Siemens' invention is the plan by which the carriers are introduced into and removed from the tube at any required station without the circulation of the air being interfered with. When the hollow cylinder is in a line with the main tube, it is plain that the carrier will not be stopped, as the tube is then continuous and uninterrupted. It has been proposed to convey letters by pneumatic dispatch between the General and Suburban Post Offices, and the Post Office authorities have even consulted engineers on the practicability of sending the Irish mails from London to Holyhead by this system. There are two lines of tube-an "up" and a "down" line-and means are provided for rapidly transferring the carriages from one to the other at the termini. Great expectations have been formed by some persons of the applications of pneumatic force. Some have suggested its use for moving the trains in the proposed tunnel between England and France.