country to Fort Du Quesne, at the junction of the Ohio and Monongahela rivers. The army was surprised by the French and their Indian allies on 11 July and nearly cut to pieces. Mackellar was severely wounded, and lost his horse and baggage, for which he eventually received 183l. as compensation.
In the spring of 1756 he was made chief engineer of the frontier forts, and was engaged in the construction of two new forts to supersede those existing at Ontario and Oswego when, in August, the enemy's appearance compelled the British troops to retire into the old forts. These Mackellar put into repair as rapidly as possible, and conducted the defence with ability. The siege of Ontario was short and decisive; the old walls broke up at every blow from a missile. The garrison abandoned the work, and crossing the water pushed into the scarcely tenable fort of Oswego. Here again Mackellar did his best, but the garrison was forced to capitulate, and Mackellar became a prisoner of war. He was taken to Quebec, and thence to Montreal. Although kept in somewhat close restraint, he managed to learn many useful particulars about the places which he was afterwards to assault. On the exchange of prisoners in 1757 he returned home, and was employed in repairing the castles, forts, and batteries in Scotland. On 14 May 1757 he was commissioned captain in the army in addition to his ordnance rank of engineer, and on 4 Jan. 1758 he was promoted sub-director and major. Mackellar was second engineer, Bastide being the chief, in the expedition under Jeffrey, afterwards baron Amherst [q. v.], for the reduction of Louisburg, which left Halifax on 28 May 1758. They arrived at Cape Breton on 2 June, and disembarked successfully on the 8th in the face of an obstinate resistance. Lighthouse Point was seized on the 12th. The camp was entrenched and ground broken against the fortress of Louisburg by Mackellar the same night. In one of the sorties Bastide, who had been wounded the previous day, was taken prisoner, and Mackellar assumed the chief conduct of the operation. The English fire soon became so hot that three of the largest of the enemy's ships were set ablaze, the approaches were driven closer and lodgments formed in the advanced works, while the citadel was in flames. On 27 July the garrison (6,537 strong), without awaiting the assault, laid down their arms. With the capture of Louisburg the whole island of Cape Breton fell to the British, whose loss was only 523 killed and wounded. Wolfe, who was present as brigadier-general, abused the chief engineer for taking so long over the business, but the success of the undertaking with so small a loss was a laurel for Mackellar.
On 13 May 1759 Wolfe, who had been appointed to the supreme command of the land forces in North America, sailed for the St. Lawrence, and took Mackellar as his chief engineer. The expedition arrived at the island of Orleans, opposite Quebec, on 26 June. Mackellar threw up batteries both on the west front of the island and also at Point Levi, and on 10 July opened fire on both the upper and lower towns of Quebec. The lower town was soon reduced to ruins, but little impression was made on the upper, and Wolfe, growing impatient, made an attack in force from Montmorency on 31 July, during which Mackellar was severely wounded. The attempt failed, and Mackellar, notwithstanding his wound, continued to direct the siege operations. Wolfe was eager to storm, but was dissuaded by Mackellar, who knew from observations, made when a prisoner, the many obstacles that must interpose between the assault on the walls and the capture of the citadel. On 13 Sept. Wolfe attacked the city from above, and Mackellar was with him when he fell on the heights of Abraham. The city still held out, and Mackellar broke ground for a regular attack from the favourable position gained by the British. On 17 Sept. the French capitulated. A journal of the expedition signed ‘P. M.,’ and written it is believed by Mackellar, is printed in the ‘Corps Papers of the Royal Engineers,’ 1847, contributed by Lieutenant-general G. G. Lewis [q. v.], by whom, however, it is wrongly attributed to Major James Moncrieff [q. v.] of the engineers, who was not commissioned until 1762. Mackellar remained as chief engineer with Brigadier-general James Murray (1720–1794) [q. v.], who took command of the city, and during the autumn and spring he strengthened the fortifications. In April 1760 the French, ten thousand strong, advanced on Quebec. Murray met them at Sillery, with Mackellar in command of his artillery. Murray was defeated and driven back to Quebec, and Mackellar was dangerously wounded. The French besieged Quebec, and Mackellar, as soon as he was convalescent, directed the defence until the advance of the British fleet up the St. Lawrence caused the siege to be raised. Mackellar took part in the various services undertaken this year to complete the conquest of Canada, ending with the capture of Montreal. He then accompanied the army to Halifax, Nova Scotia, which had become a large depôt and arsenal. On 24 Nov. 1760 he was appointed chief engineer at Halifax, and while at that station he was indefatigable in instructing the troops in siege operations,
