the computation of our ephemeris was started, in the year 1849,
there were no tables which could be regarded as really
satisfactory in use. In the British Nautical Almanac the places of
the moon were derived from the tables of Burckhardt published in
the year 1812. You will understand, in a case like this, no
observations subsequent to the issue of the tables are made use
of; the place of the moon of any day, hour, and minute of
Greenwich time, mean time, was precisely what Burckhardt would
have computed nearly a half a century before. Of the tables of the
larger planets the latest were those of Bouvard, published in
1812, while the places of Venus were from tables published by
Lindenau in 1810. Of course such tables did not possess
astronomical accuracy. At that time, in the case of the moon,
completely new tables were constructed from the results reached by
Professor Airy in his reduction of the Greenwich observations of
the moon from 1750 to 1830. These were constructed under the
direction of Professor Pierce and represented the places of the
moon with far greater accuracy than the older tables of
Burckhardt. For the larger planets corrections were applied to the
older tables to make them more nearly represent observations
before new ones were constructed. These corrections, however, have
not proved satisfactory, not being founded on sufficiently
thorough investigations. Indeed, the operation of correcting
tables by observation, as we would correct the dead-reckoning of a
ship, is a makeshift, the result of which must always be somewhat
uncertain, and it tends to destroy that unity which is an
essential element of the astronomical ephemeris designed for
permanent future use. The result of introducing them, while no
doubt an improvement on the old tables, has not been all that
should be desired. The general lack of unity in the tables