completely representing observations to the last degree of
accuracy. Few, I think, have an idea how unsystematically work of
this kind has hitherto been performed. Until very lately the
tables we have possessed have been the work of one man here,
another there, and another one somewhere else, each using
different methods and different data. The result of this is that
there is nothing uniform and systematic among them, and that they
have every range of precision. This is no doubt due in part to the
fact that the construction of such tables, founded on the mass of
observation hitherto made, is entirely beyond the power of any one
man. What is wanted is a number of men of different degrees of
capacity, all co-operating on a uniform system, so as to obtain a
uniform result, like the astronomers in a large observatory. The
Greenwich Observatory presents an example of co-operative work of
this class extending over more than a century. But it has never
extended its operations far outside the field of observation,
reduction, and comparison with existing tables. It shows clearly,
from time to time, the errors of the tables used in the British
Nautical Almanac, but does nothing further, occasional
investigations excepted, in the way of supplying new tables. An
exception to this is a great work on the theory of the moon's
motion, in which Professor Airy is now engaged.

It will be understood that several distinct conditions not yet
fulfilled are desirable in astronomical tables; one is that each
set of tables shall be founded on absolutely consistent data, for
instance, that the masses of the planets shall be the same
throughout. Another requirement is that this data shall be as near
the truth as astronomical data will suffice to determine them. The
third is that the results shall be correct in theory. That is,