The reason is that the cost of carrying letters differs greatly as between one letter and another. The cost of carrying a letter from house to house in the same terrace is so small that it cannot be expressed in money: it is as near nothing as does not matter: to get a figure at all you would have to take the cost per thousand letters instead of per letter. But the cost of carrying the same letter from the Isle of Wight to San Francisco is considerable. It has to be taken from the train to the ship to cross the Solent; changed into another ship at Southampton or perhaps at Liverpool after another train journey; carried across the Atlantic Ocean; then across the continent of North America; and finally delivered at the opposite side of the world to the Isle of Wight. You would naturally expect the Postmaster-General to deliver a dozen letters for you in the same terrace for a penny, and charge you a pound or so for sending one letter to San Francisco. What he actually does for you is to deliver the thirteen letters for three-halfpence apiece. By the time these lines are in print he may be charging you only a penny apiece, as he used to before the war. He charges you less than the cost of sending the long-distance letter, and more than the cost of sending the short-distance letters; but as he has thousands of short-distance letters to send and only dozens of long-distance ones he can make up for the undercharge on the long by an overcharge on the short. This charging the same for all letters is called by economists averaging. Others call it gaining on the swings what we lose on the roundabouts. Our reason for forbidding private persons or companies to carry letters is that if they were allowed to meddle, there would soon be companies selling stamps at threepence a dozen to deliver letters within a few miles. The Postmaster-General would get nothing but long-distance letters: that is, the ones with a high cost of carriage. He would have to put up the price of his stamps; and when we found that the advantage of sending a letter a mile or two for a farthing was accompanied by the disadvantage of paying sixpence or a shilling when we wanted to write to someone ten miles off, we should feel that we had made a very bad bargain. The only gainers would be the private companies who had upset our system. And when they had upset it they would raise their short-distance prices to the traditional penny, if not higher.