We had brought the axe along, and without delay we began to cut and pile into the exit passage more limbs and branches and rocks, until the place was full to a level somewhat above high-tide mark. To facilitate further the formation of this bank, we cast many branches into the waters of the chasm just back of the newly formed sand-bar, that it might be caused if possible to rise high enough to prevent any through current and consequent scouring action even at high tide.
We tried hard to ascertain whether the sand had begun to accumulate at the breakwater across the mouth; but were unable to do so because of the lack of transparency of the water, which held in suspension a large percentage of sand and foreign matter stirred up by the swell. We both expressed our confidence that as soon as the current through the chasm was stopped, the sand would begin to silt in and fill up the main breakwater.
It was two hours after noon when we returned to the house again. After dinner we all three turned to the work of digging sweet potatoes in my old garden, and storing them under the shed. All the crops were doing finely and we found some green Indian corn just ripe enough to boil. In the cool of the evening we sat under the shed to watch for the new moon to rise, discussing the theory and probable action of sand deposit by waves.
Mr. Millward’s theory—and I believe it to be the correct one—was that the sand was held in suspension only while the water was in very considerable motion; and that it fell to the bottom almost instantly when the motion of the water ceased. He likened it to stirring sugar, not yet dissolved, in a glass of water. As soon as the stirring stopped the sugar fell to the bottom. “Thus, for example,” he explained, “when a wave comes up on the beach in front of us, it is more or less charged with sand; the sand is deposited just when the wave has spent its force and paused before the return flow. But, of course, the sand so deposited on the naked beach would be picked up again and carried back, to be again brought up, and so on in ceaseless round.” And the reason, he insisted, why the sand gathered at the last breakwater in the chasm instead of at the first one, was simply because at that place the current and consequent motion were least.
Each morning I walked up the beach to the chasm, sometimes alone and sometimes accompanied by Mr. Millward. In two days the breakwater at the exit was completely covered with sand, which rose above the level of high tide, and the sand had already begun to silt into the chasm back of the exit. On the third day we found a great shark embayed in the chasm and dashing around the old hull every little while, as though in a flurry of excitement. Each time a wave would break in he would endeavor to swim out, following the retreating water,—for now there was no longer a current through,—but the trees and limbs prevented him. Mr. Millward said he seemed like an evil spirit set to guard the galleon and its treasure; and indeed it would have been a dangerous thing for any one to attempt exploration of the wreck while this man-eating sentinel patrolled the narrow water where she lay.
The sight of this voracious fish reminded me very forcibly of the great danger which would have attended any attempt to reach the hull by diving when the galleon lay out in the sea. Had I brought my diving-apparatus safely to the island, as I originally intended to do, it is quite possible, and even probable, that I should have found a grave among the gastric fluids of some such shark. Strangely enough, in all the many times I had looked at the wreck through the water-glass while it lay out in the sea, I had never seen a single shark, though other fish had been visible in considerable numbers and variety. But doubtless, as is the treacherous nature of this tiger of the sea, he was lying there concealed and instinctively watching the boat in expectation of prey. It made me shudder involuntarily to think of the possible encounter that I might have had. As the shark imprisoned in the chasm was no present inconvenience to us, we allowed him to remain undisturbed where he was.
As we stood on the rock which adjoined the shore, watching the frothing of the surges through the breakwater at the mouth of the chasm, I pointed out to Mr. Millward that every few minutes, at intervals of about every third wave, the water rushing back met the incoming wave exactly at the breakwater and the resulting interference produced there a temporary quiet in the waters.
“Now,” said I, in reference to this fact, “if your theory is right we ought to be getting a discharge of sand at the breakwater every time there is such a meeting of the waters there.”
“Of course,” said he, “there is no doubt of it; and we shall soon be having a bar at this point. Whether this bar will rise high enough to stop the water materially from coming in before the whole chasm has silted full of sand is something we cannot determine except by waiting to find out by actual test.”
“Nor does it greatly matter,” I added, “for in either event the galleon would be safely housed.”