He could not.

A gag was fixed firmly in his mouth, his arms were bound to his side, his legs to the tree on which he sat, and his rifle had been left standing between his knees and against his shoulder, as if held by one who watched.
The unfortunate sentinel gazed up at his chief with wide-open, appealing eyes, and, leaving him with the men, who were now crowding around he ran to the other sentinel.

Pedro, only to find him gagged and bound, exactly like his comrade.

It was some minutes before either could speak, after they were cut loose and their gags removed, and then their tales were the same.
"I watched.

I watched well, Captain," said Luiz, "by the Holy Virgin I swear it! Never in this whole terrible night, not for a moment, have my eyes closed.