990 TbajYSactto.xs of the American Institute. 



turned up at tlie ends, so that the water is kept at tlie required level, 

 say, up to the middle of tlie tubes. Tlie water enters the upper story, 

 and fills it to the required level, and then ovei-flows into tlie next 

 story below, and so on until the lowest story is filled to its proper 

 level, as shown by a gauge, and i-egulated in the usual way. The 

 water overflows on one side, and the steam flows off" on the other 

 side, the floors being upturned higher on this side, so tliat no water 

 may go over with the steam. A heater, kept full of water, may be 

 placed above the boiler, or it may be in a separate compartment, 

 through which the smoke may pass, downward or sidewise. I think 

 it best to give the smoke only an upward movement, and for that rea- 

 son I put the heater above the boiler ; but in this case the pipe which 

 connects the heater must dip under the surface of the water in the 

 upper story, so that the heater may be kept full of water. And to 

 provide for cases in which steam may be made in the heater, such as 

 when the pump is stopped, there are dams in the heater pipes to pre- 

 vent the water from running out ; only a tenth or less of the water 

 can be driven into the boiler ; and the steam made in the heater will 

 go down into the boiler mixed with more or less water. 



There is one point which, it appears to me, is favorable to high 

 pressure in boilers whose tubes are exposed to flame ; the density of 

 steam, I think, increases its heat-receptive power. If we use steam 

 of three times the usual density, it will take up heat muck faster. 

 IIow much faster I do not know, from any reports of experiments. 

 But I conjecture that its receptivity is as its density multiplied by 

 the difierence of temperature between the steam and the iron. The 

 temperature of the iron, in heaters for air, • Tredgold assumes as a 

 mean between the external air and the smoke. I doubt his accuracy, 

 and think that ho sliould have considered the densities of the cold 

 and the hot air or gases. But it is idle to attempt, without experi- 

 ment, to learn precisely how much density may affect the receptivity. 

 In case of atmospheric pressure, there is probably not much error in 

 assuming that, with thin plates, the teinperature of the surfaces cov- 

 ered with water is very nearly that of the water ; and the tempera- 

 ture of the surfaces in contact with steam is a mean between the 

 temperature of the steam and that of the flame or smoke. But if th<3 

 density of the steam be ten or twenty times greater, I believe it will 

 take up heat so much faster that the metal will be sufficiently pro- 

 tected against heat, even if the steam be dry. 



The advantages of high pressure which arc made available by safe 



