There was no time to be lost. The Captain on the bridge had to decide quickly. Checking his first impulse to ram the submarine--he remembered the possibility of having the propeller blades smashed and the rudder buckled--he bore away a couple of points, at the same time ordering the Q.F. guns to open fire. A moment later he countermanded the order, for a destroyer, observing the pole-like object in the slanting rays of the early morning sun, starboarded helm and charged straight for the periscope. Her youthful lieutenant-commander, in his zeal, had but one thought--to smash the submarine's hull with the destroyer's knife-like stem before the former could fire her torpedo at the increasingly favourable target that the _Rebound_ was momentarily presenting. In vain the battleship signalled to her to stand clear and destroy the periscope by means of gunfire and then finish off the blinded submarine with depth-charges. All on the destroyer's bridge had eyes for nothing but the hostile periscope. The _Rebound_ could do nothing, Already the destroyer was masking her quick-firers. A warning blast from the syren did attract attention, but only when it was too late. The destroyer's bows hit the periscope fairly and squarely. There was no rending of steel, no release of air and oil from the submarine, for the simple reason that there was no submarine there. The periscope was a dummy, but to it were attached two mines by means of long spans of wire. Five seconds later, the mines, swung inwards by the strain upon the spans, exploded simultaneously on either side of the destroyer. Before the upheaval of smoke and spray had dispersed, the luckless destroyer had vanished, leaving half a dozen men swimming aimlessly in an ever increasing pool of oil. Up dashed another destroyer, the survivors were picked up, and the little craft hurried on ahead of the battleship, with paravanes towing in order to detonate any other mines that might be in the vicinity. The lesson, obtained at a price, was not thrown away. It proved that the Rioguayans had not resolved to defend the river by means of submarines, otherwise they would not have indiscriminately sown mines which would prove a menace not only to the British surface craft but to their own submarines.