A little later the submarine began the descent into the lower ocean depths. From three hundred and fifty feet she went to four hundred, and when the hand on the gauge showed four hundred and fifty there was a tense moment. If anything went wrong now there would be serious trouble.
But Tom Swift and his men had done their work well. The M. N. 1 stood the strain, and when the gauge showed four hundred and ninety feet Mr. Damon gave a faint cheer.
"Bless my apple dumpling, Tom!" he replied, "this is wonderful."
"Oh, we've been deeper than this," replied the young inventor, "but under different conditions. I'm glad to see how well she is standing it, though."
Suddenly, as the needle pointer on the depth gauge showed five hundred and two feet, there came a slight jar and vibration that was felt throughout the craft.
"What's that?" suddenly and nervously cried Mr. Hardley. "Have we struck something?"
"Yes, the bottom of the ocean," answered Tom quietly. "We are now on the floor of the Atlantic, though several hundred miles, and perhaps a thousand, from the treasure ship. We bumped the bottom, that's all," and as he spoke he brought the submarine to a stop by a signal to the engine room.
And there, as calmly and easily as some of the masses of seaweed growing on the ocean floor around her, rested the M. N. 1. It was a test of her powers, and well had she stood the test, though harder ones were in store for her.
And inside the submarine Tom and his party were under scarcely greater discomfort than they would have been on the surface. True, they were confined to a restricted space, and the air they breathed came from compression tanks, and not from the open sky. The lights had to be kept aglow, of course, for it was pitch dark at that depth. The sunlight cannot penetrate to more than a hundred feet. But sunlight was not needed, for the craft carried powerful electric lights that could illuminate the sea in the immediate vicinity of the submarine.
"Are you going to stay here long?" asked Mr. Hardley, when Tom had spent some time making accurate readings of the various instruments of the boat. "Of course, I realize that you are the commander, but if we don't get to the treasure ship soon some one else may loot her before we have a chance. She's been given up as a hopeless task more than once, but the lure of the millions may attract another gang."