a few days only, the principal question would be solved. From the time that the intermediate thermal spring between Barret and Aix should begin to flow to Sextius alone, there would be, simultaneously, a considerable diminution of the

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quantity of water, and a considerable increase in the temperature of the baths. A comparative chemical analysis of the respective waters, if performed with that scrupulous accuracy of which we have now many examples, would be very interesting. Nor should it be forgotten to repeat the experiment mentioned by the locksmith Dauphin, by employing lime, or bran, or some tinctorial matter, were it only for determining the velocity of the fluid in the subterranean passages which it traverses, in passing from Barret to Sextius.

The temporary turning-off of the waters of Barret, is the most decisive mode of obtaining the solution of that very ancient problem of physical geography to which thermal springs have given rise; but should this deviation be impossible, there still seems to be a method of attaining the object. The waters of Sextius are said to diminish in dry, and to increase in rainy, weather. It is very improbable that the increase and decrease should follow exactly and simultaneously the same relations in the cold, and nearly superficial, waters of Barret, and in those of the thermal spring nearer the town. If a mixture of these waters does take place, we ought, therefore, to expect, that great variations of temperature would be observed at Sextius.

It may be seen, by this single instance, how much the authorities have erred in suppressing the office of Inspector of Thermal Waters, under the idea that nothing in that department remains to be discovered. I now add, in conclusion, that the data on which my plan of experiment is founded, have been derived from a manuscript memoir presented fifteen years ago to the Academy by M. Robert, which has not, in my opinion, met with the attention which it deserves.

MEAN HEIGHT OF THE BAROMETER.-A few years ago a positive denial would have been given to the assertion, that there is any permanent difference between the barometrical heights corresponding to different regions of the globe at the level of the sea. At present such differences are regarded as not only possible, but even probable. The officers of the Bonite ought therefore to preserve their barometers with the most scrupulous care in such excellent order, that all their observations, made in every port, may be compared. Notice should never fail to be made of the exact height of the cistern of the barometer above the level of the sea.

OF THE INFLUENCE OF DIFFERENT WINDS ON THE HEIGHTS OF THE BAROMETER.—As soon after the memorable discovery of Torricelli as meteorologists directed their attention to the observation of the barometer, they perceived that, in general, certain winds produced a rapid ascent of the mercurial column, while the opposite ones produced a contrary effect, in a manner as equally decided. The difficulty was, to determine the numerical value of these influences. It was necessary, in order to eliminate entirely all transient and accidental influences, and to obtain the true measure of permanent causes, to operate upon great numbers; it was necessary to obtain long series of good observations made in the same locality; it was necessary to group the winds according to their precise directions; and, finally, to separate effects purely thermometrical.

Burckhardt undertook this labour, availing himself of twenty-seven years of observations which Messier had made at Paris, from 1773 to

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1801. If we designate by the letter H. the mean height of the barometer at Paris, that is to say, the height determined by the average of all the observations, the means corresponding to the different winds, according to Burckhardt's calculations, will be as follows:

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It will be seen, from the mere inspection of this table, that the direction of the wind occasions a variation in the state of the barometer at Paris of 3mm, 1 (1·23 Eng. in.) above the mean, and of 2mm, 6 (1·03 Eng. in.) below it, forming a total variation of 5mm, 7 (2.26 Eng. in.); and that the opposite winds, combined two by two, give a mean height which, in extreme cases, scarcely differs by half a millimetre (0·19 Eng. in.) from the mean of all the observations.

M. Bouvard has presented to the Academy the results of an investigation analogous to that of Burckhardt; it is founded on the observations of the barometer made at the Observatory of Paris from 1816 to 1831, and leads, in general, to the same conclusions. By assigning to the letter H. the signification which we gave it in the preceding table, we shall have the following barometrical heights, corresponding to the different directions of the winds:

mm. Eng. In. Obs.

South H. minus 3, 7 (1·46). (2944) | North

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South-west

3, 0 (1.18)

(2847) North-east

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0, 8 (0.31) (3402) East
plus 2, 0 (0.78). (1533) | South-east

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mm. Eng. In. Obs.

H. plus 3, 2 (1·27). (2140)

The daily observations at nine o'clock in the

+ 3, 2 (1.27). (1390) + 1, 7 (0·67). (1248) minus 1, 7 (0·67). ( 890)

morning, at mid-day,

and at three in the afternoon, have all concurred in the formation of these numbers. Almost exactly the same results will be obtained by employing only the maxima heights of nine o'clock, and the minima heights of three o'clock.

In this instance, as well as in the table of Burckhardt, half the sums of the heights corresponding to the opposite winds are nearly equal to H., that is to say, to the total mean. The highest mean effect of the wind is 6mm, 9 (2·73 Eng. in.), which surpasses the result afforded by the observations of Messier by 1m, 2 (0·47 Eng. in.)

Both these tables tend to establish a fact with which meteorologists cannot be too strongly impressed,—that in order to obtain in our climates the mean height of the barometer, it is indispensable to admit into the calculation an equal number of observations corresponding to winds of opposite directions.

The tables which we have just transcribed, suggest many scientific questions; they lead us to inquire in what manner this influence of winds on the atmospheric pressure, varies with the position of places, with their greater or less distance from the sea, with their latitude, &c. In the mean time, till data sufficiently numerous be obtained to enable us to attempt the solution of these various meteorological problems, I shall here present to the reader the results of two series of very accurate observations, which were communicated to the Academy by MM. Schuster and Gambart.

The first were made at the School of Artillery and Engineers at Metz, the others at the Observatory of Marseilles.

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The difference between the extremes is sensibly less than in the observations at Paris. At the same time. it would be premature to draw general conclusions from this fact, which may perhaps be purely accidental.

The following seems more decisive:—

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Although this table is incomplete, and founded on observations of only five years' continuance, and although the north and north-east winds are entirely omitted, there results from it no less important a consequence than this, that if the direction of the winds exercises, at Marseilles, any influence on barometrical heights, that influence is very slight, and ought not always, in the case of winds of similar denominations, to have the same sign as in the north of France. Thus, while at Paris the southwest wind depresses the barometer considerably below the mean, its influence at Marseilles is positive; on the other hand, the north-west wind, which causes a considerable rise in the barometer at Paris, is that which produces the lowest depression at Marseilles.

When observations such as these have been made at many different places, they will probably place meteorologists in a condition to explain a phenomenon which has hitherto baffled all their efforts.

OF THE DIURNAL VARIATIONS OF THE BAROMETER.-Numerous memoirs have been published on the diurnal variation of the barometer. This phenomenon has been studied from the equator to the regions in the vicinity of the pole,—at the level of the sea,-on the immense plateaus of America,- —on the insulated summits of the highest mountains, and the cause, notwithstanding, remains in obscurity. It is still necessary, therefore, to multiply observations on the subject. In our climates, the vicinity of the sea appears to manifest itself by a sensible diminution in the extent of the. diurnal oscillation; does the same thing take place between the tropics?

OBSERVATIONS ON RAIN.-Navigators occasionally speak of rains which fall on their vessels while traversing the equinoctial regions, in terms which would lead us to suppose that it rains much more abundantly at sea than on land. But the truth still remains in the domain of mere conjecture; so seldom has the trouble been taken to procure exact measurements. These measurements, however, are by no means difficult. Captain Tuckey, for example, made many during his unfortunate expeVOL. II.

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dition to the river Zaïre, or Congo. We know that the Bonite will be provided with a small udometer (rain-gauge.) It seems, therefore, expedient to recommend the commander to cause it to be placed on the stern of the vessel, in such a situation that it can neither receive the rain collected by the sails, nor that which falls from the cordage.

Navigators would add greatly to the interest of these observations, if they would observe at the same time the temperature of the rain, and the height from which it falls.

In order to obtain the temperature of rain with some degree of accuracy, it is necessary that the mass of the water should be considerable, relatively to the size of the vessel which contains it. A metal udometer will not answer for this purpose. It would be infinitely preferable to take a large funnel of some light stuff, very close in its texture, and to receive the water which runs from the bottom in a glass, whose sides are thin, and which contains a small thermometer. So much for the temperature. The elevation of the clouds in which the rain is formed cannot be determined but during the time of a storm; then, the number of seconds which elapse between the appearance of the flash and the arrival of the sound, multiplied by 1142-the velocity with which sound is propagated-gives the length of the hypothenuse of a right-angled triangle, whose vertical side is precisely the height required. This height may be calculated, if, by means of a reflecting instrument, we obtain the angle formed with the horizon by a line which, passing from the eye of the observer, terminates in that quarter of the cloud where the lightning first showed itself.

Let us suppose, for an instant, that there falls on the vessel rain whose temperature is below that which the clouds should possess, according to their height, and the known rate of the decrease of atmospheric heat; every one will understand the consequences which such a result would produce in meteorology.

Let us suppose, on the other hand, that during a day of hail (for it hails in the open sea), the same system of observations had proved that hail-stones were formed in a region where the atmospheric temperature was higher than the point at which water congeals,-science would thus be enriched with a valuable result, which every future theory of hail must necessarily account for.

We could adduce many other considerations to demonstrate the utility of the observations we have proposed; but the two preceding must suffice.

RAIN IN A PERFECTLY CLEAR SKY.-There are some extraordinary phenomena, concerning which science possesses but few observations; and for the reason, that those who have had the opportunity of witnessing them avoid describing them, from an apprehension that they might be regarded as undiscerning visionaries. In the number of these phenomena we may rank certain rains of the equinoctial regions.

Sometimes it rains between the tropics when the atmosphere is perfectly pure, and the sky of the most beautiful azure! The drops are not very numerous, but they are larger than the greatest rain-drops in our climates. The fact is certain; we have the evidence of M. von Hum

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