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Page:Popular Science Monthly Volume 32.djvu/498

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THE POPULAR SCIENCE MONTHLY.

pheric tides, due to the moon's attraction, must exist, unless the whole theory of gravitation be wrong, and in a few cases they have been successfully traced in the barometrical records; but in general they are totally obliterated by the ordinary and very much larger disturbances due to other causes. The heating effect of the moon's rays has been the subject of several careful experiments. Melloni, in 1846, started the investigation, and since then Piazzi Smyth (on Teneriffe) and Lord Rosse (at Parsonstown) have endeavored to make precise determinations, with results that place beyond doubt the fact that moonlight does contain a minute proportion of heat-rays, mostly of the dark sort. More recently. Professor Langley's experiments with the bolometer have confirmed that conclusion. In the face of such results, insignificant though they admittedly are by comparison with the effects popularly attributed to lunar influence, it is not correct to say that science absolutely discountenances the notion of any connection between the moon and the weather. For although a barometrical fluctuation so slight as to defy most efforts to discover it, and a thermometrical effect so infinitesimal as to require a very elaborate as well as delicate apparatus to detect it, cannot in any sense be called "weather," it is not unfair to assume—granted the physical influence—that it may work upon the atmosphere in ways to which our instrumental results afford no clew.

We have an example of this in the circumstance which no less careful an observer than Sir John Herschel remarked, "without any knowledge of such a tendency having been observed by others"—the circumstance that the sky is clearer, generally speaking, about the time of full moon than when she is in her quarters. Humboldt mentions this as a fact well known to the pilots and seamen of Spanish America. The explanation has been suggested that clear nights are more conspicuous when the moon is full than when the stars alone diffuse their feeble glimmer, and that clearness in the one case is likely to arrest the attention and be remembered more readily than in the other. One might be disposed to accept the explanation did not Herschel plainly state the tendency to disappearance of clouds under the full moon as a meteorological fact; and he was too experienced an observer to be easily misled by an illusion of the memory. Now, both Lord Rosse's experiments with the three-foot mirror, and those of Professor Langley with the bolometer, have proved that the lunar heat-rays are chiefly dark rays; and Tyndall has shown that "dark heat" is very ready to undergo absorption. It may, therefore, be inferred that much of the heat sent to us by the moon—the quantity of which varies with her phase—is absorbed by the aqueous vapor in the higher regions of the atmosphere; and the direct result of this must be to raise the temperature of the air above the clouds, cause increased evaporation from their surface, and so effect, in a certain measure, their dispersion. Again, a necessary consequence of the dispersion of