Bab 737: 235. Quite reasonable, right?

Sound travels quickly through water, and its attenuation rate is much lower than when it propagates through air.

In order to ensure that the sound wave attack from the mechanos would not injure himself or the magic yacht, Noland Lee adjusted the direction of the High Sound Array, making it emit at a 30-degree angle in front.

A faint knocking sound came from some part of the ship's hull, contrasting sharply with the silent cabin.

Noland looked towards where the sound came from; it was a sea monster tapping the yacht's outer armor, trying to test the reality of the invisible object.

Finding an invisible object floating in the same spot where the Tide Spy disappeared was enough to make the sea monsters link the two.

Noland wrote a few lines in his diary and handed it to Hadley Grant:

"Drop these dozen or so metal balls I have overboard as quickly as possible."

Hadley nodded and reached for the metal balls on the tabletop with his right hand.