and retired. The port-fire burned out, and he again advanced and relit it. It again failed, and, procuring a musket, Walker went to the vicinity of the gate and fired into the powder, exploding it at once and blowing in the gate. The attacking party rushed in and slew the enemy within. Walker was severely wounded by a bullet in the left thigh, and, before he completely recovered from the wound, was nearly carried off by cholera. He was promoted to be captain on 4 Dec. 1857, and for his services in the mutiny received the medal, with clasp for Delhi, and the brevet rank of major on 19 Jan. 1858, with a gratuity of one year's pay on account of his wound.
Returning to his survey duties, he resumed work on the Indus series, which was completed in 1860, and he was afterwards employed in the Jogi Tila meridional series. In 1860 he again served under Sir Neville Chamberlain in the expedition against the Mahsud Waziris, and was present at the attack of the Barara Tanai. His services were noticed by the general in command and by the Punjab government, and he received the medal and clasp. Here again he made every effort to extend the survey, and sent a map which he had made of the country to the surveyor-general.
In September 1860 Walker was appointed astronomical assistant, and on 12 March 1861 superintendent of the great trigonometrical survey of India. In the next two years the three last meridional series in the north of India were completed, and Walker's first independent work was the measurement of the Vizagapatam base-line, which was completed in 1862. The accuracy achieved was such that the difference between the measured length and the length computed from triangles, commencing 480 miles away at the Calcutta base-line and passing through dense jungles, was but half an inch. He next undertook a revision of Lambton's triangulation in the south of India, with remeasurements of the base-lines.
On 27 Feb. 1864 Walker was promoted to be lieutenant-colonel, and went home on furlough by way of Russia, establishing very friendly relations with the geodesists of the Russian survey, which led to the supply of geographical information from St. Petersburgh and to a cordial co-operation between the survey officers of the two countries. On 27 Feb. 1869 he was promoted to be brevet colonel. About this time it was decided to undertake the great work entitled ‘Account of the Operations of the Great Trigonometrical Survey of India,’ to consist of twenty volumes. The first nine were published under the supervision of Walker, and the first appeared in 1871. It contains his introductory history of the early operations of the survey, and his account of the standards of measure and of the base-lines. The second volume, also mainly written by Walker, consists of an historical account of the triangulation, with descriptions of the method of procedure and of the instruments employed. The fifth volume is an account of the pendulum observations by Walker. In 1871–2, when at home on leave from India, he fixed, in conjunction with Sir Oliver Beauchamp Coventry St. John [q. v.], the difference of longitude between Tehran and London. He was retained at home to make a thorough investigation of the condition of the plates of the Indian atlas, and wrote an important memorandum on the projection and scale of the atlas. In 1873 he began to devote his attention to the dispersion of unavoidable minute errors in the triangulation, with the result that no trigonometrical survey is superior to that of India in accuracy.
Walker's work as superintendent of the great trigonometrical survey was as much that of a geographer as of a geodesist. At his office at Dehra Dun explorers were trained, survey parties for every military expedition organised, and native surveyors despatched to make discoveries, while their work was reduced and utilised. Many valuable maps were published, and Walker's map of Turkistan went through many editions. To Walker also was due the initiation of a scheme of tidal observations at different ports on the Indian coast. He elaborated the system and devised the method of analysing the observations. In connection with these tidal observations, he further arranged an extensive scheme of spirit levelling, connecting the tidal stations by lines of levels sometimes extending across the continent.
On 2 June 1877 Walker was made a companion of the Bath, military division. On 1 Jan. 1878 he was appointed surveyor-general of India, retaining the office of superintendent of the great trigonometrical survey; on 31 Dec. of the same year he was promoted to be major-general, and on 10 May 1881 to be lieutenant-general. He retired from the service on 12 Feb. 1883, and received the honorary rank of general on 12 Jan. 1884.
Walker became a fellow of the Royal Geographical Society in 1859, and in 1885 was elected a member of its council. In 1885 also he was president of the geographical section of the British Association at Aberdeen. He was elected a fellow of the Royal Society in 1865, was made a member of the Russian geographical society in 1868, and of the French
