ASTRONOMICAL PHENOMENA AND PROGRESS. 



93 



Measurements of Arcs of Parallel, and of 

 Meridian. Under the direction, during his life- 

 time, of F. G-. W. Struve, and of Sir Henry 

 James and others, the work of measuring the 

 immense arc of the parallel running from Orsk, 

 on the Ural River, to Valentia, on the western 

 coast of Ireland, has now been for some years 

 going forward. This arc embraces about 75 

 of longitude, thus forming more than one-fifth 

 of an entire -parallel. In carrying out these 

 determinations of the true differences of longi- 

 tudes, great aid is derived from the employment 

 of the electric telegraph. Under the arrange- 

 ments of Professor Argelander and M. Otto 

 Struve for continuing this work, observations 

 were made during the summer and fall of 1864, 

 with a view to determine the exact differences 

 of longitude of Greenwich with Bonn, with 

 Nieuport, and with Haverford-West these 

 operations being directed by Colonel Forsch 

 and Captain Zylinski (of the Russian service), 

 and Dr. Thiel (attached to the University of 

 Bonn). 



Another survey of an arc of parallel is also 

 being carried carried on in India, this being 

 commonly spoken of as the great Indian Arc, 

 and the work as the Indian Trigonometrical 

 Survey. 



Dr. Otto Torrell reported, in the fall of 1863, 

 the progress made during the summer of that 

 year in the surveys preliminary to the measure- 

 ment of an arc of the meridian at Spitzbergen. 

 The proposed arc is 4 in length, extending 

 from Ross Island, at the extreme north, to 

 Hope Island, at the extreme south, of Spitzber- 

 gen. The surveys in 1863 extended over 1 50' 

 of this distance, being chiefly directed to the 

 selection of suitable stations, and of a favorable 

 locality for the measurement of a base line : 

 they were to be continued in the summer of 

 1864. 



Parallax and Distance of the Sun, and of the 

 Moon. In the Memoirs of the Royal Astronom- 

 ical Society, vol. 33, Mr. E. J. Stone has a paper 

 on the " Determination of the Sun's Mean Equa- 

 torial Horizontal Parallax, from Declination 

 Observations of Mars and Stars, made during 

 the Opposition of 1862, at the Royal Observa- 

 tory, Greenwich ; the Royal Observatory, Cape 

 of Good Hope ; and the Government Observa- 

 tory, Williamstown, Victoria." In this paper, 

 presented May 13th, 1864, Mr. Stone's conclu- 

 sion is, that the sun's mean horizontal parallax 

 is, without doubt, already determined to the first 

 decimal place, namely, as 8".9 ; while he thinks 

 the value 8". 943, has the greatest probability in 

 its favor. Mr. Hansen, by calculations from 

 lunar theory, has lately arrived at the value 

 8".9159; Dr. Winnecke, at that of 8".964. 



Among the modes of estimating the sun's 

 distance, however, independent of parallax, are 

 those based on determinations of the velocity 

 of light. Professor Phillips, in his address 

 before the British Association, 1865, alludes to 

 M. Foucault's wonderful achievement in deter- 

 mining the velocity of light in its movement 



" through all the vast planetary space of millions 

 and thousands of millions of miles, more exactly 

 than had been inferred by astronomers from 

 observations of the satellites of Jupiter," though 

 the experimental proof requisite to this result 

 had been condensed "into his own apartment," 

 and " within a tract of thirty feet." M. Fou- 

 cault's achievement is, indeed, a remarkable 

 one, and his -numerical result doubtless gives 

 very closely, the velocity of light in the dense 

 atmosphere at the earth's surface. A corre- 

 spondent of the Reader, October 7th, 1865, calls 

 attention, however, to the admitted fact that 

 light, in passing through material media, moves 

 faster as the medium is rarer ; and he suggests 

 the probability that the velocity of light in the 

 inter-planetary spaces, where no material me- 

 dium (at least of the ordinary sort) is supposed 

 to exist, may be greater than it is in the 

 denser portions of our atmosphere. He con- 

 cludes that, taken alone, the result of M. 

 Foucault'a experiment cannot be considered a 

 sufficient warrant for the deduction that the 

 estimated distance of the earth must be mate- 

 rially reduced. Still, the tendency of late years 

 to admit an increase in the value of the solar 

 parallax coincides with the result of Foucault's 

 experiment, in requiring a diminution in the 

 previous estimates of the sun's distance ; and it 

 may be said that this is the conclusion to which, 

 on a variety of grounds, the minds of astrono- 

 mers are now tending. 



Meantime, Mr. Breen has corrected Adams' 

 constant of the lunar parallax, by 0".3S, in- 

 creasing it to 3422". 70. The constant so in- 

 creased is adopted in the Nautical Almanac for 

 1867. Its acceptance implies a diminution of 

 the moon's mean distance from the earth equal 

 to about 26 miles.' 



Preparation for Observing the Transit of 

 Venus, 1882. Whatever values for the solar 

 parallax and the sun's distance may be finally 

 arrived at, it has become admitted that, as de- 

 duced from observations on the two transits of 

 Venus last occurring those of the years 1761 

 and 1769 the former was too small, and the 

 latter, accordingly, too great. Authorities state 

 that the two transits of Venus across the sun's 

 disc next to occur, will take place in the years 

 1874 and 1882 ; but, having met with no ac- 

 count of preparations for observing that of 1874, 

 the writer infers that this transit is not ex- 

 pected to be one favorable for observation. The 

 importance, then, which must at this time at- 

 tach to the observation of the more favorable 

 one, that of 1882, may be understood from 

 what has been stated in the preceding section. 



A proposal for a new expedition toward the 

 North Pole having been, in the early part of 

 1865, discussed before the Royal Geographical 

 Society, London, the Astronomer Royal ad- 

 dressed to its President, Sir R. I. Murchison, a 

 letter in which he favors, instead, a South Polar 

 expedition. This, he argues, while it might 

 afford information on a point of vital importance 

 to astronomy the question whether the proper 



