Shovelling parallels of latitude

Not everyone was paying rapt attention as our geography teacher, S E ('Sam') Osborn, taught about co-ordinates on the earth's surface.  He unfurled maps and showed the parallels of latitude and meridians of longitude. He drew on the blackboard, carefully explaining the dimensions of each 10-degree division.

One boy, Alexander Rhind, was almost dozing as the teacher explained, emphasizing his consonants with precision as he always did.

"Parallels of latitude are lines marked on the earth's surface. They are made by men going round the earth with shovels to dig them out."

Suddenly he called, "Rhind! How are parallels of latitude formed?"

The boy jerked up.

"Er, men with shovels," he volunteered quickly.

The teacher gave him a withering look of derision. "You foolish child," he remarked.