1799. Educated by a private tutor, he entered the military college of the East India Company at Addiscombe in 1814, and passed out as an engineer cadet for the Bengal service. He arrived at Calcutta on 18 Sept. 1818, and went to Cawnpore. In 1820 he joined the recently formed corps of Bengal sappers and miners, commanded by Major (afterwards Sir) Thomas Anburey, at Allahabad. On 28 Jan. 1821 he took command of the detachment of sappers at Asirgarh, and in March visited his eldest brother, Alexander, of the Bengal artillery, at Mhow. In the following year he was engaged in the construction of a road between Asirgarh and Nagpur, and later between Nagpur and Chapara. From March to June 1823 he was employed in dismantling and blowing up the fort of Mandla. He was appointed adjutant of the Bengal sappers and miners on 29 May of this year, and on 5 Sept. he was promoted to be lieutenant.
In March 1824 war was declared with Burma, and in the following September Thomson went to Calcutta to join the pioneer department, for active service under the orders of Captain Schalch. On 14 Dec. he left Calcutta for Chittagong, where a force of eleven thousand men, under Brigadier-general Morrison of the 44th foot, had been assembled to penetrate to Ava through Arakan. Thomson was appointed field-engineer to the force and placed in command of the pontoon train. On 10 Jan. 1825 he started with Morrison's force by a route along the sea-coast, and, after crossing the Mayu estuary, a little to the west of the modern port of Akyab, advanced north-east through a difficult country, and crossed the Kala-daing or Great Arakan river. Thomson was almost always in front on reconnaissance duty, and the forests being too thick and the rivers too deep to allow of any other way of travelling, he went on foot and suffered greatly from fatigue. The approach to Arakan lay across a narrow valley, bounded by a range of hills crowned with stockades and garrisoned by nine thousand Burmese. An attack on 29 March failed, but on 1 April Thomson assisted in the assault and capture of the stockades, and Arakan was taken.
Thomson was mentioned by Morrison in his despatch of 2 April 1825 (London Gazette, 1 Oct. 1825), for having ‘displayed zeal and practical proficiency in the performance of his duty.’ On 7 May 1825 he was appointed executive engineer, south-eastern division of the public works department, and he was busy with the erection of cantonments in Arakan at the close of the rainy season. The division suffered very heavily from the pestilential climate. Thomson was sent to survey and report upon the best situation in the islands near the mouth of the Beatong river for cantoning the division. He returned to Bengal in September 1826.
On 7 Oct. 1826 Thomson was appointed executive engineer in the public works department at Nimach, and was employed in building a fort there. He was promoted to be captain in the Bengal engineers on 28 Sept. 1827. On 6 Dec. he was appointed to the Bengal sappers and miners, and on 21 Feb. 1828 he returned to the public works department as executive engineer of the Rohilkhand division. In February 1829 Thomson took furlough to Europe, married, and returned to India in November 1831. On 9 Dec. 1831 he was appointed to survey the country between Bankura and Shirghatti, and to estimate the cost of the construction of a road from Jemor to the Karamnassa river. He was next placed in charge of the construction of the grand trunk road between Bardwán and Benares. In 1834 he had the additional duty of constructing barracks at Hazaribagh for a European regiment; in this work, despite occasional conflict with the authorities, he adopted successful methods of his own for the utilisation of convict labour.
In March 1837 Thomson was appointed to the command of the Bengal sappers and miners at Delhi, and to be at the same time executive engineer of the Delhi division of the public works department, a combination of duties which he did not think was for the good of the service. On 13 Sept. 1838 he was selected to be chief engineer of the army of the Indus assembling at Karnal for the invasion of Afghanistan. He marched from Delhi with two companies of sappers and miners on 20 Oct. to Karnal, thence on 9 Nov. to Firozpur, and on to Bhawalpur (230 miles), where he arrived on 29 Dec. Rohri, on the left bank of the Indus, was reached on 24 Jan. 1839, and the fort of Bakkar, on a rocky island between Rohri and Sakkar, on the right bank, was seized without opposition on 29 Jan., and preparations made by Thomson to bridge the river. The channel between Rohri and Bakkar is some 360 yards wide, and that between Bakkar and Sakkar about 130 yards, and in both the water ran like a millstream. Thomson had asked the political officer to collect beforehand at Rohri materials for bridging, but when he arrived none were there. By great exertion he procured boats, cut down and split palm trees, made grass cables, constructed anchors of small trees joined together and loaded with stone, made nails on
