f
the satellites proceeding outwards from the planet is almost the same
size as our moon; the other three bodies are larger; the third being the
greatest of all (about 3,560 miles in diameter). Owing to the minuteness
of the satellites as seen from the earth, it is extremely difficult to
perceive any markings on their surfaces, but the few observations made
seem to indicate that the satellites (like our moon) always turn the
same face towards their primary. Professor Barnard has, with the great
Lick refractor, seen a white equatorial belt on the first satellite,
while its poles were very dark. Mr. Douglass, observing with Mr.
Lowell's great refractor, has also reported certain streaky markings on
the third satellite.

A very interesting astronomical discovery was that made by Professor
E.E. Barnard in 1892. He detected with the 36-inch Lick refractor an
extremely minute fifth satellite to Jupiter at a distance of 112,400
miles, and revolving in a period of 11 hrs. 57 min. 22.6 secs. It can
only be seen with the most powerful telescopes.

The eclipses of Jupiter's satellites had been observed for many years,
and the times of their occurrence had been recorded. At length it was
perceived that a certain order reigned among the eclipses of these
bodies, as among all other astronomical phenomena. When once the laws
according to which the eclipses recurred had been perceived, the usual
consequence followed. It became possible to foretell the time at which
the eclipses would occur in future. Predictions were accordingly made,
and it was found that they were approximately verified. Further
improvements in the calculations were then perfected, and it was sought
to predict the times with still greater accuracy. But when it came to
naming the actual minute at which the eclipse should occur, expectations
were not always realised. Sometimes the eclipse was five or ten minutes
too soon. Sometimes it was five or ten minutes too late. Discrepancies
of this kind always demand attention. It is, indeed, by the right use of
them that discoveries are often made, and one of the most interesting
examples is that now before us.

The irregularity in the occurrence of the eclipses was at length
perceived to observe certain rules. It was noticed that when the earth
was near to Jupiter the eclipse generally occurred before the predicted
time; while when the earth happened to be at the side of its orbit away
from Jupiter, the eclipse occurred af