The shore line beyond was absolutely unknown to them, but it extended to the west as far as they could see, and when night set in the faint mist prevented them from judging how much farther it ran in that direction.
Without proper instruments at sea, distance is always a difficult matter to judge, and the boys were constantly venturing guesses as to the distance traveled. The start was made shortly after nine o'clock, and it was now past six in the evening.
"How far do you think we have gone during the day?" was George's question.
The Professor made a mental calculation before replying. "If we have traveled at the same speed during the entire course that we made during the first three miles to the cape, we have gone about thirty miles."
"Do you know it is three miles to the cape?"
"Yes, it is approximately that distance. I measured it by triangulation some time ago, using our house and Observation Hill as the base line."
The boys had neglected to take this precaution.
"So that we might have some means to observe the speed our boat could make. If we knew the speed of the wind, we might be able to calculate our distance."
"But the wind has been coming from the north and we are sailing due west. Would not the difference in the speed of the wind make a difference in the speed of the boat?"
"It was for that reason I stated if our rate of travel was the same we would have made that distance. The wind has been variable at different points along the coast, so that our average may have been four miles per hour."
"At what speed has the wind been during the day; I mean the average speed?"
"If the wind had been coming from the east we could have made much better time, and we might then have been near the mouth of the West River," was Harry's conclusion.
"Why do you think we should have made better time?" asked the Professor.
"Because we should then have been going with the wind."
"You are entirely wrong in your assumption. Sailing ships travel faster when tacking than when sailing with the wind."
The boys looked at the Professor in astonishment.
"It does not seem possible," replied George, "that any movement of the wind pushing sidewise could be more effective than a pressure straight ahead. Can you explain the reason for the statement?"