480 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 suc- 

 cessfully 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 

 llosse (at Parsonstown) have endeavored to make precise determina- 

 tions, 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, insignifi- 

 cant though they admittedly are by comparison with the effects popu- 

 larly 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 ap- 

 paratus 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 men- 

 tions 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 in- 

 ferred 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 



