TRANSACTIONS OF SECTION A. 321 
enables us with fair probability to extend the latter back to 1750. With some 
change of phase the periods of high and low maxima correspond nearly with 
the fluctuations above. The eleven-year oscillation is naturally eliminated 
from the group results for the Earth and Mercury. One might expect it to 
be present in the lunar curve, but owing to its shorter period we should 
probably not obtain a coefficient of over half-a-second. Notwithstanding this 
fact, it is a valid objection to the hypothesis that there is no evidence of it 
in the moon’s motion. Reasons may exist for this: but until the mechanism 
of the action can be made more definite it is hardly worth while to belabour 
the point. 
The hypothesis presents many difficulties. Even if one is disposed to admit 
provisionally a correlation between the four curves—and this is open to con- 
siderable doubt—it is difficult to understand how, under the electron theory 
of magnetic storms, the motions of moon and planets can be sensibly affected. 
I am perhaps catching at straws in attempting to relate two such different 
phenomena with one another, but when we are in the presence of anomalies 
which show points of resemblance and which lack the property of analysis 
into strict periodic sequences some latitude may be permissible. 
In conclusion, what, it may well be asked, is the future of the lunar theory 
now that the gravitational effects appear to have been considered in such 
detail that further numerical work in the theory is not likely to advance our 
knowledge very materially? What good purpose is to be served by continuous 
observation of the moon and comparison with the theory? I believe that the 
answer lies mainly in the investigation of the fluctuations already mentioned. 
I have not referred to other periodic terms which have been found because 
the observational evidence for their real existence rests on foundations much 
less secure. These need to be examined more carefully, and this examination 
must, I think, depend mainly on future observations rather than on the records 
of the past. Only by the greatest care in making the observations and in 
eliminating systematic and other errors from them can these matters be fully 
elucidated. If this can be achieved and if the new theory and tables serve, 
as they should, to eliminate all the known effects of gravitation, we shall 
be in a position to investigate with some confidence the other forces which 
seem to be at work in the solar system and at which we can now only guess. 
Assistance should be afforded by observations of the sun and planets, but 
the moon is nearest to us and is, chiefly on that account, the best instrument 
for their detection. Doubtless other investigations will arise in the future. 
But the solution of the known problems is still to be sought, and the laying 
of the coping stone on the edifice reared through the last two centuries can- 
not be a simple matter. Even our abler successors will hardly exclaim, with 
Hotspur, 
‘ By heaven, methinks, it were an easy leap 
To pluck bright honour from the pale-faced moon.’ 
They, like us and our predecessors, must go through long and careful investi- 
gations to find out the new truths before they have solved our difficulties, and 
in their turn they will discover new problems to solve for those who follow 
them :— 
‘For the fortune of us, that are the moon’s men, doth ebb and flow like 
the sea, being governed, as the sea is, by the moon.’ 
1914. 7 
