MILITARY AERONAUTICS SQUlER. 143 



of resistance both to hull-shaped bodies and to aero-surfaces gliding 

 at small angles of flight." 



Relative dynamic and huoyant support. — Peter Cooper-Hewitt has 

 given careful study to the relative behavior of ships in air and in 

 water. He has made a special study of hydroplanes, and has pre- 

 pared graphic representations of his results which furnish a valuable 

 forecast of the problem of flight. 



Without knowing of Helmholtz's theorem, Cooper-Hewitt has inde- 

 jDendently computed curves for ships and hydroplanes from actual 

 data in water, and has employed these curves to solve analogous prob- 

 lems in air, using the relative densities of the two media, approxi- 

 mately 800 to 1, in order to determine the relative values of support 

 by dynamic reaction and by displacement for various weights and 

 speeds. 



An analysis of these curves leads to conclusions of importance, 

 some of which are as follows: 



The power consumed in propelling a displacement vessel at any 

 constant speed, supported by air or water, is considered as being two- 

 thirds consumed by skin-resistance, or surface resistance, and one- 

 third consumed by head resistance. Such a vessel will be about 10 

 diameters in length, or should be of such shape that the sum of the 

 power consumed in surface friction and in head resistance will be a 

 minimum (torpedo shape). 



The power required to overcome friction due to forward movement 

 will be about one-eighth as much for a vessel in air as for a vessel of 

 the same weight in water. 



Leaving other things out of consideration, higher speeds can be 

 obtained in craft of small tonnage by the dynamic reaction type than 

 by the displacement tjq^e, for large tonnages the advantages of the 

 displacement of type are manifest. 



A dirigible balloon carrjdng the same weight, other things being 

 equal, may be made to travel about twice as fast as a boat for the 

 same power or be made to travel at the same speed with the expendi- 

 ture of about one-eighth of the power. 



As there are practically always currents in the air reaching at 

 times a velocity of man}^ miles per hour, a dirigible balloon should 

 be constructed with sufficient power to be able to travel at a speed of 

 about 50 miles per hour, in order that it may be available under prac- 

 tical conditions of weather. In other words, it should have sub- 

 stantially as much power as would drive a boat, carrying the same 

 weight, 2.") miles an hour, or should have the same ratio of power 

 to size as the Lusitania. 



Motors. — It is the general opinion that any one of several types of 

 internal combustion motors at present available is suitable for use 

 Avith dirigible balloons. With this type lightness need not be ob- 



