de part of astronomical instruments for finding
the altitude of a heavenly body, and after the pendulum clock had
been invented by Huyghens, that the time of an occultation could be
fixed with the required exactness. Thus it happens that from 1640
to 1670 somewhat coarse observations of the kind are available, and
after the latter epoch those made by the French astronomers become
almost equal to the modern ones in precision.

The question that occurred to me was: Is it not possible that such
observations were made by astronomers long before 1750? Searching
the published memoirs of the French Academy of Sciences and the
Philosophical Transactions, I found that a few such observations were
actually made between 1660 and 1700. I computed and reduced a few of
them, finding with surprise that Hansen's tables were evidently much
in error at that time. But neither the cause, amount, or nature of
the error could be well determined without more observations than
these. Was it not possible that these astronomers had made more
than they published? The hope that material of this sort existed
was encouraged by the discovery at the Pulkowa Observatory of an
old manuscript by the French astronomer Delisle, containing some
observations of this kind. I therefore planned a thorough search
of the old records in Europe to see what could be learned.

The execution of this plan was facilitated by the occurrence,
in December, 1870, of an eclipse of the sun in Spain and along the
Mediterranean. A number of parties were going out from this country
to observe it, two of which were fitted out at the Naval Observatory.
I was placed in charge of one of these, consisting, practically,
of myself. The results of my observation would be of importance
in the question of the moon's motion, but, although the eclipse was
ostensibly the main object, the proposed search of the records was
what I really had most in view. In Paris was to be found the most
promising mine; but the Franco-Prussian war was then going on, and
I had to wait for its termination. Then I made a visit to Paris,
which will be described in a later chapter.

At the observatory the old records I wished to consult were placed
at my disposal, with full liberty not only to copy, but to publish
anything of value I could find in them. The mine proved rich beyond
the most sanguine expectation. After a little prospecting, I found
that the very observations I wanted had been made in