"How far did you travel down the chasm ?" "Ten miles, at least," replied Rod.
"And you discovered no fall ?" "No." With a splinter picked up from the floor Wabi measured the distances between the different points on the diagram.
"There is no doubt but what this map was drawn by John Ball," he said after a few moments of silent contemplation.

"Everything points to that fact.

Notice that all of the writing is in one hand, except the signatures of Langlois and Plante, and you could hardly decipher the letters in those signatures if you did not already know their names from this writing below.

Ball wrote a good hand, and from the construction of the agreement over the signatures he was a man of pretty fair education.
Don't you think so?
Well, he must have drawn this map with some idea of distance in his mind.

The second fall is only half as far from the first fall as the third fall is from the second, which seems to me conclusive evidence of this.