Santiago in Chili, M. Houzeau, of the Brussels Observatory, derived a
solar parallax of 8.907", and a distance of 91,727,000 miles.[784] But
the "probable errors" of this determination amounted to 0.084" either
way: it was subject to a "more or less" of 900,000, or to a total
uncertainty of 1,800,000 miles. The "probable error" of the English
result, published in 1887, was less formidable,[785] yet the details of
the discussion showed that no great confidence could be placed in it.
The sun's distance came out 92,560,000 miles; while 92,360,000 was given
by Professor Harkness's investigation of 1,475 American
photographs.[786] Finally, Dr. Auwers deduced from the German
heliometric measures the unsatisfactorily small value of 92,000,000
miles.[787] The transit of 1882 had not, then, brought about the desired
unanimity.

The state and progress of knowledge on this important topic were summed
up by Faye and Harkness in 1881.[788] The methods employed in its
investigation fall (as we have seen) into three separate classes--the
trigonometrical, the gravitational, and the "phototachymetrical"--an
ungainly adjective used to describe the method by the velocity of light.
Each has its special difficulties and sources of error; each has
counter-balancing advantages. The only trustworthy result from celestial
surveys, was at that time furnished by Gill's observations of Mars in
1877. But the method by lunar and planetary disturbances is unlike all
the others in having time on its side. It is this which Leverrier
declared with emphasis must inevitably prevail, because its accuracy is
continually growing.[789] The scarcely perceptible errors which still
impede its application are of such a nature as to accumulate year by
year; eventually, then, they will challenge, and must receive, a more
and more perfect correction. The light-velocity method, however,
claimed, and for some years justified, M. Faye's preference.

By a beautiful series of experiments on Foucault's principle, Michelson
fixed in 1879 the rate of luminous transmission at 299,930 (corrected
later to 299,910) kilometres a second.[790] This determination was held
by Professor Todd to be entitled to four times as much confidence as any
previous one; and if the solar parallax of 8.758" deduced from it by
Professor Harkness errs somewhat by defect, it is doubtless because
Glasenapp's "light-equation," with which it was combined, errs slightly
by excess. But all earlier effor