Page:Dictionary of National Biography volume 02.djvu/442

 nisable order the stars composing them. A paper on the subject, read by him before the Royal Astronomical Society 12 May 1843, was appended to the report of a committee (consisting of Herschel, Whewell, and Baily) appointed by the British Association in 1840 to consider the subject (Report, 1844, p. 34), and was also reprinted in his introduction to the ‘Catalogue.’

The reform of the ‘Nautical Almanac’ was another of the benefits derived by science from his zeal. It was rendered inevitable by his strictures on its deficiencies in 1819, 1822, and 1829, and the admiralty having, on the death of the superintendent, Dr. Thomas Young, 10 May 1829, submitted the matter to the Astronomical Society, Baily formed one of the deliberating committee, and drew up the report upon which the present National Ephemeris was modelled (Mem. R. A. S. iv. 449).

In view of Captain Foster's proposed expedition, Baily devised, in 1828, a simplified kind of convertible pendulum (described in Phil. Mag. iv. 137), of which two specimens, of iron and copper respectively, formed part of the scientific equipment of the Chanticleer. The accidental death of her commander (5 Feb. 1831) threw upon him the onerous duty of digesting and completing (by swinging the pendulums in London) the numerous observations made in both hemispheres; and his elaborate and admirable report, presented to the admiralty and ordered to be printed at the government expense, filled the entire seventh volume of the ‘Royal Astronomical Society's Memoirs.’ The general result of 20,000 experiments gave 1/289.48 for the ellipticity of the earth, showing a most satisfactory agreement with Sabine's of 1/288.40.

Meanwhile Baily had prosecuted independently a research entitling him to a distinguished share of merit in the determination of the length of the seconds' pendulum. Bessel pointed out in 1828 (Abhandlungen Kön. Ak. der Wiss. Berlin, 1826, p. 32) that, in the received ‘correction for buoyancy,’ no allowance was made for the expenditure of force in setting the particles of surrounding air in motion. In order to estimate with precision this neglected element of reduction, Baily had a vacuum-apparatus erected in his house, and there carried out, in 1831–2, a series of most delicate experiments on eighty-six pendulums of every variety of form and material, of which the details were communicated to the Royal Society 31 May 1832 (Phil. Trans. cxxii. 399). It appeared thence that the value of the new correction, while varying very sensibly with the shape and size of the pendulum, was in many cases more than double the old. The subject of the length of the seconds pendulum led naturally to that of the national unit of length, defined by act 5 George IV in terms of that (as it had now proved) uncertain quantity. Baily accordingly obtained in 1833 from the Royal Astronomical Society authority to construct for them a tubular scale of five feet (see his admirable report, Mem. R. A. S. ix. 35), the accuracy of which had been ascertained by repeated comparisons with the standard yard, when the latter was irreparably injured in the conflagration of the houses of parliament 16 Oct. 1834. A commission of seven, appointed 11 May 1838 to consider the best means of replacing it, included him amongst its members; and to him was entrusted in 1843, by the unanimous desire of his colleagues, the actual reconstruction of the standards of length, in the preparatory experiments for which laborious task he was arrested by fatal illness.

The most arduous and conspicuous labour of his life has still to be adverted to. This was the repetition of the ‘Cavendish experiment’ for measuring the density of the earth. The principle of this research depends upon the comparison between the observed attractive effects of masses of ascertained weight and density with the known force of gravity at the earth's surface; but its adequate execution is attended by difficulties of the most baffling description. A remark made by Professor De Morgan at the council-table of the Royal Astronomical Society occasioned the appointment, in 1835, of a committee to consider the matter; but no progress was made until Baily offered his services in 1837, and the treasury granted 500l. towards expenses. The operation, conducted in an upper room of his house, twelve feet square, lasted from October 1838 to May 1842, and resulted in establishing, within narrow limits of error, that our globe is composed of materials, on an average, 5.66 times as heavy as water (Mem. R. A. S. xiv. table vii.) Nevertheless, in spite of precautions incredibly minute, the experiments were vitiated during eighteen months by an unknown cause of error. Ultimate success seemed scarcely to be hoped for, yet Baily resolved to persevere; and to this determination, Lord Wrottesley remarked (Mem. R. A. S. xv. 280), it is due that his memoir (occupying the entire fourteenth volume of Mem. R. A. S.) ‘is hardly less valuable as a lesson upon the nature and use of the torsion pendulum in measuring small forces than as a determination of the mean density of the earth.’ It was at length suggested by Professor Forbes that the anoma-