188 
Proceedings of the Royal Society 
addition to these changes in the illumination of the earth as seen 
from the moon, we would also observe that the brightness of the 
illuminated part of the earth would vary from time to time, and 
from point to point. On some days more light would be reflected 
from the earth to the moon than on other days, giving rise to bright 
days and dark days. These changes would be produced by the 
changes in our atmosphere, as it is very obvious that when our 
atmosphere is covered with clouds more light will be reflected to 
the moon than when the air is clear and cloudless, as clouds reflect 
more light than either land or sea. From these considerations we 
see that the brightness of the dark body of the moon is mainly 
determined — other things being equal — by the amount of cloud in 
our atmosphere. 
The dark body of the moon being visible, or, as it is generally 
expressed, “ the old moon seen in the arms of the new,” is one of 
our well known and oldest indications of coming bad weather. If the 
explanation given above of the illumination of the dark side of the 
moon is correct, then, it gives an explanation of the old saying, 
and we see that this lunar indication, unlike many others, has a 
sound physical basis. “ The old moon seen in the arms of the new ” 
indicates that to the west of us there are at the time vast areas of 
clouds. ISTow that is the direction from which we receive most of 
our weather, and the probability is that many of these clouds are 
over the ISTorth Atlantic Ocean, and will travel eastwards and north- 
wards, and unburden themselves over the north-west of Europe. 
If these conclusions are correct, we might look upon the dark 
side of the moon as an outlying signal station, capable of giving us 
indications of the more or less cloudy condition of the earth’s atmo- 
sphere, and with properly constructed instruments, the signals might, 
with some practice, come to be perfectly intelligible and accurate. 
There is, however, one unfortunate circumstance which will make 
the readings of these signals at certain times extremely difficult. The 
brightness of the dark side of the moon will not only be determined 
by the more or less cloudy condition of our atmosphere, but also by 
the amount of the illuminated side of the earth turned towards the 
moon, and, as this varies from day to day, the interpretation of the 
signals will be difficult and probably impossible when the moon 
approaches the full, as the illuminated part of the earth then turned 
