falling well within the scope of observation. This Professor Langley’s experiments (when completely worked out) will, by their unequalled precision, enable him to determine with some approach to finality. Pouillet valued the “solar constant” at 1·7 “calories”; in other works, had calculated that, our atmosphere being supposed removed, vertical sunbeams would have power to heat in each minute of time, by one degree centigrade, 1·7 gramme of water for each square centimetre of the earth’s surface. This estimate was raised by Crova to 2·3, and by Violle in 1877 to 2·5;[19] Professor Langley’s new data bring it up (approximately as yet) to three calories per square centimetre per minute. This result alone would, by its supreme importance to meteorology, amply repay the labors of the Mount Whitney expedition. Still more unexpected is the answer supplied to the question: Were the earth wholly denuded of its aëriform covering, what would be the temperature of its surface? We are informed in reply that it would be _at the outside_ 50 degrees of Fahrenheit below zero, or 82 of frost. So that mercury would remain solid even when exposed to the rays—undiminished by atmospheric absorption—of a tropical sun at noon.[20] The paradoxical aspect of this conclusion—a perfectly legitimate and reliable one—disappears when it is remembered that under the imagined circumstances there would be absolutely nothing to hinder radiation into the frigid depths of space, and that the solar rays would, consequently, find abundant employment in maintaining a difference of 189 degrees[21] between the temperature of the mercury and that of its environment. What we may with perfect accuracy call the _clothing function_ of our atmosphere is thus vividly brought home to us; for it protects the teeming surface of our planet against the cold of space exactly in the same way as, and much more effectually than, a lady’s sealskin mantle keeps her warm in frosty weather. That is to say, it impedes radiation. Or, again, to borrow another comparison, the gaseous envelope we breathe in (and chiefly the watery part of it) may be literally described as a “trap for sunbeams.” It permits their entrance (exacting, it is true, a heavy toll), but almost totally bars their exit. It is now easy to understand why it is that on the airless moon no vapors rise to soften the hard shadow-outlines of craters or ridges throughout the fierce blaze of the long lunar day. In immediate contact with space (if we may be allowed the expression) water, should such a substance exist on our enigmatical satellite, must remain frozen, though exposed for endless æons of time to direct sunshine.