304 Introduction to the Study of Science 



combination of steering and propelling by a motor made 

 possible the present airship of the Zeppelin type. Such an 

 airship, combining motor and balloon, is called a dirigible. 

 The characteristic shape of the Zeppelin dirigible is determined 

 by the conditions which it must meet. What are these condi- 

 tions ? How does the form of the balloon help in meeting them ? 



II. THE AIRPLANE 



182. Early experiments. The belief had long been held 

 by a few individuals that a " heavier-than-air " machine, lifted 

 by its own power and sustained by the pressure of the air 

 against its surfaces, after the fashion of a kite, could be made to 

 work successfully. In short, a machine, resembling somewhat 

 a bird in form, seemed altogether possible. Dr. Lilienthal, a 

 German, experimented with wings having an area of about 160 

 square feet, or about one square foot for each pound of his 

 weight. By running down a hill, up which the wind was blow- 

 ing, he found the current sufficiently strong to lift and float him. 

 Unfortunately the wings collapsed when he was in mid-air and 

 the fall killed him. Others tried the same plan, with fatal 

 results in most cases. These experiments, however, brought 

 knowledge of important conditions of equilibrium in air, upon 

 which any flying machine depends. As a child must learn to 

 maintain balance and walk before he can run, so man must 

 master the conditions of maintaining equilibrium in the air 

 before he can fly. 



Two remarkable series of experiments were being carried on 

 while Dr. Lilienthal and others were trying the wing-sails. Sir 

 Hiram Maxim took up Jbhe problem and tried first to find the 

 best possible fabric for the planes, testing silk, cotton and woolen 

 fabrics, crepe, linen, and other materials. His tests proved that 

 lifting power of linen is much greater than its tendency to drift 

 horizontally in the direction with air currents. He then worked 

 to construct a steam engine of maximum power and minimum 

 weight. Finally he built a machine, weighing about 7500 



