arison with the
earth, the intensity of the gravitation on the neighbouring planet would
be different from the attraction on the surface of the earth. We have
already alluded to the small gravitation on the moon, and in a lesser
degree the same remarks will apply to Mars. A body which weighs on the
earth two pounds would on the surface of Mars weigh rather less than one
pound. Nearly the same exertion which will raise a 56-lb. weight on the
earth would lift two similar weights on Mars.

The earth is attended by one moon. Jupiter is attended by four
conspicuous moons. Mars is a planet revolving between the orbits of the
earth and of Jupiter. It is a body of the same general type as the earth
and Jupiter. It is ruled by the same sun, and all three planets form
part of the same system; but as the earth has one moon and Jupiter four
moons, why should not Mars also have a moon? No doubt Mars is a small
body, less even than the earth, and much less than Jupiter. We could not
expect Mars to have large moons, but why should it be unlike its two
neighbours, and not have any moon at all? So reasoned astronomers, but
until modern times no satellite of Mars could be found. For centuries
the planet has been diligently examined with this special object, and as
failure after failure came to be recorded, the conclusion seemed almost
to be justified that the chain of analogical reasoning had broken down.
The moonless Mars was thought to be an exception to the rule that all
the great planets outside Venus were dignified by an attendant retinue
of satellites. It seemed almost hopeless to begin again a research which
had often been tried, and had invariably led to disappointment; yet,
fortunately, the present generation has witnessed still one more attack,
conducted with perfect equipment and with consummate skill This attempt
has obtained the success it so well merited, and the result has been the
memorable detection of two satellites of Mars.

This discovery was made by Professor Asaph Hall, the distinguished
astronomer at the observatory of Washington. Mr. Hall was provided with
an instrument of colossal proportions and of exquisite workmanship,
known as the great Washington refractor. It is the product of the
celebrated workshop of Messrs. Alvan Clark and Sons, from which so many
large telescopes have proceeded, and in its noble proportions far
surpassed any other telescope ever devoted to the same research. The
object-glass measures