NO. 6 LANliLEV MEMOI1! ON MECHANICAL FLIGHT 



57 



water, so that by proper regulation at the needle valve, just that quantity would 

 be delivered which could be converted into steam in its passage through the 

 coils, and be ready for use in the engines as it left the boiler at the farther ex- 

 tremity. The results obtained from this were an improvement over those from 

 the original coil, and a third set of coils (G, in Plate 13) was made. This boiler 

 consisted of three flattened tubes superposed one over another. 



These two boilers were tried by placing them in a charcoal fire and turning 

 on an alcohol blast, while water from a reservoir under constant air pressure 

 was forced through them past a pin valve. The result was that the two-stranded 

 coil supplied steam at from 10 to 40 pounds pressure to run the engines at about 

 400 revolutions per minute. The pressure rose steadily for -about 40 seconds 

 and then suddenly fell away, though the coils were red-hot, and neither the water 

 nor the alcohol was exhausted — apparently because of the irregularity of the 

 supply of water, due to the time taken by it after passing the valve to fill the 

 considerable space intervening between that point and the boiler. 



An attempt was made to overcome this difficulty by putting a stop-cock di- 

 rectly in front of the boiler so that the water, while still under the control of the 

 needle valve, could be turned in at once; the alcohol blast was also arranged to 

 be turned on or off at pleasure, and provision was made, by taking out the end 

 of the flue inclosing the boiler, to provide for an increased air supply. With this 

 arrangement a flame eight or nine inches long was obtained, but a test showed 

 that not more than 25 grammes of water per minute passed through the tubes, 

 which was not enough. 



Further tests with these boilers were so far satisfactory as to show that 

 with the flattened-tube Serpollet boiler, comprising from 60 to 80 feet of tubing, 

 from 80 to 100 pounds pressure of steam could be maintained, but not steadily. 

 As there were difficulties in flattening the tubes to make a boiler of this sort, a 

 compromise was effected in the construction of the one shown at H (Plate 13), 

 which was made of light copper tubes 5 mm. in diameter, laid up in three lengths 

 of 6 metres each. The ends of these coils were so attached to each other that the 

 water entering at one end of the smallest coil would pass through it and then 

 enter the middle coil, whence it passed through the third or outer coil. Two 

 sets of these coils were made and placed in the thin sheathings shown in the 

 photograph. Repealed experiments with these boilers demonstrated that the 

 pressure did not rise high enough in proportion to the heat applied, and that 

 even the pressures obtained were irregular and untrustworthy. The principal 

 difficulty still lay in maintaining an active and uniform circulation through the 

 coils, and for this purpose the water reservoir under constant air pressure had 

 proved itself inadequate. This pointed to a return to the use of the force pump, 

 the construction of which had hitherto presented so many special difficulties that 

 it had been temporarily abandoned. 



