At this moment a piercing cry for help rent the air, and the twins instantly rested upon their oars, and listened eagerly for a repetition of the cry. Alas! there was none; the silence of night was again upon the river. "Oh, Ralph!" said William, "that was a genuine cry for aid; it came from some poor creature in deadly peril. Oh! what can we do?" "We will respond to it, by the help of God," replied Ralph; "it came, surely, from that dark wherry which I see yonder preparing to shoot the bridge." "I thought so also," said William, "and methought I recognized the voice of him who called for help; it rang into my very soul, and, if I err not, it was the voice of our friend Diego!" "To the rescue! to the rescue!" cried Ralph in reply, and in an instant they had turned their boat down-stream and were following the suspicious wherry. Their light boat soon brought the heavier wherry into full view. They could see that there were three men on board of her; two were rowing, the third held the tiller. "What are our plans, William?" said Ralph; "do you take the lead, and I will second you promptly." "Agreed," replied his brother. "I propose, then, that we follow that wherry whithersoever it goes. If those men have a captive on board, they will soon seek to lodge him in durance—that will be our moment of attack. "For the present we keep within reach of them, but sufficiently far off to disarm their suspicions. "Leave the boat to me, I will row, and do you keep a vigilant eye on their movements. Loosen your poignard in its sheath—I will do the same— for this matter will not be decided without bloodshed, and may God defend the right!" "Amen," said Ralph solemnly, yet with a distinct sound of joyous exultation in his voice. No fear, no misgiving, found place in their brave young souls! On the contrary, they rejoiced in the thought and belief that this was a call from Heaven, that they were God's ministers in carrying out a work of mercy and justice! A minute later both boats shot beneath London Bridge at a furious pace, the temporarily imprisoned tide hurling them on its strong bosom down-stream. "They are making for the Surrey side," said Ralph; "it strikes me that they are going aboard one of those wretched hulks which line the shore; if so, what then?" "I think you are right," replied his brother; "they would not dare to land their victim on shore, where they would at once encounter the watchmen. If these men are Thames pirates, as I strongly suspect, then these dark black hulks are their fitting and foul nests.