On August 8 Rawlinson had attacked on the south with the Fourth Army. On August 22 Byng followed on his left with the Third Army. Four days later Horne took it up in turn south of the Scarpe with his First Army. The general function of that Army was to co-operate with the attack of the Third Army on its right, and to cover the flank of that Army from the north. Therefore the First and Eighth Corps stood fast, while the Canadians, who had been brought up from the southern sector, advanced. They formed the right unit of the First Army, and were in touch to the south with Fergusson's newly formed Seventeenth Corps, which was the left unit of the Third Army. As only the Canadians were seriously engaged up to the end of September the narrative of the First Army can be easily summarised. It will be necessary first to explain some preliminary operations. After its return from its hard fighting in the French line, where it had co-operated {139} with the Sixty-second Division in attacking the great German salient upon the Ardres, the famous Fifty-first Highland Division had only a few days' rest before it was in action once more. It was now placed on the north bank of the Scarpe in the Arras sector, The 170th Brigade of the Fifty-seventh Lancashire Territorial Division lay to the south of the stream, and this was placed under the command of Carter-Campbell, General of the Fifty-first, so that he had four brigades under him, with instructions to advance along the line of the river, as opportunity served, in order to screen the left of the attack now about to break out in the south. Carter-Campbell covered from Feuchy in the south to the east of Bailleul, about 8000 yards.