another, Tau Ophiuchi, had _closed up_ into apparent singleness; while
the motion of a third, Xi Ursae Majoris, in an obviously eccentric orbit,
was so rapid as to admit of being traced and measured from month to month.

It was from the first confidently believed that the force retaining
double stars in curvilinear paths was identical with that governing the
planetary revolutions. But that identity was not ascertained until
Savary of Paris showed, in 1827,[112] that the movements of the
above-named binary in the Great Bear could be represented with all
attainable accuracy by an ellipse calculated on orthodox gravitational
principles with a period of 58-1/4 years. Encke followed at Berlin with
a still more elegant method; and Sir John Herschel, pointing out the
uselessness of analytical refinements where the data were necessarily so
imperfect, described in 1832 a graphical process by which "the aid of
the eye and hand" was brought in "to guide the judgment in a case where
judgment only, and not calculation, could be of any avail."[113]
Improved methods of the same kind were published by Dr. See in
1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance with
stellar orbits is steadily gaining precision, certainty, and extent.

In 1825 Herschel undertook, and executed with great assiduity during the
ensuing eight years, a general survey of the northern heavens, directed
chiefly towards the verification of his father's nebular discoveries.
The outcome was a catalogue of 2,306 nebulae and clusters, of which 525
were observed for the first time, besides 3,347 double stars discovered
almost incidentally.[116] "Strongly invited," as he tells us himself,
"by the peculiar interest of the subject, and the wonderful nature of
the objects which presented themselves," he resolved to attempt the
completion of the survey in the southern hemisphere. With this noble
object in view, he embarked his family and instruments on board the
_Mount Stewart Elphinstone_, and, after a prosperous voyage, landed at
Cape Town on the 16th of January, 1834. Choosing as the scene of his
observations a rural spot under the shelter of Table Mountain, he began
regular "sweeping" on the 5th of March. The site of his great reflector
is now marked with an obelisk, and the name of Feldhausen has become
memorable in the history of science; for the four years' work done there
may truly be said to open the chapter of our knowledge as regards the
souther