by Professor Airy are found to hold good,* and his methods are still adopted without alteration.

Professor Airy proposed a series of pendulum experiments for the determination of that difficult question-the Density of the Earth. In 1826 Mr. Airy made his first experiments. In company with Professors Whewell and Sedgwick, and Mr. Sheepshanks, the experiments were repeated in 1828 in Dolcoath Mine, near Camborne, at that time one of the deepest mines in this country. The results obtained, notwithstanding the advantages supposed to be presented by the neighbouring hill-Carn Brea, and the great depth to which the miners had reached, were not considered satisfactory. The attempts were frustrated by accidents (one of fire, and one of water) having no connection with the essential parts of the experiment, but which live as a traditional story among the miners, by no means favourable to the purity of science. This problem has, however, since been solved, with every appearance of the strictest accuracy, by Professor Airy, in the Harton Colliery, near South Shields, in 1854, in which two stations could be found with exactly the same vertical, but at 1256 feet difference of height. The results obtained gave the mean specific gravity of the earth as 6.566. The Schehallien experiments by Dr. Maskelyne gave 4713 for the mean result. The experiments of Cavendish and those of Reich gave 5438; and Mr. Bailey's celebrated observations with the Torsion Balance gave the mean density of the earth as 5.660. Mr. Airy's results give therefore a much higher density to our planet than any of those previously given; and he expresses his belief "that the value now presented is entitled to compete with the others on, at least, equal terms."

On October 16, 1834, occurred the fire at the Houses of Parliament, in which the national standards of length and weight were destroyed. Mr. Airy was then appointed Chairman of the Commission constituted to consider the general question of standards of length and weight. Their Report was presented to Parliament in 1841. In 1843 a committee of scientific men were appointed to superintend the construction of new parliamentary standards. Of this committee the Astronomer Royal was chair

* 66 'Discussion of the observed Deviations of the Compass in several Ships, Wood-built and Iron-built, with general Tables for facilitating the Examination of Compass-deviations," Phil. Trans., 1855.

man.

The Reports of both these committees were drawn up by the Astronomer Royal. His elaborate and valuable "Account of the Construction of the New National Standard of Length, and of its principal Copies," will be found in the Philosophical Transactions for 1857.

In 1844, Professor Airy planned and executed the complicated operations for determining the longitude of Valencia in Ireland by means of chronometers: a determination which he has since twice repeated by the electric telegraph. He also determined the longitudes of Brussels and Paris (in addition to several others) by the telegraph. The latter determinations are now combined with other continental determinations: and, by use of the whole series, an arc of longitude is formed, extending from Valencia to the eastern boundary of Europe beyond the Volga. The longitude of Valencia is combined with the arc of longitude from Valencia to Newfoundland, found by use of the Atlantic telegraph; and thus are ascertained the differences of longitude between Greenwich and the principal American observatories.

Professor Airy has given considerable attention to the testing and improvement of marine chronometers; and to him we are mainly indebted for the diffusion, by the aid of the electric telegraph, of accurate time-signals. Some of these consist in dropping time-balls, as at Deal; some are made public, by the firing of guns, as at Newcastle and Shields; some are simple needlesignals. As Astronomer Royal he has been continually at the service of the country in very varied capacities. Mr. Airy conducted the astronomical observations, preparatory to the definition of the boundary between Canada and the United States; and he also aided in tracing the Oregon territory. He has aided the Government in carrying out the Act applying to measures for the sale of gas. He was consulted in every stage of the manufacture of the clock and bells of the New Palace of Westminster; and Mr. Airy has rendered essential aid to the Royal Commission for the examination of Light-houses.

In 1845 a Royal Commission of three members, of whom Mr. Airy was second, was appointed to investigate the subject of gauge of railways. Mr. Airy joined in the Report which the Commission made in 1846, and shortly afterwards addressed a public letter to Sir Edward Ryan on the same subject. The influence of the "broad gauge" was sufficient to prevent the Government

of the day from fully adopting the principles of the Report and letter; but, under the influence of the progress of railways, the broad-gauge authorities have now spontaneously acted in the direction that was there recommended, and the principles of the Commission are completely carried out.

In the Cambridge Transactions,' 'The Philosophical Transactions of the Royal Society,' 'The Memoirs of the Royal Astronomical Society,' will be found numerous memoirs from Mr. Airy's pen. Among these are some important papers on Astronomical Chronology, and some on Terrestrial Magnetism, based on the magnetic observations made at Greenwich. In the Philosophical Magazine,' and in the 'Athenæum,' have appeared many valuable contributions. Some of the latter bear the signature A. B. G. In the latter journal his "Investigation of the Place of Cæsar's Landing in Britain," and his replies to the recommendations of the University Commissioners, should be especially mentioned.

Professor Airy contributed the excellent article on "Gravitation" to the Penny Cyclopædia;' and the articles "Trigonometry," "The Figure of the Earth," and "Tides and Waves," to the Encyclopædia Metropolitana.' All these articles have been separately published; and it should be noted, in connection with the last-named article, that he has published a memoir on "The Tides on the Coast of Ireland."

In 1848 Professor Airy delivered a series of lectures at the Museum of Ipswich; the object of which (in his own words) was "to explain the methods by which the distances of the sun, moon, and stars are measured by a yard-measure; and the weights of the sun and planets are ascertained by a pound weight." These oral lectures were taken down in short-hand, and have been republished under the title of " The Ipswich Lectures." The work has gone through several editions.

Professor Airy has lately (1866) published, for the use of Students in the University of Cambridge, small works on the Theory of Errors of Observations and on Partial Differential Equations, intended (as were the Mathematical Tracts in 1826) to promote the study of Physical, as distinguished from Pure, Mathematics.

So large an amount of labour, and all of it so important, not merely to science, but to society, could not be allowed to pass without its honorary rewards. Mr. Airy has received the Lalande Medal of the French Institute, the Copley Medal of the Royal

Society for optical researches, and the Royal Medal for his tidal investigations. On two occasions the Royal Astronomical Society have presented Mr. Airy with their medal,--in the first instance, for the discovery of an inequality of long period in the movements of Venus and the Earth; and, in the second, for the reduction of planetary observations. Edinburgh, Oxford, and Cambridge have conferred on him the honorary degrees of LL.D. and D.C.L.; and the Institution of Civil Engineers have elected him honorary member, and have lately awarded him a medal for the theory of a new construction of bridges of great span. Professor Airy is a Foreign Correspondent of the Institute of France, and he holds honorary titles from many other Continental and American societies.