Then each must needs set his watch with Doctor Joe's, which was a little slower than Andy's and a little faster than David's.
Doctor Joe made some mental calculations.

Both David and Andy had observed their watches, and there could be no doubt of the length of time it had required them to come from Flat Point to Lem's cabin.

They had consumed four hours, but their progress had been exceedingly slow.
Indian Jake had doubtless travelled much faster in his light canoe, but, at best, with the wind against him, he could hardly have paddled from Lem's cabin to Flat Point in less than two hours.

He had arrived one hour after sunset.

If Lem were correct as to the time when the shooting took place, Indian Jake could not be guilty.
But still there was, with but one hour or possibly a little more in excess of the time between sunset and Indian Jake's arrival at camp, an uncertain alibi for Indian Jake.