DALBY, ISAAC, one of the many self-taught men of this country, who have attained considerable eminence in mathematical science by the mere force of genius, and in defiance of the obstacles opposed by fortune to their progress, was born in Gloucestershire, in the year 1744, and he appears to have been instructed in the rudiments of Latin and arithmetic at a grammar-school in that county. By his friends he was destined to be a clothworker, but his taste leading him to the study of mathematics, be laboured, by the aid of Stone's Euclid,' Simpson's 'Algebra,' and Martin's 'Trigonometry,' to qualify himself to be an usher in a country school. In that capacity he was employed for about three years, when be opened a school on his own account in another par t of the country, but meeting with no success, he came up to London in 1772. Hera he received the appointment of usher to teach arithmetic in Archbishop Tenison's grammar-school near Charing Cross, and while fulfilling the duties of that employ ment be became known to many of the most celebrated men of science in town. Among these were Dr. Maskeline, the astronomer royal, Dr. Hutton and Mr. Bonnyeastle, both of the Royal Military Academy at Woolwich, tho Rev. Messrs. Crakelt and Lawson, and Mr. Landen, Mr. Wale; mathematical master of Christ's Hospital, and Mr. Witohel, master of the Royal Naval Academy at Portsmouth. Mr. Bayly, who hail been employed iu making astronomical observations iu a building erected near Highgate by the Hon. Topham Beauclerk for philoso phical purposes, being engaged to sail with Captain Cook, Dalby, after having been about a year at the school above mentioned, was appointed to succeed him. .In this situation, besides his duties as observer and librarian, he performed, under Dr. Fordyce, that of experimenter iu chemistry ; and amidst these employments he found time to make himself acquainted with the French lauguage and revive his know. ledge of Latin. In 1751, Mr. Beauclerk's establishment being broken up, and the library, instruments, &c., euld, Dalby was engaged to make a c-ttalogue of the library of Lord Beauchamp ; and in the following year he was appoiuted mathematical master of the Naval School at Chelsea. This was supported by voluntary contributions, and it sue• erected for a time under the management of Mr. Jonas }leeway; but the subscriptions falling off, the Institution was given up.
In 1767 Mr. ltamsden, the distinguished maker of philosophical instruments, to whom for several years Dalby bad been known, reoommended him, as an assistant, to Major-General Roy, who was then employed In the trigonometrical observations for connecting the meridians of Greenwich and Paris; and during that and the following year he was employed in extending the triangulation through Kent and part of Sussex to the coast opposite Franco. Dalby was subset.
quenely employed iu making the computations preparatory to the publication of the account of the proceedings; and on this occasion he was lod to apply a theorem (ascribed to Albert Girard) to the purpose of computing the emcee; of the three angles of a spherical triangle above two right angles. The account was published in the 'Philosophical Transactions for 1790; and in the volume for the RAMO year is a paper by Dalby on the figure of the earth, iu which it is proved that the 'excess' I; without 'sensible error, the same whether the earth be a sphere or a spheroid. General Roy died in 1790, and le the following year Dalby was engaged, together with Colonel Williams and Captain (since Major-General) Mudge, to carry on the survey of England. The operations commenced by a remesauroment of the original base on Hounslow Heath, and before Mr. Dalby quitted that service the trisugillation was extended through the southern counties of England to the Laud's End. The accounts of the survey were published in the ' Philosophical Transactions ;' but in l793 Dalby, together with Colonel Mudge, made a revision of General Roy's papers, and connected the operations of that officer with those which had subsequently taken place to the eud of 1796: those form the subjects of a volume which was published separately.
In the year 1799, on the formation of the Royal Military College at nigh Wycombe, Dalby was appointed professor of mathematics iu the senior department of that institution. He continued to hold that appointment during the years that the department to which he belonged remained at High Wycombe, and subsequently to its removal to Farnham in Surrey ; but in 1S20, it bring then uuited to the junior department at Sandhurst in Berkshire, his infirmities obliged him to resign. He continued however to reside at Farnham till his death, which took place October 14, 1521, when he was iu the eighty-firet year of his age.
His attention to his duties was unremitting ; and besides his con tributions to the Ladies' Diary' and other works, ho wrote for the use of the Military College, a valuable Course of Mathematics,' iu 2 vols., which, with successive improvements, extended to a sixth edition.
(Leybourn, Mathematical Repository, vol. v.)