Proceedings of the Polytechnic Association. 991 



boilers are, as to the boilers : the quantity of water is less, so that the 

 time and fuel required to get up steam may be considerably econo- 

 mized ; the volume of steam is less, and the water is more solid, and 

 takes up heat faster than water mixed with more foam. Williams 

 says that the receptivity of plates covered with water is 10, covered 

 with foam, 8, and covered with steam, 6 ; and the room occupied is 

 less ; an^ the cost of small lap-welded tubes is much less per square 

 foot of surface than that of large ones. Against these, it may be said 

 that the smoke from a high pressure boiler will go away hotter : but 

 this would improve the draft ; and it need not go away hotter if the 

 heater be arranged so that the coolest water is exposed to the coolest 

 smoke. The advantage, as to the engine, is that expansion may be 

 carried out more advantageously. And there are now, in England 

 and in this country, engineers of high reputation who are working 

 on the Woolf system, of high and low pressure cylinders. This sys- 

 tem will be the more effective if we increase the pressure to 300, 400, 

 or 500 pounds. The external surfaces of tubes are greater than the 

 internal ; and, as the surfaces exposed to heat determine the rate of 

 vaporization, the cost of tubes will be less than when the smoke goes 

 inside. Lastly, tubes that are staggered and have the draft across 

 them makes one-half more steam than when set in rows, and proba- 

 bly much more than when used as flues. And as to the danger of 

 burning the upper half of a tube that is but half full of water, it must 

 be observed that the upper half receives but a quarter of the heat in 

 any case, and less when the tubes are staggered. D. K. Clark reports 

 ah experiment with a square flue, in Mdiich the upper surface, with 

 water on it and heat under it, made as much steam as the two sides, 

 and the bottom made none at all. From these facts I infer that the 

 upper halves of the tubes would be of little use — about a sixth of the 

 whole — in making steam if covered with water, and that they will 

 not be burnt. And their use in superheating or drying the steam 

 will be greater than their use in making saturated steam. There 

 will be no loss in leaving the upper halves of the tubes uncovered by 

 water, as I think ; but water may be carried higher if the tubes be 

 larger." 



Adjourned for two weeks. 



