other influences by which those figures are affected. For example,
if an observation were to be perfect, then the telescope with which
the observation is made should be perfectly placed in the exact
position which it ought to occupy; this is, however, never the case,
for no mechanic can ever construct or adjust a telescope so perfectly
as the wants of the astronomer demand. The clock also by which we
determine the time of the observation should be correct, but this is
rarely if ever the case. We have to correct our observations for
such errors, that is to say, we have to determine the errors in the
positions of our telescopes and the errors in the going of our
clocks, and then we have to determine what the observations would
have been had our telescopes been absolutely perfect, and had our
clocks been absolutely correct. There are also many other matters
which have to be attended to in order to reduce our observations so
as to obtain from the figures as yielded to the observer at the
telescope the actual quantities which it is his object to determine.

The work of effecting these reductions is generally a very intricate
and laborious matter, so that it has not unfrequently happened that
while observations have accumulated in an observatory, yet the
tedious duty of reducing these observations has been allowed to fall
into arrear. When Airy entered on his duties at Greenwich he found
there an enormous mass of observations which, though implicitly
containing materials of the greatest value to astronomers, were, in
their unreduced form, entirely unavailable for any useful purpose.
He, therefore, devoted himself to coping with the reduction of the
observations of his predecessors. He framed systematic methods by
which the reductions were to be effected, and he so arranged the work
that little more than careful attention to numerical accuracy would
be required for the conduct of the operations. Encouraged by the