72 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



eadb containing about 17 turns, wound on a mandrel 5 cm. in diameter. The turns 

 are spaced so as to allow about 1 em. clear between successive turns. The outer 

 coil envelopes both, and in this there are about 3 cm. between successive turns, 

 and 8 turns in all. Length, say, :'.() cm., breadth 16 cm., thickness 10 cm., give 

 the external dimensions of the boiler. The shell space between outer and inner 

 Layers of tubing must nowhere be less than 1 cm. When so wound, the inner 

 coils (here as in other boiler forms) raise about 80 per cent or more of the steam ; 

 the outer or enveloping coil, while not quite useless, make the most effective 

 frame work for the boiler jacket which has been devised. The coils are brazed 

 together by blind tubes, as shown in Diagram 2, to keep the whole in shape. 

 Weight with couplings and cover when complete 535 grammes. 



" The cover is preferably of mica, through which the flame within the boiler 

 may be seen, and in which lightness, nonconduction, and resistance to the disin- 

 tegrating effects of high temperature are met with in a pronounced degree. This 

 jacket is held down by copper bands and the end band is continuous with the 

 long smoke-slack, as will presently be shown. 



" The wide form of boiler with two coils within the envelope is not abso- 

 lutely essential. The same amount of steam can be generated from one coil in 

 an envelope in other respects equal to Diagram 1 if a sufficiently hot flame be 

 passed axially through the coils. Such a flame, however, is unstable, and for 

 this reason two milder flames with a good air access are to be preferred on prac- 

 tical grounds even if the weight is thereby increased. 



" To further understand the boiler construction it is advisable to consider 

 the action of the flame. Inasmuch as wide tubes must be used, the problem of 

 evaporating water as fast as possible is equivalent to getting heat into the cur- 

 rent (water and steam circulating through the coils) as fast as possible from 

 without. If, therefore, t is the mean temperature of the fluids within the coils, 

 and T the effective temperature surrounding the tube, then the rate at which 

 heat will flow into the tubes is proportional to T-t. Now t the temperature of 

 the steam is nearly constant (100-150°) whereas T the effective flame tempera- 

 ture may vary from 800° to, say, 1600°. It is for this reason that the heat 

 sponged up by the boiler depends almost directly on the flame temperature. 



" What conditions, therefore, will make the flame effectively hot? 



"(1) The coils must obviously be brought as nearly into the flame as feasi- 

 ble: for this purpose the cylindrical helix is better than any other form. But 



"(2) The turns and coils must not be so crowded together as to chill the 

 flame into imperfect combustion in various parts of its extent. Hence the loose 

 form of winding. Again 



"(3) There must be oxygen enough to allow complete combustion, and 



"(4) The flame itself must be hot and the radiation checked by good 

 jacketing. 



" To take up the last points: the effective heat of the flame depends not 

 only on the combustion heat of the fuel used ; it depends also, among other things, 

 on the speed with which this combustion takes place. A flame burning from a 

 low pressure of alcohol gas will be at low temperature as compared with a flame 

 burning from high pressures of the gas. [f the (lame be burnt from a Bunsen 

 burner in the usual way it is an interesting question to know how flame tempera- 

 ture will vary with gas pressure. At present we know it merely in steam pres- 

 sures ineidently produced in a given engine (No. 4) as for instance: 



Flame pressure, 10 lbs., 20 lbs.. .10 lbs.l . „ 



n, irv ,, OA ,, , nA ., !■ in the running engine. 



bteam pressure, 40 lbs., 80 lbs., 120 lbs. J & s 



