At that instant a big column of water shot upward and a dull rumbling could be heard. A few seconds later the little boat rocked violently from the effects of the waves. Then the sea became calm, and the Porpoise could be seen dancing up and down on the heaving billows.
"Now we can go back in safety," spoke Mr. Henderson, and Tom and Bill bent to the oars.
"What happened?" asked Mrs. Johnson. "The rock seemed to disappear before the explosion occurred."
"That's exactly what it was," explained the inventor. "By some strange freak of nature the volcanic mass dropped back into the ocean a little before I was ready to blow it to pieces. In settling down it lowered the ship. Then the explosion occurred beneath the waves. If I had waited a little while I need not have risked the dynamite."
"Well, there was no guarantee that the rock would go back where it came from," remarked Jack.
"No, we had to act," agreed the professor. "But now let us go aboard."
They rowed back to the Porpoise, which they found had sustained no damage from the queer experience it had been through.
The motors were set in motion and in a little while the craft was again moving through the water out of the dangerous vicinity of the volcanic area.
"Who has the two watches to-night?" asked Mr. Henderson after supper had been served.
"Washington and Andy," answered Jack, who kept track of the matter.