Spot On The Table

A boy, recently home from school, wished to give his father an exhibition of his precocity. 
He pushed a large circular table into the corner of the room, as shown in the illustration, so that it touched both walls, and he then pointed to a spot of ink on the extreme edge.

"Here is a little puzzle for you, pater," said the youth. 
"That spot is exactly eight inches from one wall and nine inches from the other. 

Can you tell me the diameter of the table without measuring it?"

The boy was overheard to tell a friend, "It fairly beat the guv'nor;" but his father is known to have remarked to a City acquaintance that he solved the thing in his head in a minute. 
I often wonder which spoke the truth.

See answer

Math Genius