He smoked for a minute or two without troubling to make his meaning clear; and then the _souffleur_ itself gave the answer. Armadale's ear caught the sound of a deep gurgle from the heights above their head; then came a noise like a giant catching his breath; and at last from the Blowhole there shot up the column of spray, towering white and menacing in the moonlight. As it fell, Sir Clinton pressed the self-starter. “That bolts the back door, you see, inspector. I only hope he's been caught on the threshold. Now, I think, we can go back to the hotel and see if we can pick up one or two useful things.” He turned the car on the last strip of sand before the rocks and swung it round towards Neptune's Seat. After a little searching, he found a spot from which he could ascend to the road without straining his springs. “I had the curiosity to examine that Blowhole cave at low tide once, inspector,” he explained as he drove up towards the hotel. “The thing works this way. The entrance is low, and the tide fills it soon. The air in the cave can still get out by a narrow tunnel leading up to the Blowhole. But in a minute or two this second tunnel's mouth gets filled up, and there's no escape from the cave. The sides are smooth, and the tide rises quickly, so that fellow will either drown or else he'll creep into the Blowhole tunnel to escape. The tide rises a bit farther, and compresses the air in the cave. At that stage the _souffleur_ begins to work. Intermittently, you get the air-pressure in the cave big enough to blow through the Blowhole tunnel, carrying the layer of water there in front of it; and that mixture of water and compressed air makes the jet. So, you see, if that fellow's in the cave, he must be swimming round like a rat in a pail; and if he's in the tunnel, he must be suffering agonies as the jet comes up and tears at him. You know what sort of force it has. And if he can't cling on to the rocks of the tunnel, he'll be battered against the sides as the jet carries him before it, and he'll probably be severely injured by the time it spits him out at the top.” “Good Lord!” said the inspector, as the realisation of the thing crept into his mind. “That's a nasty trap to fall into. He's going to get his gruel, sure enough.” They had reached the hotel, and Sir Clinton dispatched the constable to bring ropes, if any were available. “You don't seem eager to get him out, sir,” the inspector ventured, as they were waiting.