on, on the principle of least
squares--that is, by the mathematical rules of probability--of all the
various quantities upon which the great datum depends. It thus summed up
and harmonised the whole of the multifarious evidence bearing upon the
point, and, as modified in 1894,[795] falls very satisfactorily into
line with the Cape determination. We may, then, at least provisionally,
accept 92,870,000 miles as the length of our measuring-rod for space.
Nor do we hazard much in fixing 100,000 miles as the outside limit of
its future correction.

FOOTNOTES:

[Footnote 748: Airy, _Month. Not._, vol. xvii., p. 210.]

[Footnote 749: Mars comes into opposition once in about 780 days; but
owing to the eccentricity of both orbits, his distance from the earth at
those epochs varies from thirty-five to sixty-two million miles.]

[Footnote 750: J. D. Cassini, _Hist. Abregee de la Parallaxe du Soleil_,
p. 122, 1772.]

[Footnote 751: The present period of coupled eccentric transits will, in
the course of ages, be succeeded by a period of single, nearly central
transits. The alignments by which transits are produced, of the earth,
Venus, and the sun, close to the place of intersection of the two
planetary orbits, now occur, the first a little in front of, the second,
after eight years less two and a half days, a little behind the node.
But when the first of these two meetings takes place very near the node,
giving a nearly central transit, the second falls too far from it, and
the planet escapes projection on the sun. The reason of the liability to
an eight-yearly recurrence is that eight revolutions of the earth are
accomplished in only a very little more time than thirteen revolutions
of Venus.]

[Footnote 752: _Die Entfernung der Sonne: Fortsetzung_, p. 108. Encke
slightly corrected his results of 1824 in _Berlin Abh._, 1835, p. 295.]

[Footnote 753: Owing to the ellipticity of its orbit, the earth is
nearer to the sun in January than in June by 3,100,000 miles. The
quantity to be determined, or "mean distance," is that lying midway
between these extremes--is, in other words, half the major axis of the
ellipse in which the earth travels.]

[Footnote 754: _Month. Not._, vol. xv., p. 9.]

[Footnote 755: _The Distance of the Sun from the Earth determined by the
Theory of Gravity_, Edinburgh, 1763.]

[Footnote 756: _Opera_, t. iii., p. 326.]

[Footnote 757: _Comptes Rendus_, t. xlvi., p. 882. The parallax 8.95"
derived