282 



SCIENCE 



[N. S. Vol. XXIX. Jvo. 73$ 



airship, yet the experience already avail- 

 able in the construction and performance 

 of such ships built on different plans is 

 sufficient to enable the engineer to proceed 

 with the design of a dirigible balloon to 

 accomplish definite results along fairly 

 accurate lines. In the case of this class 

 of lighter-than-air ships the following gen- 

 eral equation obtains: 



■.y(a — <T/n) 



where' 



W ^weight of balloon, envelope, car and aero- 

 nauts, 



y = volume of balloon, 



a = density of the air, 



n = density of air as compared with gas, 



w = weight of air displaced by car and aeronauts 

 and envelope of balloon. 



If we call the weight of the gas in the 

 balloon M, then we can write this equation 

 in the following manner: 



W + M = w + nU, 



from which we find that 



M-= (W — w/n — 1) 



and 



F=[(W — w)/<r] [w/(«,— 1)], 



thus obtaining the volume of gas required. 

 If the volume of the gas-bag, car, aero- 

 nauts, etc. = V, then w = vo- ; so that the 

 preceding equation may be written 



Thus far, certainly, no dirigible baUoon 

 has ever been developed which has attained 

 an independent speed greater than forty 

 miles per hour. It will readily be admitted 

 that an airship so designed as to reach a 

 speed of fifty or sixty miles per hour would 

 be regarded as a most decided step forward 

 in the art, since this difference of velocity 

 is just the increment needed to place such 

 craft on a practical basis capable of ma- 

 neuvering in the air in all ordinary weather. 

 This advancement, although requiring much 



consideration, would fully compensate in 

 practical results. 



The first point to be decided upon in the 

 design of an airship is the method of main- 

 taining the shape of the gas-bag against the 

 pressure encountered at the maximum 

 velocity to be attained. There are two 

 schools of design in this respect, each 

 having its adherents. One maintains the 

 shape of the gas-bag by a rigid interior 

 frame, and the other by means of the in- 

 ternal pressure of the gas itself. 



Upon the selection of the type depends 

 to a large extent the particular shape of the 

 envelope. If the envelope is to maintain 

 its shape by interior pressure of gas, evi- 

 dently it must be so designed that the maxi- 

 mum pressure of the air developed at the 

 speed contemplated shaU not be sufficient 

 to cause deformation of any part of the 

 envelope. This can be effected only by 

 making the uniform internal pressure at 

 least equal to the maximum external pres- 

 sure. Since the maximum external pres- 

 sure occurs over the prow of the airship, 

 this, evidently, is the particular part which 

 must receive most careful attention with 

 this system. 



The desirable shape of head would evi- 

 dently be one where the distribution of 

 external pressure due to air resistance at 

 the velocity used is uniform. In addition 

 to preventing deformation of the gas-bag, 

 a prime requisite also is that the shape 

 shall be such that the total resistance, com- 

 prising head resistance and skin-friction, 

 shall be a minimum for a given displace- 

 ment and velocity. 



This immediately forces the question of 

 the law of resistance of the air. On this 

 subject there are numerous aerodynamic 

 data for low velocities, and also for very 

 high velocities, but such data are incom- 

 plete for the range of velocities here con- 

 sidered. 



In fact, the law of resistance of the air 



