For long journeys over land, therefore, the aëroplane must come to earth for replenishment of fuel every five hundred miles. Even for this distance it cannot take more than one and one-half to two tons beyond the weight of fuel and crew. If heavier loads are to be transported, more machines must be used. Finally there comes a point at which a single airship, carrying a heavy freight over five hundred miles, is more economical than several aëroplanes. For non-stop flights of over one thousand miles the same considerations make the airship always more economical than the aëroplane. Over the ocean the flying boat can beat the dirigible in time and cost up to five hundred miles. Even at one thousand miles it is a commercial proposition, but it must then have all in its favor. For longer distances the airship has no competitor. It may be deduced that in years to come, when the world's airways are in general operation, heavier-than-air machines will bring freight to the great airports, there to be transferred to dirigibles and by them carried to the earth's uttermost ends. The time for this seeming Utopia is not yet, however, although a group of airship interests in England are now planning airship services that may eventually set London within two and a half days of New York, one and a half days of Cairo, four of Rio de Janeiro, five and a half of Cape Town and seven of Australia. But first must come bold expenditure, very careful organization, many-sided research and improved invention. Although no claim is made that present-day airships can compete for reliability with railroad trains and ocean liners, there is no doubt that a sufficient number of passengers are prepared to pay relatively higher rates for the great saving in time taken for long distance journeys, particularly over the ocean. The demand would be mainly for the carriage of express freight and mail matter and for passenger traffic to serve people who wish to get from center to center in the shortest possible time. Another use for large airships would be the carrying of freight of high intrinsic value, such as valuable ores, from places otherwise inaccessible, or not provided with other means of direct transport. To meet the requirements of various purposes for which airships may be utilized, dirigibles of four kinds are projected: First, the airship of moderate size and high speed for carrying express, mails and passengers. Secondly, the air liner solely for passenger traffic, of a large size and speed. Thirdly, the large airship of comparatively slow speed, and great carrying capacity, for general transport. Fourthly, the small non-rigid airship for private purchase and upkeep as an aërial yacht. [Illustration: THE VICKERS AEROPLANE WORKS AT WEYBRIDGE, ENGLAND] [Illustration: COMFORT CAN BE ENJOYED IN AIR TRAVEL TO-DAY]