With a deep thumping explosion the howitzer launched its shell. The muzzle velocity of the weapon was lower than that of a standard artillery piece and Arthur could see the faint dark smudge that marked the passage of the shell as it arced across the river towards the enemy battery sheltering behind the rocks. There was a sudden puff of smoke in the air just above the enemy guns and Arthur saw an entire gun crew topple to the ground, directly beneath the point where the shell had burst and scattered its lethal fragments of iron.

‘Right on target!’ Harris cried out. ‘Range is good. Fire at will.’

Two of the next four rounds killed more of the men working the French guns, and then, as Arthur watched, their officer began to shout and gesticulate and the survivors hurriedly began to limber their guns and withdraw back up the track, though not before one shell struck down two horses in their traces, causing the whole team to veer sharply to one side so that horses, riders, limber and gun toppled over the edge of the track and tumbled down the slope in a shower of small rocks and dust before splashing into the river.

Arthur saw that the men in the convent were now out of danger, and the barges were safely and steadily feeding fresh troops into the fight. By contrast the French battalions joining the melee at the foot of the cliff were forced to undergo a steady hail of grapeshot fired at them from across the river before they even faced the muskets of the men of the Third Foot. It was no wonder that their attacks on the convent were half-hearted, Arthur reflected.

A fresh column of French troops had left the city and were making for the track to reinforce the assault on the convent. Three battalions of them, Arthur calculated. He turned to take a quick glance at the rest of Oporto and noticed that a small crowd of people had emerged on to the quays to the left of the remains of the demolished bridge. He examined them through his telescope and saw that they were civilians. More and more of them appeared, rushing out from side streets and racing towards the boats that Soult had ordered to be moved to the north bank. There was no sign of any French soldiers along the quay and Arthur guessed at once that Soult had been forced to strip men from that part of the city to send them against Waters and the men in the convent. The Portuguese swarmed aboard the boats and the first of them began to row across towards the south bank.

‘Poor devils,’ Harris muttered as he stood beside Arthur. ‘Taking their chance to escape from the French, I imagine.’

‘Escape be damned,’ Arthur replied. ‘They’re coming to help us get across.’