On the 20th of July all the batteries opened.
In the night between the 20th and 21st of July, early in the evening, the enemy abandoned the circular redoubt: a working party of seven hundred men had been prepared to open a parallel across the isthmus, but the night proving extremely dark, tempestuous, and rainy, the men dispersed among the ruined buildings of St. Martin, and not more than two hundred could be collected together; therefore only about one-third of the parallel and the right approach to it were opened.
On the 21st of July, Sir Thomas Graham sent a flag of truce with a summons to the governor, but he would not receive it.
In the night between the 21st and 22nd of July, the left communication and the remainder of the parallel across the isthmus were opened; the parallel near its left crossed a drain level with the ground, four feet high and three feet wide, through which ran a pipe to convey water into the town. Lieutenant Reid ventured to explore it, and at the end of 230 yards, he found it closed by a door in the counterscarp, opposite to the face of the right demi-bastion of the hornwork; as the ditch was narrow, it was thought that by forming a mine at this extremity of the drain, the explosion would throw earth sufficient against the escarpe, only twenty-four feet high, to form a road over it; eight feet at the end of the aqueduct was therefore stopped with filled sand-bags, and thirty barrels of powder, of ninety pounds each, were lodged against it, and a saucisson led to the mouth of the drain.
On the 23rd of July the breach between the two towers, about one hundred feet in length, being considered practicable, the fire of all the guns was concentrated on a part of the wall to its left to effect a second breach, and by evening, that also was considered practicable on a front of thirty feet. At the same time, the four ten-inch mortars and the sixty-eight-pounder carronades were turned on the defences and on the houses in rear of the breach, to prevent the enemy working to form an obstacle to them.
The breaches were to have been stormed at daylight on the 24th, at which time the tide was out, and the troops were formed in readiness; but the houses at the back of the breach being on fire, it was supposed they would prevent the advance of the troops when they had gained the summit, and in consequence the order was countermanded.
The next night a trench was opened in advance of the parallel, to contain a firing party on the hornwork, during the assault.
The assault was ordered to take place at daylight on the 25th; the storming party, about 2,000 men, were to assemble in the trenches, and the explosion of the mine was to be the signal to advance.
The distance of the uncovered approach, from the trenches to the breach, was about three hundred yards, in face of an extensive front of works, over very difficult ground, consisting of rocks covered with sea-weed, and intermediate pools of water; the fire of the place was yet entire, and the breach was flanked by two towers, which, though considerably injured, were still occupied.
At five A.M. the mine was sprung, and destroyed a considerable length of the counterscarp and glacis, and created so much astonishment in the enemy posted on the works near to it, that they abandoned them for the moment, and the advance of the storming party reached the breach before any great fire was brought to bear on them: on their attempting to ascend the breach, the enemy opened so heavy a fire, and threw down such a number of shells, &c., from the towers on the flanks, and from the summit of the breaches, that the men began to waver, and in a short time the assaulting party had returned into the trenches, with the loss of nearly one hundred killed and four hundred wounded.