tudying profoundly the
movements of the planets; and from his study of the past he had ventured
to predict the future. Kepler announced that in the year 1631 the
planets Venus and Mercury would both make a transit across the sun, and
he assigned the dates to be November 7th for Mercury, and December 6th
for Venus. This was at the time a very remarkable prediction. We are so
accustomed to turn to our almanacs and learn from them all the
astronomical phenomena which are anticipated during the year, that we
are apt to forget how in early times this was impossible. It has only
been by slow degrees that astronomy has been rendered so perfect as to
enable us to foretell, with accuracy, the occurrence of the more
delicate phenomena. The prediction of those transits by Kepler, some
years before they occurred, was justly regarded at the time as a most
remarkable achievement.

The illustrious Gassendi prepared to apply the test of actual
observation to the announcements of Kepler. We can now assign the time
of the transit accurately to within a few minutes, but in those early
attempts equal precision was not practicable. Gassendi considered it
necessary to commence watching for the transit of Mercury two whole days
before the time indicated by Kepler, and he had arranged an ingenious
plan for making his observations. The light of the sun was admitted into
a darkened room through a hole in the shutter, and an image of the sun
was formed on a white screen by a lens. This is, indeed, an admirable
and a very pleasing way of studying the surface of the sun, and even at
the present day, with our best telescopes, one of the methods of viewing
our luminary is founded on the same principle.

Gassendi commenced his watch on the 5th of November, and carefully
studied the sun's image at every available opportunity. It was not,
however, until five hours after the time assigned by Kepler that the
transit of Mercury actually commenced. Gassendi's preparations had been
made with all the resources which he could command, but these resources
seem very imperfect when compared with the appliances of our modern
observatories. He was anxious to note the time when the planet appeared,
and for this purpose he had stationed an assistant in the room beneath,
who was to observe the altitude of the sun at the moment indicated by
Gassendi. The signal to the assistant was to be conveyed by a very
primitive apparatus. Gassendi was to stamp on the floor when t