By one of those inexplicable quirks of puzzle land, it is necessary that you cross a desert in a steam-engine powered car. Your steam car is very reliable, but it can't carry enough coal to cross the whole desert without refuelling. Fully loaded with coal, it can travel just half way across. The car has a very efficient condenser, so water for the boiler is not a problem. You also have adequate supplies of food and drinking water. So you hatch a cunning plan... You will drive out some distance into the desert, leave some coal there, and then drive back to the starting point. You aim is to establish fuel depots out in the desert, enabling you to refuel on subsequent journeys. By this means, you hope to drive out further into the desert than would otherwise be possible. There is another constraint. You have just 8 full loads of coal at your starting point, and there is no prospect of more being delivered till next year. You decide to get as far as possible in your car, and (if necessary) abandon it and walk whatever distance remains. The desert is 1000 miles wide. What is your best strategy?