y planets of comparative
insignificance, there could be no irregularity in its motion. If it were
once started with a velocity of 1,000 miles a minute, then it must
preserve that velocity. Neither the lapse of centuries nor the mighty
length of the journey could alter it. The path of Sirius would be
inflexible in its direction; and it would be traversed with unalterable
velocity.

[Illustration: Fig. 92.--The Orbit of Sirius (Professor Burnham).]

The fact that Sirius had not been moving uniformly was of such interest
that it arrested the attention of Bessel when he discovered the
irregularities in 1844. Believing, as Bessel did, that there must be
some adequate cause for these disturbances, it was hardly possible to
doubt what the cause must be. When motion is disturbed there must be
force in action, and the only force that we recognise in such cases is
that known as gravitation. But gravity can only act from one body to
another body; so that when we seek for the derangement of Sirius by
gravitation, we are obliged to suppose that there must be some mighty
and massive body near Sirius. The question was taken up again by Peters
and by Auwers, who were able to discover, from the irregularities of
Sirius, the nature of the path of the disturbing body. They were able to
show that it must revolve around Sirius in a period of about fifty
years, and although they could not tell its distance from Sirius, yet
they were able to point out the direction in which it must lie. Fig. 92
shows the orbit of Sirius as given by Mr. Burnham, of Yerkes
Observatory.

The detection of the attendant of Sirius, and the measures which have
been made thereon, enable us to determine the weight of this famous
star. Let us attempt to illustrate this subject. It must, no doubt, be
admitted that the numerical estimates we employ have to be received with
a certain degree of caution. The companion of Sirius is a difficult
object to observe, and previous to 1896 it had only been followed
through an arc of 90 deg.. We are, therefore, hardly as yet in a position to
speak with absolute accuracy as to the periodic time in which the
companion completes its revolution. We may, however, take this time to
be fifty-two years. We also know the distance from Sirius to his
companion, and we may take it to be about twenty-one times the distance
from the earth to the sun. It is useful, in the first place, to compare
the revolution of the companion around Sirius wit