Proceedtnos of the Polytechnic Association. 989 



locomotive boilers ; and, witli only the thickness now adopted for 

 diirabilitj, tubes may safely be worked at 1,000 pounds pressure utitil 

 they fail from partial corrosion and leakage. And this type of 

 boiler, which has the water in small chambers, brings up the question 

 of pressure disincumbered with the question of safety. We can work 

 safely with any pressure now contemplated ; and the higher the pres- 

 sure, the better for the boilers as to the quality of steam. 



" Fairbairn has for many years recouunended increased pressure, 

 and has said that he expects to see 500 pounds used. Alban has worked 

 up to 1,000 pounds, with no disadvantage but the decomposition of the 

 hemp packing of his piston. Perkins, with metal packing, for along 

 time worked at 1,500 pounds pressure. In a few cases in this country, 

 locomotives have been worked at 200 pounds, and steamboats at 250 to 

 300. The chief trouble apprehended is that the side-valves will have 

 too much friction, under extreme pressures, and will cut their seats. 

 But there are balanced valves that promise relief from this ; and, up 

 to a high limit, not yet reached in locomotives, the friction of the 

 valve decreases as the pressure increases. 



If^ we 'increase the pressure we may decrease the <liameter of the 

 tubes. If seven -inch tubes will make dry stealn at 140 pounds, with 

 fifteen feet of surface per cubic foot of water per hour, Z^ inch tubes 

 will make dry steam with 280 pounds pressure. But four or five inches 

 is the least diameter that we want to use, having cost of construction 

 in view. We may say, then, that if we adopt 250 to 300 pounds pres- 

 sure, which has been used without harm to the engines, we may, in 

 tubes of economical size, and with perfect safety, fulfill the conditions 

 which Watt laid down for his own practice, after careful trials, in 

 which he was not embarrassed by the questions of safety, weight, 

 compactness, or any of the difficulties that have arisen in marine and 

 locomotive practice. 



The means by which I propose to extricate the steam from the 

 water, so that it may not carry water with it, are, first, a separate 

 water level in each tier of tubes, so that in each tube there shall be an 

 ample separating surface and sufficient steam room ; and, second, a 

 sufiiciently wide and deep passage for the steam, so that it may move 

 over the water so slowly as not to ripple it, or drive away bubbles, 

 or in any way take up particles of water. There are forty-eight 

 tubes, seven inches diameter and seven feet long, arranged in eight 

 tiers over each other. These tubes are screwed into a flat box, set 

 on encL The box is divided into eight stories, by floors which ai'e 



