THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 175 
but not so very much lower; at any rate the test of experience has shown that they 
are sufficient. 
As stated in Professor Hovgaard’s paper, the forces to be sustained by the airship are 
three-fold—the weight of the ship, the buoyancy, and gas pressure—all of which would be 
quite definitely known in any given case, and the aero-dynamic forces due to atmospheric pres- 
sure, when the ship is being propelled in service. These last forces are gradually becoming 
known by experiment, and fuller knowledge of them will be available in the near future. 
As to the theory of the longitudinal strength of which Professor Hovgaard treats in this 
paper, it is quite clear that the ship, as a whole, bends like a truss or beam, but unfortunately 
the number of parts is so great that it is impossible to apply the method of least work or 
other method, based on the theory of elasticity to determine the stresses. 
The best we can do is to use approximate methods and check them up by careful rea- 
soning, with the expectation of some day getting a sufficient number of strain measurements 
on airships in service to arrive at more correct results. 
Professor Hovgaard has added very much to our knowledge of the theory of the stresses 
on which there has been a lot of loose reasoning in the past. His studies have helped our 
committee to get a much closer idea of what the stresses are in this complex structure, and 
the present paper constitutes a decided advance on what has previously been published on 
the subject. 
Mr. Ermer A. Sperry, Member (Communicated) :—I wish to add my word of appre- 
ciation of the splendid work that Professor Hovgaard has done in the researches and calcu- 
lations connected with the preparation of his paper. 
When contemplating the labyrinth of detail wherein maximum strength and minimum 
weights have to be kept constantly before the designer as the goal of his achievement, it is 
of course perfectly apparent to the experienced engineer that neither of these can be completely 
attained. As in the case of most other engineering, it must necessarily be a matter 
of compromise. 
From these considerations my mind runs out to the performance of the finished structure 
and especially the limitations in the prescription for its tests. In fact, no one can think in 
terms of either rigid or semi-rigid airships without having prominently rise before him the 
nightmare of the disastrous failure of the R-38. Nothing is more apparent than the fact 
that demands can very easily be made on these airships that they cannot be expected to 
withstand, and, as one who for many years has been required to consider just how far to go 
in tests of new constructions and new developments, I wish to put out a word of warning. 
It should be approached with great caution and, in addition to very complete knowledge of 
the minutest detail as well as the structure as a whole, a certain amount of sympathy for 
structures based on ripe experience, in the same sense that one can have quite complete sym- 
pathy with a machine and intuitively sense its limitations so as not to make demands upon it, 
especially at first, that it cannot be expected to easily fulfill, letting subsequent experience 
guide as to where the increased severity of the tests should stop. 
In preparing tests, I have an innate feeling that someone seasoned and experienced should 
be consulted or depended upon to make the final decision, who would prescribe treatment that 
is calculated to fit the ensemble, including its appendages, and also its environment as a whole, 
all those unseen and more subtle forces acting both from without and within. Why should 
not our author, who has made such an extensive study of all the features, be among those to 
be considered? It is perfectly apparent that the untrained enthusiast often imagines, inasmuch 
