SECTION XXV.

ATMOSPHERE OF THE PLANETS AND THE MOON. CONSTITUTION OF THE SUN. ESTIMATION OF THE SUN'S LIGHT. — HIS INFLUENCE ON THE DIFFERENT PLANETS. TEMPERATURE OF SPACE.-INTERNAL HEAT OF THE EARTH.-ZONE OF CONSTANT TEMPERATURE. -HEAT INCREASES WITH THE DEPTH.-HEAT IN MINES AND WELLS. THERMAL SPRINGS. CENTRAL HEAT.-VOLCANIC ACTION. THE HEAT ABOVE THE ZONE OF CONSTANT TEMPERATURE ENTIRELY FROM THE SUN. THE QUANTITY OF HEAT ANNUALLY RECEIVED FROM THE SUN.ISOGEOTHERMAL LINES. -DISTRIBUTION OF HEAT ON THE EARTH. -CLIMATE.- LINE OF PERPETUAL CONGELATION. CAUSES AFFECTING CLIMATE. ISOTHERMAL LINES. EXCESSIVE CLIMATES. THE SAME QUANTITY OF HEAT ANNUALLY RECEIVED AND RADIATED BY THE EARTH.

THE ocean of light and heat perpetually flowing from the sun, must affect the bodies of the system very differently, on account of the varieties in their atmospheres, some of which appear to be very extensive and dense. According to the observations of Schröeter, the atmosphere of Ceres is more than 668 miles high, and that of Pallas has an elevation of 465 miles. These must refract the light and prevent the radiation of heat like our own. But it is remarkable that not a trace of atmosphere can be perceived in Vesta; and that Jupiter and Saturn have very little. The action of the sun's rays must be very different on these bodies from what it is on the earth, and the heat imparted to them quickly lost by radiation; yet it is impossible to estimate their temperature, since the cold may be counteracted by their central heat, if, as there is reason to presume, they have originally been in a state of fusion, possibly of vapour. The attraction of the earth has probably deprived the moon of hers; for the refractive power

of the air at the surface of the earth, is at least a thousand times as great as refraction at the surface of the moon. The lunar atmosphere, therefore, must be of a greater degree of rarity than can be produced by our best air-pumps; consequently no terrestrial animal could exist in it. This was confirmed by M. Arago's observations during the last great solar eclipse, when no trace of a lunar atmosphere was to be seen.

The sun has a very dense atmosphere. What his body may be, it is impossible to conjecture; but he seems to be surrounded by a mottled ocean of flame, through which his dark nucleus appears like black spots often of enormous size. These spots are almost always comprised within a zone of the sun's surface, whose breadth measured on a solar meridian, does not extend beyond 30° on each side of his equator, though they have been seen at the distance of 3910. From their extensive and rapid changes, there is every reason to suppose that the exterior and incandescent part of the sun is gaseous. The solar rays probably arising from chemical processes that continually take place at his surface, or from electricity, are transmitted through space in all directions; but notwithstanding the sun's magnitude, and the inconceivable heat that must exist at his surface, as the intensity both of his light and heat diminishes as the square of the distance increases, his kindly influence can hardly be felt at the boundaries of our system, or at all events it must be but feeble.

The direct light of the sun has been estimated to be equal to that of 5563 wax candles of moderate size, supposed to be placed at the distance of one foot from the object. That of the moon is probably only equal to the light of one candle at the distance of twelve feet. Consequently the light of the sun is more than three hundred thousand times greater than that of the moon. Hence the light of the moon imparts no heat. Professor Forbes is

convinced by recent experiments that the direct light of the moon is incapable of raising a thermometer one threehundred-thousandth part of a centigrade degree, at least in this climate. The intensity of the sun's light diminishes from the centre to the circumference of the solar disc; but in the moon the gradation is reversed.

In Uranus, the sun must be seen like a small but brilliant star, not above the hundred and fiftieth part so bright as he appears to us; but that is 2000 times brighter than our moon; so that he is really a sun to Uranus, and may impart some degree of warmth. But if we consider that water would not remain fluid in any part of Mars, even at his equator, and that in the temperate zones of the same planet even alcohol and quicksilver would freeze, we may form some idea of the cold that must reign in Uranus.

The climate of Venus more nearly resembles that of the earth, though, excepting perhaps at her poles, much too hot for animal and vegetable life as they exist here; but in Mercury, the mean heat arising only from the intensity of the sun's rays must be above that of boiling quicksilver, and water would boil even at his poles. Thus the planets, though kindred with the earth iǹ motion and structure, are totally unfit for the habitation of such a being as man, unless, indeed, their temperature should be modified by circumstances of which we are not aware, and which may increase or diminish the sensible heat so as to render them habitable.

It is found by experience, that heat is developed in opaque and translucent substances by their absorption of solar light, but that the sun's rays do not sensibly alter the température of perfectly transparent bodies through which they pass. As the temperature of the pellucid planetary space can be but little affected by the passage of the sun's light and heat, neither can it be sensibly raised by the heat now radiated from the earth; consequently its temperature

must be invariable, at least throughout the extent of the solar system. The atmosphere, on the contrary, gradually increasing in density towards the surface of the earth, becomes less pellucid, and therefore gradually increases in temperature, both from the direct action of the sun, and from the radiation of the earth. Lambert had proved that the capacity of the atmosphere for heat varies according to the same law with its capacity for absorbing a ray of light passing through it from the zenith, whence M. Svanberg found that the temperature of space is 58° below the zero point of Fahrenheit's thermometer. From other researches, founded upon the rate and quantity of atmospheric refraction, he obtained a result which only differs from the preceding by half a degree. M. Fourier has arrived at nearly the same conclusion from the law of the radiation of the heat of the terrestrial spheroid, on the hypothesis of its having nearly attained its limit of temperature in cooling down from its supposed primitive state of fusion. The difference in the result of these three methods, totally independent of one another, only amounts to the fraction of a degree.

The cold endured by Sir Edward Parry one day in Melville Island was 55° below zero; and that suffered by Captain Back on the 17th of January, 1834, in 62° 46′ of north latitude, was no less than 70° below the same point. However, M. Poisson attributes this to accidental circumstances, and by a recent computation, he makes the temperature of space to be 8° above the zero of Fahrenheit. This he considers greatly to exceed the temperature of the exterior strata of the atmosphere, which he conceives to be deprived of their elasticity by intense cold, and he thus accounts for the decrease of temperature at great elevations, and for the limited extent of the atmosphere.

maintains the ethereal medium at a higher temperature than it would otherwise have, and must eventually increase it, but by a quantity so evanescent that it is hardly possible to conceive a time when a change will become perceptible.

The temperature of space being so low, it becomes a matter of no small interest to ascertain whether the earth may not be ultimately reduced by radiation to the temperature of the surrounding medium; what the sources of heat are; and whether they be sufficient to compensate the loss, and to maintain the earth in a state fit for the support of animal and vegetable life in time to come. All observations that have been made under the surface of the ground concur in proving, that there is a stratum at the depth of from 40 to 100 feet throughout the whole earth, where the temperature is invariable at all times and seasons, and which differs but little from the mean annual temperature of the country above. According to M. Boussingault, indeed, that stratum at the equator is at the depth of little more than a foot in places sheltered from the direct rays of the sun; but in our climates it is at a much greater depth. In the course of more than half a century, the temperature of the earth at the depth of 90 feet in the caves of the Observatory at Paris, has never been above or below 53° of Fahrenheit's thermometer, which is only 2° above the mean annual temperature at Paris. This zone, unaffected by the sun's rays from above, or by the internal heat from below, serves as an origin whence the effects of the external heat are estimated on one side, and the internal temperature of the globe on the other.

As early as the year 1740, M. Gensanne discovered in the lead mines of Geromagny, three leagues from Béfort, that the heat of the ground increases with the depth below the zone of constant temperature. A vast number of observations have been made since that time in the mines of Europe and America, by MM. Saussure, Daubuisson,