On the Limar Cloud-Period. 317 



evidence of the cloud-dispersing power of the moon. Herschel 

 wrote as follows : " Though the surface of the full moon exposed to 

 us must necessarily be very much heated — possibly to a degree much 

 exceeding that of boiling water — yet we feel no heat from it, and 

 even in the focus of large reflectors it fails to affect the thermometer. 

 No doubt, therefore, its heat (conformably to what is observed of 

 that of bodies heated below the point of luminosity) is much more 

 readily absorbed in traversing transparent media than direct solar 

 heat, and is extinguished in the upper regions of our atmosphere, 

 never reaching the surface of the earth at all. Some probability is 

 given to this by the tendency to disappearance of clouds tmder the 

 full moon — a meteorological fact (for as such we think it fully entitled 

 to rank), for which it is necessary to seek a cause, and for which 

 no other rational explanation seems to offer." Herschel further 

 remarks that the '' fact " is based on his own observation, quite 

 independently of any knowledge of such a tendency having been 

 observed by others : " Humboldt, however, in his personal narrative, 

 speaks of it as well known to the pilots and seamen of Spanish 

 America." * 



The cloud-dispersing power of the moon may be as well known to 

 the pilots and seamen of Spanish America as the dependence of the 

 changes of the weather upon the changes of the moon. And it may, 

 for all one knows, be equally baseless. However, in the large Table 

 at the end, I have given the average decrease of cloudiness from 

 afternoon to night, i.e. — 



XIY . + XYI. + XYIII. XX. + XXII. 

 ^~ 3 2 ' 



at Habana (Belen) during the lunar month, derived from four years 

 of observation.! There appears from this to be a fairly well-defined 

 maximum of decrease about three or four days before full moon, but 

 a, distinct minimum at the time of full moon. The critical reader 

 will judge for himself w^hether this supports Humboldt's statement. 

 Three-day averages of the decrease of cloud from afternoon to night 



* Sir J. F. W. Herschel, Outlines of Astronomy, 1851, Art. 432. The italics in 

 the quotation are Herschel's. Herschel thought that Arago's result {i.e. , slightly 

 more rain at new moon than at full) was " part and parcel of the same meteoro- 

 logical fact." An old idea in England was that a change of the moon near the 

 Pleiades brought rain. 



t These are reduced from Padre Gangoiti's excellent annual, Observaciones 

 Meteorologicas y Magneticas. The beautiful printing of these reports is unsur- 

 passed throughout the world. 



