"Yes, our aërial ships are large and stanch; but it would be foolhardy to attempt to reach the pole in one of them. We, of course, depend on their lightness to overcome gravitation; now, the lightest gas we can get is hydrogen, and this we use. With our vessels filled with this gas, we have no trouble in making from twenty to fifty, and even a hundred miles per hour, according to the wind. But here comes in the greatest factor in aërial navigation; how to make up the gas discharged in changing altitudes and lost by exudation through the skin of the balloon. In nearly every great city large quantities of hydrogen are kept in store for filling the balloons of such vessels as may arrive and require replenishment. So long as a vessel is kept within a day's journey of one of these cities, it is easy to keep sufficient gas in the balloon, and thus to travel from point to point; but as there are no hydrogen works north of latitude fifty-four degrees fifteen minutes, and as the distance from there to the pole is over 2,200 miles, and the same distance back again, and as, again, the speed of an aërial ship depends upon the direction of the wind, and its velocity--the maximum speed in a perfectly tranquil atmosphere being only forty-five miles per hour--it will easily be seen that a period of one hundred hours, and perhaps very many more, would elapse ere the ship could return to the starting point. As a fact, the loss of hydrogen will be so great that, unless replenished, the vessel will lose its carrying power ere thirty hours have passed. Thus you see, Junius, it is impossible to use the aërial ship to reach the pole." "But can you not carry material to keep your supply of hydrogen up to the amount required?" asked Cobb, eagerly. "No. The amount would be too great to manufacture in the time which would be at one's command; besides, the apparatus would be too heavy for the balloon to carry." "Then, I understand that, if you could manufacture this gas in sufficient quantities on the ship, and by light apparatus, you could go anywhere?" Cobb spoke the words slowly, as if lost in some deep thought. "Certainly," replied Hugh. "But that is a discovery which I doubt much will ever be accomplished!" "Perhaps." "Perhaps?" "Yes, I said perhaps," returned Cobb, with a complaisant smile. Then, inquiringly: "Will you show me your finest aërial ship to-morrow?" "Of course you will see it if we start to-morrow, as we have agreed." "But do not agree to start to-morrow. Show me your ship, as I have not seen them closely, and I will be ready to start soon after." "Well, if you wish it, Junius, I will do so; but I do not understand the reason for your request." "You will see," quietly returned Cobb.