318 TRANSACTIONS OF SECTION A. 
In spite of the fact that Harvard Observatory has undertaken to continue for 
the present the work of photographing the moon’s position, I believe that this 
method should find a permanent home in a national observatory. It has already 
shown itself capable of producing the accuracy which the best modern observa- 
tions of Greenwich can furnish, and no higher praise need be given. If this home 
could be found in the southern hemisphere, and more particularly in Australia, 
other advantages would accrue. ; 
But we should look for more than this. In an observatory whose first duty 
might be the securing of the best daily records of the sky, the positions of the 
sun, stars, planets, a couple of plates of the moon on every night when she is 
visible would be a small matter. What is needed is an organisation so con- 
structed as to be out of the reach of changing governmental policy with a 
permanent appropriation and a staff of the highest character removed from all 
political influences. It could render immense service to astronomers, not only in 
the Empire but all over the world. The pride which every Englishman feels who 
has to work with the records of the past furnished by Greenwich would in course 
of time arise from the work of a similar establishment elsewhere. Those of us 
who live in a community which, reckoning by the age of nations, is new, know 
that, in order to achieve objects which are not material, sacrifices must be made; 
but we also know that such sacrifices are beneficial, not only in themselves, but 
as exerting an indirect influence in promoting the cause of higher education and 
of scientific progress in every direction. In saying this I am not advocating the 
cause of the few, but of the majority; the least practical investigations of yester- 
day are continually becoming of the greatest practical value to-day. 
No address before this section is complete without some speculation and a 
glance towards the future. I shall indulge in both to some small extent before 
closing. I have shown you what the outstanding residuals in the moon’s motion 
are: they consist mainly of long-period fluctuations in the mean longitude. I 
have not mentioned the secular changes because the evidence for them does not 
rest on modern observations but on ancient eclipses, and these are matters too 
debatable to discuss in the limited time allotted to me for this address. It may 
be said, however, that the only secular motion which is capable of being deter- 
mined from the modern observations and is not affected by the discussion of 
ancient eclipses—namely, the secular motion of the perigee—agrees with its 
theoretical value well within the probable error. With this remark I pass to the 
empirical terms. 
These unexplained differences between theory and observation may be 
separated into two parts. First, Newcomb’s term of period between 250 and 300 
years and coefficient 13/, and, second, the fluctuations which appear to have an 
approximate period of 60 to 70 years. The former appears to be more important 
than the latter, but from the investigator’s point of view it is less so. The force 
depends on the degree of inclination of the curve to the zero line or on the 
curvature, according to the hypothesis made. In either case the shorter period 
term is much more striking, and, as I have pointed out on several occasions, it is 
much more likely to lead to the sources of these terms than the longer period. 
It is also, at least for the last sixty years, much better determined from observa- 
tion, and is not likely to be confounded with unknown secular changes. 
Various hypotheses have been advanced within the last few years to account 
for these terms. Some of them postulate matter not directly observed or matter 
with unknown constants; others, deviations of the Newtonian law from its 
exact expression; still others, non-gravitational forces. M. St. Blancat ° 
examines a variety of cases of intramercurial planets and arrives at the conclusion 
that such matter, if it exists, must have a mass comparable with that of Mercury. 
Some time ago I examined the same hypothesis and arrived at similar results. 
The smallest planet with density four times that of water, which would produce 
the long inequality. must have a disc of nearly 2” in its transit across the sun 
and a still larger planet would be necessary to produce the shorter period terms. 
But observational attempts, particularly those made by Perrine and Campbell, 
have always failed to detect any such planet, and Professor Campbell is of the 
opinion that a body with so large a disc could hardly have been overlooked. If 
® Annales de la Faculté des Sciences de Toulouse, 1907. 
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