To thoroughly understand how a complete system of shallow underground works we must go abroad to Paris, Buda-Pesth, Boston, and New York. I may remark that in describing this system a certain amount of repetition is inevitable. PARIS Paris has its “Twopenny Tube,” or rather its equivalent. On July 30th, 1900, Londoners for the first time travelled by a deep-level line from the City to Shepherds Bush, a distance of 5·77 miles; and a few days earlier, on the 19th of the same month, the Electrical Chemin de Fer Métropolitain de Paris--the main channel of an elaborate system that links together every district of the capital--was opened for traffic. This chief artery connects at the fortifications, the Porte Maillot with the Porte de Vincennes, a distance of 6·6 miles. In other words, it crosses Paris diagonally from north-west to south-east; from a point at the north of the Bois de Boulogne to another at the north of the Bois de Vincennes; the eighteen stations (including terminals) on the main line being Porte Maillot, Rue D’Obligado, Place de L’Etoile (Arc de Triomphe), Avenue de L’Alma, Rue Marbœuf, Champs Elyseés, Place de la Concorde, Tuileries, Palais Royal, Louvre, Châtelet, Hotel de Ville, St. Paul, Place de la Bastille, Gare de Lyons, Rue de Reuilly, Place de la Nation, and Porte de Vincennes. On the Métropolitain there is a three-minutes’ service of trains during the day, and a six-minutes’ service at night. On the London Tube the intervals vary from two-and-a-half to three-and-a-half minutes in the day, while at night they are the same as in Paris, both railways being open for some twenty hours out of the twenty-four. In Paris two classes of passengers are provided for: first and second. The former are called upon to pay 2½_d._, the latter 1½_d._, for any length of journey. Up to nine a.m., second-class, or workmen’s tickets, are issued for 2_d._, the return half being available for the remainder of the day.