If a stranger were allowed to travel in one of these carriages, the first thing that would probably take his notice would be the brilliant light which fills the interior; and the necessity for a good light to enable men, standing on a vibrating and oscillating floor, to read quickly all sorts of manuscript addresses, will be understood by whoever has attempted to peruse writing by the light derived from the ordinary oil-lamps of a railway carriage. Yet for years the light supplied in the Travelling Post-office has been given by improved oil-lamps, though more recently gas has been introduced in some of the carriages. The next thing he would notice would likely be the long series of pigeon-holes occupying the whole of one side of the vehicle, divided into groups--each box having a name upon it or a number, and a narrow table running along in front of the boxes, bearing a burden of letters which the sorters are busily disposing of by putting each one in its proper place--that is, in the pigeon-hole, from which it will afterwards be despatched. Then hanging on the walls or lying under the table will be seen canvas bags and canvas sacks, each having its name stencilled in bold letters on its side; and somewhere about the floor great rolls of black leather, with enormously strong straps and buckles--the expanse of leather in each roll being almost sufficient to cover an ox. The use of these hides of leather will be described further on. The _raison d'être_ of the travelling post-office is to circumvent time,--to enable that to be done on the way which, without it, would have to be done before the train started or after it arrived at its destination, at the expense of time in the doing, and to collect and dispose of correspondence at all points along the route of the train--which correspondence would otherwise in many cases have to pass through some intermediate town, to be detained for a subsequent means of conveyance. The T.P.O. is one of the most useful parts of the machinery of the Post-office. Among the smaller things that might be observed in the carriage would be balls of string for tying bags or bundles of letters, cyclopean sticks of sealing-wax, a chronometer to indicate sure time, a lamp used for melting the wax, and various books, report-forms, seals, &c.