(introduction...)

Farming with animal power is costly.

The farmer spends money on buying his animals and tools, on feeding and looking after the animals, on mending the tools.

The animals get old, the tools wear out.

After five or six years, you have to buy new oxen and new tools.

The farmer must know how much money he spends on farming with animal power.

He must know what it costs him.

The farmer also knows how much money he makes from working by hand.
He knows how much money he gets from working with animals.

So he knows how much more money he gets from the use of animal power.

The extra money earned with the animal power, less the money spent on it, is the income from animal power.