The railway post-office proper, is now extended over nearly every considerable line of railway in the kingdom. It comprises a number of divisions or sections, named generally from the locality through which they extend, or the railway travelled over, as the Bangor and Leeds division, the Caledonian Railway post-office. The four principal or trunk mails, three of them being divided into two sections, are (1) the North-Western Railway post-office, travelling between London and Carlisle; (2) the Irish Mail, between London and Holyhead; (3) the Great Western, between London and Exeter; and (4) the Midland, between Bristol and Newcastle-on-Tyne. Most of these divisions have _day_- as well as _night_-mails running over them daily. Four trains a-day, being two in each direction, are therefore the usual proportion of mails on the chief lines of railway. As London is the _heart_ of the postal system, so these four principal mails may be termed its _main arteries_, while as veins in the great system, there are a number of smaller divisions of the railway post-office that have not been enumerated. Again, at other parts or points not important or extensive enough for travelling offices, railway trains are arranged to wait the arrival of the trunk mails; and thus, to continue the figure, our letters--the life-blood of a nation's commerce and sociality--are conveyed to the remotest corners of the country. It may be imagined that a proper control of this vast machinery, extending through almost every county in the kingdom, with its scattered staff of officials, will be difficult; but the efficient working of the whole is nevertheless as thoroughly and promptly maintained as in any other department where personal supervision is more direct. Each divisional part has distinct officers allotted to it, the number of _clerks_ being regulated according to the number of mails running over the division in the course of a day, and the number of _sorters_ according to the amount of sorting duties to be performed. Each mail travels under the charge of one clerk, while each division is locally superintended by one senior clerk. The entire direction, however, of all the travelling officers is vested in the Inspector-General of Mails, who also presides over the Mail Office at St. Martin's-le-Grand. We may here further state, that the _length_ of the divisions--the extent of one of which forms a post-office journey or "trip"--varies slightly, averaging about 170 miles; the average _time_ taken to perform the journeys being between five and six hours. As a rule, the night-mails travel during the night-time, or between eight P.M. and six A.M.; the day-mails generally speaking throughout the day.