Offices 209 21,551 Post roads (miles) 5,642 178,670 Revenue of Post-office £22,800 £1,207,000 Expenses of Post-office £22,800 £1,207,000 Expenses of Post-office £15,650 £1,130,000 Mileage of trains 4,560 Total length of 100 miles would be a very simple formula:-- Magnification = focal length of.