SECT. III.] 



CONDENSATION OF STEAM. 



119 



flame to boil off a cubic foot of water in an hour ;" 1 which is only the rule of 

 Smeaton in general terms. 



206. The proportion of the bottom surface, or that within the immediate 

 effect of the fire and flame, seems to have been subjected to no fixed rule : the 

 proportions used in practice vary from 3 to 5 feet of bottom surface for each cubic 

 foot of water boiled off per hour. Mr. Millington seems to have first indicated 

 the use of measuring the power of a boiler by its bottom surface ; and gives as 

 examples, that a boiler for 20 horse power is usually 15 feet long and 6 wide, 

 having 90 feet of surface, or 4|- feet to 1 horse power ; a boiler for a 14 horse 

 power, 60 feet of surface = 4*3 feet to 1 horse power. 2 I have observed boilers 

 to be incapable of supplying the proposed quantity of steam when they had less 

 than 4 feet ; and that those were effective which have the proportion assigned 

 by the rule above, provided they also had a proper quantity of flue surface. 



207. In regard to high pressure steam, some interesting trials were made by 

 Mr. Wood 3 with steam carriage engines, which show the disadvantage of attempt- 

 ing to form steam by intensity of heat instead of quantity of surface. 



The first was with a steam carriage boiler 8 feet in length, and 3 feet 9 inches 

 in diameter, with a tube 20 inches diameter, passing through its length, which 

 contains the grate for the fuel from whence the smoke passes along to an upright 

 tube at the end, serving as a chimney : the pressure of the steam in the boiler 

 was limited to 50 Ibs. per square inch above the atmosphere. 



The whole surface of the tube forming the fire-place and flue would be only 

 40 feet ; and it was the same in all the trials, but of this not more than two-thirds 

 or 27 feet could be effective as fire surface. 

 208. 



The mean intensity of the fire must have been equal to 1200, to produce this 

 effect ; and the fuel consumed is somewhat more than double the quantity which 

 ought to have generated the same quantity of steam. 



209. In another trial the length of the boiler was 9 feet 2 inches, its diameter 

 ^ feet, the diameter of the tube 22 inches, and the force of the steam limited 



1 Robison's Mechan. Phil. vol. ii. p. 147. 

 3 Treatise on Rail Roads, p. 249. 



Epitome of Natural Philosophy, p. 266. 



