288 



SCIENCE 



[N. S. Vol. XXIX. No. 738 



orem, Cooper-Hewitt has independently- 

 computed curves for ships and hydroplanes 

 from actual data in water, and has em- 

 ployed these curves to solve analogous 

 problems in air, using the relative densities 

 of the two media, approximately 800 to 1, 

 in order to determine the relative values 

 of support by dynamic reaction and by dis- 

 placement for various weights and speeds. 



An analysis of these curves leads to con- 

 clusions of importance, some of which are 

 as follows : 



The power consumed in propelling a dis- 

 placement vessel at any constant speed, 

 supported by air or water, is considered as 

 being two thirds consumed by skin-resist- 

 ance, or surface resistance, and one third 

 consumed by head resistance. Such a ves- 

 sel will be about ten 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 (tor- 

 pedo 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 considera- 

 tion, higher speeds can be obtained in craft 

 of small tonnage by the dynamic reaction 

 type than by the displacement type, for 

 large tonnages the advantages of the dis- 

 placement of type are manifest. 



A dirigible balloon carrying 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 expenditure of 

 about one eighth of the power. 



As there are practically always currents 

 in the air reaching, at times, a velocity of 

 many 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 practical conditions of 



weather. In other words, it should have 

 substantially as much power as would drive 

 a boat, carrying the same weight, 25 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 combus- 

 tion motors at present available is suitable 

 for use with dirigible balloons. "With this 

 type, lightness need not be obtained at the 

 sacrifice of efficiency. In the aeroplane, 

 however, lightness per output is a prime 

 consideration, and certainty and reliability 

 of action is demanded, since if by chance 

 the motor stops, the machine must imme- 

 diately glide to the earth. A technical dis- 

 cussion of motors would of itself require an 

 extended paper, and may well form the 

 subject of a special communication. 



Propellers.— The fundamental principles 

 of propellers are the same for air as for 

 water. In both elements, the thrust is di- 

 rectly proportional to the mass of fluid set 

 in motion per second. A great variety of 

 types of propellers have been devised, but, 

 thus far only the screw-propeller has 

 proved to be of practical value in air. 

 The theory of the screw-propeller in air is 

 substantially the same as for the deeply 

 submerged screw-propeller in water, and 

 therefore does not seem to call for treat- 

 ment here. There is much need at present 

 for accurate aerodynamic data on the be- 

 havior of screw-propellers in air, and it is 

 hoped that engineers will soon secure such 

 data, and present them in practical form 

 for the use of those interested in airship 

 design. 



Limitations. — Euclid's familiar "square- 

 cube" theorem connecting the volumes and 

 surfaces of similar figures, as is well known, 

 operates in favor of increased size of 

 dirigibles, and limits the possible size of 

 heavier-than-air machines in single units 

 and with concentrated load. 



It appears, however, that both funda- 



