Читать книгу A Voyage of Discovery to the Southern Hemisphere - François Péron - Страница 8

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Meteorological Observations—State of the Thermometer—State of the Hygrometer—Of the Barometer—Agreement between them—The Winds—Their Agreement with the Barometer—Atmospheric Phenomena—General Results.

THE meteorological observations were made with the thermometers of Dolland and Mossy; barometers made by the last mentioned artist, and hygrometers by Richer. To compare them as exactly with each other as possible, I made it an invariable rule from the beginning of our voyage to take an observation four times each day, at the hours most opposite; that is to say, at six in the morning and at six in the evening, at midnight, and at twelve at noon in the open air, and on the poop of our vessel, and as often besides as I conveniently could. This first series of my labours furnished me with the the following general results:

1st, The temperature of the thermometer rose progressively as we drew near the equator.

2d, It sunk progressively as we went from the equator.

3d, The proportion of its rising and sinking was not equal in both hemispheres, the mean degree of the heat in the southern hemisphere having been weaker than in the correspondent northern latitudes.

4th, In other respects, every thing being as usual, the temperature of the atmosphere between the tropics is weaker in open sea than in the interior of the continents or even oil the islands. We did not really experience more than 23° of heat under the line, and the mean degree is much below this last point.

5th, The variations of the temperature become weaker, and not so frequent the nearer we approach the equator, and vice versa.

6th, Not only the variations of the temperature are more inconsiderable from one day, or even from one month to another, between the tropics, and in open sea, but also, in general, there is but little difference between the temperature of the day taken in the shade at noon, and that of the night at midnight: thus, forty observations of this sort, taken from the 22d of November to the 1st of December,1800, produced me a sum total of 909, 6° of heat: noon produced 233, and midnight 222, 7, which makes scarcely one degree of difference for each day; a phenomenon so much the more remarkable, as it is known from the experiments of Miller, Beze, Pison, Halley, Lister, &c. that the difference of heat in the day and night is greater in the equinoctial regions, on shore, and that we ourselves should have occasion to draw conclusions from these facts suited to the objects of our enquiries.

We shall now proceed to describe the state of the hygrometer, being the first time that this valuable instrument crossed the seas: it was thought beforehand that its observation would furnish many important results; and we shall see that these expectations were well founded.

7th, Every thing in other respects being as usual, the it hygrometer shews a proportion of humidity so much the stronger, the nearer we approach the equator.

8th, The absolute proportion of the atmospheric humidity becomes so much the less, the nearer we approach either of the poles.

9th, The variations of the hygrometer are so much the weaker and less frequent, as we observe this instrument nearer the equator, and vice versa.

10th, The hygrometer, in the midst of the equatorial legions, remains almost invariably in a state Of extreme saturation. We proceed next to the state of the barometer.

11th, The barometer in general sinks the more we approach the equator.

12th, It rises progressively as we go farther from the equator.

13th, The variations of the barometer are neither frequent nor striking, when this instrument is observed near the equator, and vice versa.

14th, The equality of level of the place where the observation is made, the distance or absence of every cause of perturbation foreign to the atmosphere, gives to the barometer at sea, a more regular action, more easily admitting of comparison in its variations than it would do in the midst of a continent. That instrument in this respect is eminently useful to mariners, and our own experience leaves us no doubt on this head.

On the subject of the agreement between the hygrometer and the barometer we observe:

15th, That the variations of the barometer have an incontestible agreement with those of the hygrometer.

16th, The falling of the mercury corresponds in the greatest number of cases with the increase of humidity. It appears as much more considerable as the other becomes greater.

17th, The rising of the quicksilver almost always corresponds with the diminution of the humidity of the atmosphere; it is as much greater (other circumstances not interfering); as that diminution is marked more considerably by the hygrometer.

18th, The winds become by degrees lighter and more constant, as the navigator approaches the equinoctial regions, and vice versa.

The agreement of the wind with the barometer is as follows:

19th, The winds appear to exercise a real action, altogether independent of the variations of the barometer, for I have noticed frequently that the mercury fell almost suddenly 3, 4, 5, 6, or even 8 lines, in spite of the rapid diminution of, the humidity of the atmosphere; a circumstance which, after the two exclusive theories of Deluc, ought to have occasioned, on the contrary, the ascension of the mercury.

20th, This effect of the winds upon the barometer (other circumstances not interfering) appeared to me to be generally in a ratio composed of their least temperature, and of their celerity.

On the atmospheric phenomena I made the following observations:

21st, The vaporous appearance of the sky, to be observed towards the middle of the day, in all the seas of the equinoctial regions, and denominated by navigators, grey sky, thick horizon, vaporous horizon, &c. &c.; the astonishing splendour of the rising and setting sun in the same climates; the serenity of the sky during the night, so strongly contrasted with the vaporous state of the atmosphere during the day; the frequent, and almost instantaneous formation of the menacing clouds, described by so many voyagers; of the tempests known by the name of equinoctial squalls; the prodigious power of the humidity, from which it is almost impossible to preserve any thing, however valuable; the abundance of the rains, and the size of the drops which fall; all these phenomena, inexplicable even to this day in equatorial meteorology, appear to me to depend almost entirely on the hygrometrical state of the atmosphere in these parallels of latitude; and the theory of atmospheric refractions seems to me to depend in an important degree on such observations.

The general results of the above observations are:

22d, If we add to our own observations those of M. de Humboldt, on the diminution of intensity of the magnetic force towards the equator, it follows that all the great physical phenomena experience modifications, the more important in proportion as we approach towards that point: thus, the force of the weight, and the intensity of the magnetic virtue diminish; the barometer sinks, the thermometer rises; the action of the hygrometer tends to the point of saturation. The winds become lighter and more constant; the action of all the instruments becomes at the same time more regular, and their variations in consequence are less.