Page:Is Mars habitable - Wallace 1907.djvu/78

 of a total eclipse of the moon—less than two hours of complete darkness, and about twice that period of partial obscuration.

Dr. Boeddicker was unable to detect any appreciable heat at the period of greatest obscuration; but, owing to the extreme sensitiveness of the Bolometer, Mr. Very ascertained that those parts of the surface which had been longest in the shadow still emitted heat "to the amount of one per cent. of the heat to be expected from the full moon." This however is the amount of radiation measured by the Bolometer, and to get the temperature of the radiating surface we must apply Stefan's law of the 4th power. Hence the temperature of the moon's dark surface will be the $$^4\sqrt{\frac{1}{100}}=\frac{1}{3.2}$$ of the highest temperature (which we may take at the freezing-point, 491°F. abs.), or 154°F. abs., just below the liquefaction point of air. This is about 50° lower than the amount found by calculation from our most rapid radiation; and as this amount is produced in a few hours, it is not too much to expect that, when continued for more than two weeks (the lunar night), it might reach a temperature sufficient to liquefy hydrogen (60°F. abs.), or perhaps even below it.

Theory of the Moon's Origin.

This extremely rapid loss of heat by radiation, at first sight so improbable as to be almost incredible,