Though the salient was well known to be a source of weakness, it was not strongly held by the Allies at this time. Probably the Germans were aware of the fact, for suddenly they launched a furious and determined attack against the forces holding it. The only warning which the Allies received was on the 20th, when the guns of the enemy began to bombard Ypres. Huge shells from the heaviest of guns fell in the streets, which were then thronged with citizens and our own reserves. Fifteen little children were killed at their play, and a number of the townsfolk perished amidst the ruins of their houses. Our generals understood at once the meaning of this bombardment. It was meant to block the roads to our lines on the salient, and make the work of sending forward supplies and ammunition very difficult, if not impossible. It was not meant to embarrass us at Hill 60, for we had free roads leading to that position from the west. It could only be the forerunner of an attack on that part of the salient extending from the Yser Canal to the Menin road--that is, on the portion held by the French Colonials, the Canadians, the 28th Division, and Princess Pat's. Our generals viewed the bombardment with anxiety; they knew that we were ill prepared to meet the attacks which were soon to follow. Thursday, 22nd April, was a peaceful day, warm and sunny. A light, steady wind was blowing from the north-east. About five in the evening an aviator reported that he had seen a strange green cloud, higher than a man, surging across the open ground from the German lines towards the French trenches. It was the deadly poison gas chlorine, which when taken into the lungs sets up acute bronchitis and causes its victims to die in horrible agony. At every fifty feet or so along the German front a battery of twenty retorts had been established. The gas from these retorts had been pumped at high pressure into huge reservoirs from which pipes ran to the front trenches. When the nozzles were turned on, the deadly gas rushed out, and was carried by the wind towards the French lines. Special respirators had been served out to the German soldiers, who were waiting in readiness to take advantage of this foul blow. Never before had poison gas been used in this manner on the battlefield. The Germans were about to sound the deepest depths of their infamy and try to poison those whom they could not beat in fair fight.