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Uranus, and probably imparts 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, though it cannot exceed that of the surrounding space.
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 alter the temperature of perfectly transparent bodies through which they pass. As the temperature of the pellucid planetary space cannot be affected by the passage of the sun's light and heat, neither can it be raised by the heat radiated from the earth, consequently its temperature must be invariable. 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; and 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. Thus, as the temperature of space is uniform, it follows that no part of Uranus can experience more than 90° of cold, which only exceeds that which Sir Edward Parry suffered during one day at Melville Island, by 3o.
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 in motion and structure, are totally unfit for the habitation of such a being as man.
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; for which reason the light of the moon either imparts no heat, or it is too feeble to penetrate the glass of the thermometer, even when brought to a focus by a mirror. 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.
Much has been done within a few years to ascertain the manner in which heat is distributed over the surface of our planet, and the variations of climate; which in a general view mean every change of the atmosphere, such as of temperature, humidity, variations of barometric pressure, purity of air, the serenity of the heavens, the effects of winds, and electric tension. Temperature depends upon the property which all bodies possess, more or less, of perpetually absorbing and emitting or radiating heat. When the interchange is equal, the temperature of a body remains the same; but when the radiation exceeds the absorption, it becomes colder, and vice versa. But in order to
determine the distribution of heat over the surface of the earth, it is necessary to find a standard by which the temperature in different latitudes may be compared. For that purpose it is requisite to ascertain by experiment the mean temperature of the day, of the month, and of the year, at as many places as possible throughout the earth. The annual average temperature may be found by adding the mean temperatures of all the months in the year, and dividing the sum by twelve. The average of ten or fifteen years will give it with tolerable accuracy; for although the temperature in any place may be subject to very great variations, yet it never deviates more than a few degrees from its mean state, which consequently offers good standard of comparison.
If climate depended solely upon the heat of the sun, all places having the same latitude would have the same mean annual temperature. The motion of the sun in the ecliptic, indeed, occasions perpetual variations in the length of the day, and in the direction of the rays with regard to the earth; yet, as the cause is periodic, the mean annual temperature from the sun's motion alone must be constant in each parallel of latitude. For it is evident that the accumulation of heat in the long days of summer, which is but little diminished by radiation during the short nights, is balanced
by the small quantity of heat received during the short days in winter and its radiation in the long frosty and clear nights. In fact, if the globe were everywhere on a level with the surface of the sea, and also of the same substance, so as to absorb heat equally, and radiate the same, the mean heat of the sun would be regularly distributed over its surface in zones of equal annual temperature parallel to the equator, from which it would decrease to each pole as the square of the cosine of the latitude; and its quantity would only depend upon the altitudes of the sun, atmospheric currents, and the internal heat of the earth evinced by the vast number of volcanos and hot springs, in every region from the equator to the polar circles, which has probably been cooling down to its present state for thousands of ages. The distribution of heat, however, in the same parallel is very irregular in all latitudes, except between the tropics, where the isothermal lines, or the lines passing through places of equal mean annual temperature, are parallel to the equator. The causes of disturbance are very numerous; but such as have the greatest influence, according to Humboldt, to whom we are indebted for the greater part of what is known on the subject, are the elevation of the continents, the distribution of land and water over the surface of the globe, exposing different absorb