ote down certain figures and symbols, of a more or less
hieroglyphic character. It needed much comparison and examination
to find out what sort of an instrument was used, how the observations
were made, and how they should be utilized for the required purpose.

Generally the star which the moon hid was mentioned, but not in all
cases. If it was not, the identification of the star was a puzzling
problem. The only way to proceed was to calculate the apparent
position of the centre of the moon as seen by an observer at the Paris
Observatory, at the particular hour and minute of the observation.
A star map was then taken; the points of a pair of dividers were
separated by the length of the moon's radius, as it would appear
on the scale of the map; one point of the dividers was put into
the position of the moon's centre on the map, and with the other a
circle was drawn. This circle represented the outline of the moon,
as it appeared to the observer at the Paris Observatory, at the hour
and minute in question, on a certain day in the seventeenth century.
The star should be found very near the circumference of the circle,
and in nearly all cases a star was there.

Of course all this could not be done on the spot. What had to be
done was to find the observations, study their relations and the
method of making them, and copy everything that seemed necessary
for working them up. This took some six weeks, but the material I
carried away proved the greatest find I ever made. Three or four
years were spent in making all the calculations I have described.
Then it was found that seventy-five years were added, at a single
step, to the period during which the history of the moon's motion
could be written. Previously this history was supposed to commence
with the observations of Bradley, at Greenwich, about 1750; now it
was extended back to 1675, and with a less degree of accuracy thirty
years farther still. Hansen's tables were found to deviate from
the truth, in 1675 and subsequent years, to a surprising extent;
but the cause of the deviation is not entirely unfolded even now.

During the time I was doing this work, Paris was under the reign
of the Commune and besieged by the national forces. The studies
had to be made within hearing of the besieging guns; and I could
sometimes go to a window and see flashes of artillery from one of the
fortifications to the south. Nearly every day I took a walk through
the town, occasion