1864. | Astronomy. 105 
the three guineas and two guineas which our own Society charges for 
very inadequate returns. Amongst the officers elected at the Heidel- 
berg foundation meeting, are Zech of Tiibingen (President), Argelander, 
O. Struve, Bruhns, Schénfeld, &e. The secretary is Forster, of the 
Royal Observatory, Berlin, well known as an expert calculator. 
In reviewing the progress of Astronomy during the last six months, 
we shall scarcely do wrong in assigning a foremost place to some re- 
marks on the belief which has recently taken hold upon the minds of 
leading men, that it is now necessary to adopt some revised estimate 
of the sun’s distance from the carth. The precise amount of the re- 
duction to be made in the hitherto-received value is open to future 
determination, but concerning the general fact that some correction is 
requisite there seems to be no difference of opinion. The first really 
public announcement at any considerable length is due to Mr. Hind, 
who contributed a very lucid memoir on the subject to ‘The Times’ in 
the month of September last. For our present purpose no more is re- 
quisite than to give a brief recapitulation of the matter in Mr. Hind’s 
own words, followed by a few general remarks on two of his heads 
which appear to deserve comment. He thus sums up :—“ A diminu- 
tion in the measure of the sun’s distance now adopted is implied by— 
1st, the theory of the moon as regards the parallactic equation, agreeably 
to the researches of Professor Hansen and the Astronomer Royal ; 2nd, 
the lunar equation in the theory of the earth, newly investigated by 
M. Le Verrier; 5rd, the excess in the motion of the node of the orbit 
of Venus beyond what can be due to the received value of the planetary 
masses; 4th, the similar excess in the motion of the perihelion of 
Mars, also detected within the past few years by the same mathematician ; 
5th, the experiments of M. Foucault on the velocity of light; and 6th, 
the results of observations of Mars when near the earth about the 
opposition of 1862.” 
To Encke we owe the best discussion of the observations of the 
transit of Venus in 1769: he determined the value of the sun’s paral- 
lax to be 8’-5776, from which we infer the earth’s mean distance from 
the sun to be 95,283,115 miles. Now, the time occupied by a ray of 
light reaching the earth from the sun is known very exactly to be 8m. 
18s., from which a velocity of about 192,000 miles per second is de- 
ducible. Foucault of Paris, however, by the optical contrivance of a 
“turning mirror,” due to Professor Wheatstone, has concluded that this 
value is too great; that it is more precisely 185,170 (English) miles. 
Assuming that Foucault is right, and all his predecessors wrong, it fol- 
lows that the solar parallax must be 8/86. Two most singular coin- 
cidences must here be disposed of. (1) The theoretical value assigned 
by Le Verrier, irrespective of all instrumental measurements, and purely 
on physical grounds, is 8°95 ; and (2) The discussion, by Stone of Green- 
wich, of the observations of Mars (adverted to above in Mr. Hind’s 
6th point), taken by Ellery at Williamstown, Victoria, N. 8. W., give 
a value of 8’-95, with a probable error of only 0”-03. Combining the 
foregoing, we find that three different observers, working in three most 
diverse ways, have all arrived at the same general result, and more than 
