The other great plan of air navigation receiving the attention of scientists and aeronauts is the aeroplane system. Although the cohesive force of the air is so exceedingly small that it cannot be relied upon as a sufficient resisting medium through which propulsion may be accomplished alone by a counter-resisting agent like propeller blades, yet it is known what weight the air has and it has been ascertained what expanse of a thin plane is necessary without other means to support the weight of a man in the air. To this idea must be added the means of flight, of starting and maintaining a stable flight and of directing its course. Careful observation of the manner of the flight of large heavy birds, especially in starting, has led to some successful experiments. They do not rise at once, but require an initiative force for soaring which they obtain by running on the ground before spreading their wings. The action of the wings in folding and unfolding for maintaining the flight and controlling its direction, is then to be noted. It is along these lines that inventions in this system are now working. An initiative mechanism to start the ship along the earth or water, to raise it at an angle, to spread planes of sufficient extent to support the weight of the machine and its operators on the body of the air column, light engines to give the wing-planes an opening and closing action, rudders to steer by, means for maintaining equilibrium, and means when landing to float upon the water or roll upon the land, these are the principal problems that navigators of the great seas above us are now at work upon. CHAPTER XXX. ILLUMINATING GAS. “How wonderful that sunbeams absorbed by vegetation in the primordial ages of the earth and buried in its depths as vegetable fossils through immeasurable eras of time, until system upon system of slowly formed rocks have been piled above, should come forth at last, at the disenchanting touch of science, and turn the light of civilised man into day.”--_Prof. E. L. Youmans._ “The invention of artificial light has extended the available term of human life, by giving the night to man’s use; it has, by the social intercourse it encourages, polished his manners and refined his tastes, and perhaps as much as anything else, has aided his intellectual progress.”--_Draper._