great numbers
by the Paris astronomers, both at the observatory and at other points
in the city.

And how, the reader may ask, did it happen that these observations
were not published by the astronomers who made them? Why should they
have lain unused and forgotten for two hundred years? The answer to
these questions is made plain enough by an examination of the records.
The astronomers had no idea of the possible usefulness and value of
what they were recording. So far as we can infer from their work,
they made the observations merely because an occultation was an
interesting thing to see; and they were men of sufficient scientific
experience and training to have acquired the excellent habit of
noting the time at which a phenomenon was observed. But they
were generally satisfied with simply putting down the clock time.
How they could have expected their successors to make any use of
such a record, or whether they had any expectations on the subject,
we cannot say with confidence. It will be readily understood that no
clocks of the present time (much less those of two hundred years ago)
run with such precision that the moment read from the clock is exact
within one or two seconds. The modern astronomer does not pretend
to keep his clock correct within less than a minute; he determines
by observation how far it is wrong, on each date of observation,
and adds so much to the time given by the clock, or subtracts it,
as the case may be, in order to get the correct moment of true time.
In the case of the French astronomers, the clock would frequently
be fifteen minutes or more in error, for the reason that they used
apparent time, instead of mean time as we do. Thus when, as was often
the case, the only record found was that, at a certain hour, minute,
and second, by a certain clock, _une etoile se cache par la lune_,
a number of very difficult problems were presented to the astronomer
who was to make use of the observations two centuries afterward.
First of all, he must find out what the error of the clock was at the
designated hour, minute, and second; and for this purpose he must
reduce the observations made by the observer in order to determine
the error. But it was very clear that the observer did not expect
any successor to take this trouble, and therefore did not supply
him with any facilities for so doing. He did not even describe
the particular instrument with which the observations were made,
but only wr