The difficulty of transmitting from London to every part of the United Kingdom, and _vice versâ_, the innumerable quantity of letters which, like mushrooms springing up from a bed of spawn, have arisen from our sudden adoption of a penny-postage, would alone require minute calculations, involving an infinity of details; but when it is considered that besides this circulation from and to the heart of the metropolis--(the average weight of letters and newspapers carried daily by the London and North-Western Railway is seventeen tons)--there exists simultaneously a cross circulation, not only from and to every great city and town, but from every little post-office to every part of the United Kingdom and _vice versâ_, and moreover to every region on the globe, the eccentric zigzag courses of all these letters to their respective destinations may justly be compared to the fiery tracks and sparks created by the sudden ignition of a sackful of fireworks of all descriptions; of rockets, Catherine wheels, Roman candles, squibs, stars, crackers, flower-pots, some flying straight away, while others are revolving, twisting, radiating, bouncing, exploding in every possible direction and in all ways at once. To explain the mode in which all our postal arrangements are conducted would not only exceed our limits, but be foreign to our subject; we will therefore only attempt to supply our readers with a slight sketch of a very small portion of this business, namely, the transmission of letters from the metropolis by the London and North-Western Railway’s night mail. While the passengers by the Lancashire mail-train are taking their seats and making other preparations for their departure, two or three Post-office vans are seen to enter the main carriage gate of the Euston Station, and then to drive close to their tenders on the railway, which form the last carriages of the train. The servants of the Post-office, rapidly unloading their vans, remove a portion of the bags they contained into the travelling-office and the remainder into two large tenders, which, as soon as they are filled, are locked up by the guard, who then takes his place in the flying office, in which we propose to leave him to his flight for 132½ miles--only observing, however, that no sooner has he started than another flying post-office, which had been lying in ambush, advances (with its tender), and, after being loaded in a similar manner, in a quarter of an hour they are despatched to Yorkshire and the East of Scotland. * * * * *