NO. 3 LANGLEY MEMOIR ON MECHANICAL FLIGHT 71 



tlio passages is better tlian the other. This conduit will then generate more steam 

 and drive the water under force through the other passage, increasing the tem- 

 perature discrepancy between them. Eventually the hot passage reaches igni- 

 tion and either bursts or melts. This is what sooner or later takes place in 

 boilers adapted for flying machines and consisting of tubes joined in multiple 

 arc, when a single moderately strong circulating pump supplies the system. 



" To avoid these annoyances, i. e., to increase the length of life of the 

 boiler, the boiler tubes are joined in series to the effect that a single current of 

 water may flow successively through all of them. It is needful therefore to 

 select wide tubes, such as will admit of an easy circulation in consideration of the 

 length of tubing employed without straining the pump and at the same time to 

 allow sufficient room for the efflux of steam. Other considerations enter here, 

 the bearing of which will be seen presently: if the tube be too wide the difficulty 

 of coiling it on a mandrel of small diameter is increased, while at the same time 

 the tube loses strength (ccet. par.) in virtue of the increased width. 



Diagram 2. 



Diagram 1. 



Fio. 11. 



"It is from considerations such as these that, in the course of many ex- 

 periments, copper tubing about 8 mm. in diameter has been adopted. Copper is 

 selected because of its freedom from internal corrosion, easy coiling, and be- 

 cause of its availability in the market. The thinnest tube to be had (walls only 

 0.1 mm. thick) will withstand more pressure than can be entrusted to the larger 

 steam receivers in circuit with the boiler. The boiler weight is thus a negligible 

 factor, and it is quite feasible to reduce the thickness of boiler tubing, by the 

 superficial application of moderately strong nitric acid, to 200-400 grammes per 

 horse-power of steam supplied. External corrosion due to flames occurs only in 

 case of deficient water, and if the boiler be made of tubing with the walls 0.2 

 mm. thick, it is in view of the possibility of such accidents. Boilers may then 

 be tested to 25 atm. without endangering the metal. 



" Boilers are wound or coiled with regard to the two points above suggested, 

 viz.: to avoid chilling the flame the successive turns are spaced on all sides, and 

 to bring the water as nearly into the flame as possible, the diameter of the coils 

 is chosen as small as expedient. Further reasons for this will presently be ad- 

 duced. The type of boiler eventually adopted is shown in the accompanying dia- 

 grams, 1 and 2, Fig. 11. 



" Diagram 1, is a perspective diagram showing the plan of winding and Dia- 

 gram 2, an end view. The circulation is indicated. There are two inner coils 



