The patent log is an instrument for recording the distance sailed by the ship. It consists of a dial on the outside of a case, inside of which are wheels to turn the hands. Attached to the machinery is a long line, at the end of which is a rotator shaped somewhat like a ship's propeller. This rotator drags through the sea, and makes a certain number of revolutions every mile, twisting the line, and thus turning the hands on the dial, where the number of knots is marked.
"It's a new log, isn't it?"
"Yes, sir; I received it at Queenstown."
"Good. What is its percentage of error?"
"Two per cent."
Patent logs usually overrate the distance run, and the percentage of error has to be ascertained.
"It's running fully two per cent. now, I fancy," said the Captain, stepping to the speaking-tube that ran to the engine-room and calling for the chief engineer.
"Hello, Mr. Bargot! How many revolutions are you making a minute?"
A jumble of figures returned through the tube.
"And that's nineteen knots, isn't it?" said the Captain. "Good."