Russian artillery being hauled through the snow to the battlefield by long teams of horses. By September 1915 the Russians had managed to provide themselves with sufficient artillery and ammunition to meet the Germans on equal terms.] The forces in front of Vilna had only one good railway line by which they could retire eastward, and only one good road--a causeway running across the marshes towards Minsk. On 15th September it was discovered that some 40,000 German cavalry, with 140 guns, were sweeping round so as to cut the railway and capture the causeway. Here was a terrible danger. If the cavalry could hold the railway and the causeway, the Russians in the Vilna salient would be surrounded, and nothing could save them. Not an hour must be lost. Vilna was abandoned on 18th September, and the troops were hurried eastwards by means of the road and railway. Rearguards on the right fought desperate holding battles, but on the 20th the gap through which the Russians were retreating had shrunk to little more than fifty miles. The Minsk railway was in danger; only the causeway, densely packed with guns, wagons, convoys, ambulances, and troops, was clear. Suddenly, in this moment of peril, the German thrust weakened. While the cavalry were sweeping round to the rear, the artillery and infantry to the west of the fortress made no push. Owing to the bad and crowded roads they could only proceed very slowly, and thus the Russians were afforded what they most needed--time. No longer were they without arms and ammunition. The Russian factories had worked miracles, and now the Tsar's armies were able to meet the enemy on equal terms. On the evening of 20th September, when the retreating Russians were thirty miles east of Vilna, their right wing fell upon the German cavalry and drove them back with the bayonet. For some days there was heavy fighting, but by the end of the month the Russian line was straight again. Once more the Germans had been foiled. Meanwhile the army of Brest, which had never been in serious danger, had been pursued, but the pursuers were now firmly held. On the Dvina von Buelow had made but little progress, while in the south Ivanov had held his ground, and had even won victories against the German right. He had overthrown a force moving against Tarnopol and another which was advancing further south, and had won one of the most successful of the smaller battles. Of course the Germans had made counter-attacks upon him, but they had been unsuccessful, and Ivanov had advanced in some places as much as twenty miles. His captures at the end of the month amounted to 80,000 men and many guns. Thus the end of September saw the Germans held in check. They had won Vilna and Grodno, but they had failed to cut off the troops in these salients, and had not made good the line of the Dvina. Winter was almost upon them, yet they had not found a suitable position for winter quarters. Meanwhile the Russians were growing in strength every day.