You know what dead-reckoning is, don't you?
If a captain can't see the sun he allows for how fast the ship is going, and for the time run and the currents, and all that, and then reckons up where he is.

I travelled with a captain once, and so long as he stuck to dead-reckoning he was all right.

He made out we were off Cairns, and that's just where we were; because we struck the Great Barrier Reef, and became a total wreck ten minutes after.

With the cattle it's just the same.

You'll reckon the cattle that you started with, add on each year's calves, subtract all that you sell,--that is, if you ever do sell any--and allow for deaths, and what the blacks spear and the thieves steal.