RATER, HENRY, an English mathematician of some eminence, and an. excellent practical philosopher, was born at Bristol, April 16, 1777, but of his early life very little is known. He obtained a com mission in the army ; and in 1808, while holding the rank of lieutenant in the 12th regiment (infantry), he becarcie a student in the senior department of the Royal Military College, Sandhurst. During his residence at that institution he was promoted to a company in the 62nd regiment; and on quitting the college he received a certificate of the first class. He was afterwards made brigade-major of the eastern district.
Captain Kater was first engaged in making experiments to deter mine the relative merits of reflecting telescopes constructed according to the methods of Cassegrain and Gregory ; and his conclusion was that the ratio of the illuminating power of the former to that of the latter kind was two-and-a-half to one. On this subject he wrote two papers, entitled On the Light of the Cassegrainian telescope compared with that of the Gregorian,' which were published in the 'Philo sophical Transactions' for 1813.
Tho determination of the precise length of the seconds' pendulum, an object of high importance in physical science, engaged the attention of Captain Kater during several years. The methods which had previously been employed to determine accurately the centre of oscil lation in an irregular and body vibrating as a pendulum were found totally inadequate to this purpose ; but Captain Kater succeeded in surmounting the difficulty by availing himself of a property of that centre which had been demonstrated by Huyghens : this property is that, if the centre of oscillation in a suspended body be made the point of suspension, the body will perform a vibration about it in a time equal to that in which it performs a vibration about the original point of suspension. The distance between the two points, experimentally obtained, is evidently equal to the length of a mathematical pendulum vibrating in the same time as tho given pendulum. The ' knife-edge' mode of suspension was first used by Captain Rater in these experiments; and the details of the construction of the pendulum are contained in a paper which was published in the Philosophical Transactions' for 1818. A bill having been introduced into parliament for establishing a uniform system of weights and measures in this country, Captain Rater distinguished himself by the experiments which he made to ascertain the length of the second? pendulum, for the purpose of assigning the physical value of the English foot ; and these experiments gave for the length of such pendulum, in London, in vacua and when reduced to the level of the sea, 39'13929 inches. At the request of the Royal Society of London, Captain Hater proceeded, with the instruments, In July 1818, to Dunnoso In the Isle of Wight, to Arbnry Hill, Clifton, Leith Fort, Portnoy, and the island of Unit, where he made the necessary experi ments; and lie subsequently computed for those places the several lengths of the seconds' pendulum : an account of the experiments, with the computed results, was published in the 'Philosophical Transactions' for 1819. Captain Kater also investigated, by the aid
of Clairsues theorem, the diminution of terrestrial gravity from the pole to the equator; and the great accuracy with which the force of gravity may be determined by means of his pendulum suggested to him ;tie application of the latter to the Important purpose of finding the minute variations of that force in different parts of a country whose substrata consist of materials having different degrees of density.
But the name of Captain Hater will be transmitted to posterity in connection chiefly with his invention of the floating collimator, an instrument which has conferred on practical science essential benefits, its object being the determination of the position of the line of collimation in the telescope attached to an astronomical circle; and this end Is obtained by the collimator with greitter certainty than by the spirit-level, the plumb-line, or by the reflection of an object from the surface of a fluid. Accounts of Captain Katees horizontal and vertical collimators are given in the 'Philosophical Transactions' for 1825 and 182S.
The 'Philosophical Transactions' contain also a paper by Captain Kater on an improved method of dividing Astronomical Circles and other Instruments; one on the length of the French Metre estimated in parts of the English Standard ; one on a remarkable Volcanic Appearance in the Moon in February 1821 ; two papers on the com parison of British Standards of Linear Measures; one paper entitled 'An Account of Experiments made with an Invariable Pendulum belonging to the Board of Longitude ; ' and two papers on the Con struction and Adjustment of the New Standards of Weights and Measures in the United Kingdom of Great Britain and Ireland.' Besides these valuable papers, Captain Hater was the author of a large portion of the work entitled 'A Treatise on Mechanics,' con stituting one of the volumes of Dr. Lardner's Cycloptedia '—this volume being the joint production of Lardner and Kater. In it is a chapter on the subject of pendulums constructed on the principle above mentioned ; and it may be observed that, for the purpose of measuring the distance between the knife-edges, Captain Kater employed a scale furnished with powerful microscopes, to one of which a micrometer was adapted : with this apparatus the 10,000th part of an inch becomes a measurable quantity. He published in 1882 'An Account of the Construction and Verification of certain Standards of Linear Measures for the Russian Government,' 4to, London.
Captain Hater was a Fellow of the Royal Society of London, and In 1814 be received from the Emperor of Russia the decoration of the Order of St. Anne. After a life spent in philosophical research, he died in London, April 26, 1835, leaving behind him many proofs of his zeal for the promotion of physical science.