EXPLANATION OF THE PLATES. 383 



The general operation of these engines is like all other marine engines fitted with 

 Hall's condensers, excepting that the water pumped from the condensers by the air 

 pumps is not forced by the air pumps through the feed valves directly into the boilers, 

 but is delivered into a casing or cylindrical jacket surrounding the chimney, and 

 flows out at the upper part of the casing, into a stand pipe, sufficiently high to retain 

 a column of water to overcome the pressure within the boilers, like the usual mode of 

 feeding boilers in an ordinary Boulton and Watt engine. By this means the water ac- 

 quires an additional temperature of about twenty degrees before it enters the boilers, and 

 it is more perfectly separated from any bubbles of air which may arise in admixture 

 with the water, when it leaves the air pumps. 



The nominal power of these engines is 285 horses' ; a very rapid and perfect ex- 

 haustion of the cylinders is effected, as will be seen by diagrams, pgaes 385, 386, 

 taken with great care by a very good indicator adjusted for the occasion. The adran- 

 tage of Samuel Hall's process of condensing by contact instead of by injection, will 

 be very manifest on inspecting the diagram No. 2, taken in a heavy sea while the 

 vessel was rolling and exposed to the effects of half a gale of wind ; yet the vacuum 

 was as perfect as in smooth water, and the exhaustion of the cylinder, proved by ac- 

 tual experiment, amounted to 1 3 Ibs. upon each square inch of its area, which, to- 

 gether with the pressure of the steam, gives a mean force on the piston of 16*54 Ibs. 

 per square inch, and produces an absolute effect of 283'6 horses' power, or 567'2 

 horses' power with both cylinders, after deducting 2 Ibs. per square inch on the pistons 

 for the friction and power expended in working the pumps, &c., leaving an effect of 

 98 per cent, more than the nominal power of the engines ; the diagram No. 1 

 gives a still greater effect than this, being more than 100 per cent, above the nominal 

 power, and the mean effect, as ascertained by the indicator and shewn by the several 

 diagrams, amounts to 96 per cent, more than their nominal power. 



The uniform velocity of the engines in smooth water is 22 revolutions, the pistons 

 moving through 264 feet per minute when the draught of the vessel is 9 feet 6 inches 

 forward and 1 1 feet aft, at which draught the paddle wheels are immersed 3 feet 6 

 inches, measuring from the outer edges of the centre floats to the surface of the water, 

 and the total area of the paddle boards thus immersed, amounts to 102 feet in each 

 w r heel. 



The same letters of reference are employed to denote the same parts in each en- 

 graving, the use and operation of which will be understood on referring to the 

 several plates. 



Plate LII. represents a longitudinal elevation of the engines. 



Plate LIII. represents a cross section shewing the after parts of the engines and the 

 paddle wheels. 



