What did the pirates find serving as a compass? What effect did it have on them

The pirates found a skeleton serving as a compass. The pirates didn't seem overly concerned about the bones.... merely curious.

Indeed, on a second glance, it seemed impossible to fancy that the body was in a natural position. But for some disarray (the work, perhaps, of the birds that had fed upon him or of the slow-growing creeper that had gradually enveloped his remains) the man lay perfectly straight--his feet pointing in one direction, his hands, raised above his head like a diver's, pointing directly in the opposite.

"I've taken a notion into my old numbskull," observed Silver. "Here's the compass; there's the tip-top p'int o' Skeleton Island, stickin' out like a tooth. Just take a bearing, will you, along the line of them bones."

It was done. The body pointed straight in the direction of the island, and the compass read duly E.S.E. and by E.

"I thought so," cried the cook; "this here is a p'inter. Right up there is our line for the Pole Star and the jolly dollars. But, by thunder! If it don't make me cold inside to think of Flint. This is one of HIS jokes, and no mistake. Him and these six was alone here; he killed 'em, every man; and this one he hauled here and laid down by compass, shiver my timbers! They're long bones, and the hair's been yellow. Aye, that would be Allardyce. You mind Allardyce, Tom Morgan?"