NO. 3 LANGLEY MEMOIB ON MECHANICAL PLIGHT 39 



graph), so as to afford more room in tlio hull. This introduced a constructional 

 weakness which was not compensated by the added convenience, but the princi 

 pal improvement was the abandonment of the inclined propeller shafts, which 

 was clone at the suggestion of Mr. J. E. Watkins, so that the propellers were 

 carried on parallel shafts as in marine practice. These parallel shafts were 

 driven by two very small engines with cylinders 2.3 cm. in diameter by 4 cm. 

 stroke, with a collective capacity of 33 cu. cm. and without cut-off, which were 

 mounted on a cross-frame attached to the midrod at right angles near the rear 

 end of the hull. 



These engines, driven either by steam or by carbonic-acid gas developed 

 0.035 horse-power at the Prony brake, giving 750 revolutions of the 45 cm. pro- 

 pellers, and lifting about * of the total weight which it was necessary to provide 

 for in actual flight. A higher rate of revolution and a better lift were occasion- 

 ally obtained, but there was little more hope with this than with the preceding 

 models of obtaining power enough to support the actual weight in flight, although 

 such sacrifices had been made for lightness that every portion of the little model 

 had been reduced to what seemed the limit of possible frailty consistent with 

 anything like safety. Thus the midrod was lighter than that of No. 1, being 

 only 1 cm. in outside diameter. The frame was made of thin wooden strips 5 

 mm. x 3.5 mm., united by light steel rings. The cross framing carrying the en- 

 gines was also of wood, and was formed of four strips, each 7 mm. x 3 mm. 

 The shafts were but 4 mm. in diameter. 



As these engines did not give results that were satisfactory, when using 

 carbonic-acid gas, experiments were commenced to secure a boiler that would 

 furnish the requisite steam. As the " beehive " boiler had proved to be too 

 heavy, and as the steam obtained from it had been inadequate to the require 

 ments, something else had to be devised. A few of the boilers used in 1892 are 

 shown in Fig. 3. The one marked A is one of the " beehives," while an ele- 

 ment of another form tried is that marked B. It consisted of f-inch copper tubes 

 joined to a drum of 10-oz. copper. This was made in May, 1S92, and was tested 

 to a pressure of 50 atmospheres, when it burst without any tearing of the metal. 



In July another boiler like that shown at C in Fig. 3 was made. This 

 was formed of tubes 3 cm. in diameter, and weighed 348 grammes. It carried 

 about 300 grammes of water and stood a steam pressure of 125 pounds per 

 square inch, but failed to maintain sufficient steam pressure. 



Accordingly, in the same month, a third boiler like that shown at D was built. 

 It consisted of a tube 12 inches long to which were attached fifteen 1-inch tubes 

 each 7 inches long, in the manner shown. The heating surface of this boiler, 

 including the tubes and the lower half of the drum, amounted to 750 square cm., 

 and it was thought that this would be sufficient to supply steam for a flight of a 



