ORDINARY OSCILLATING PADDLE-WHEEL ENGINES. 27 



two cylinders, it will be found that a higli-prossure cylinder 

 of 40 inches diameter, cutting off at eight-tenths of the strohe, 

 with 95 lbs. initial absolute pressure, will give about 50-4 lbs. 

 moan pressure in the high-pressure cylinder and IS'S lbs. 

 mean pressure in the low-pressure cylinder, and would at a 

 correspondingly reduced piston speed of 25 revolutions, 

 equal to 300 feet per minute, develop 575 horse power in 

 each cylinder, or a total of 1,150 indicated horse power, in- 

 stead of 1,600 as formerly ; and this reduced power would 

 giv'o a speed of 12^ knots per hour at the original mean 

 dv.M of 9 feet 3 inches and displacement 1,172 tons, with 

 the original coefficient 193, instead of 14J„- knots as formerly. 

 Such an engine would require two boilers, IG feet diameter 

 X 11 fecit long, with a grate area of 146 square feet, and a 

 total heating" surface of 4,500 squa,re feet, and the con- 

 sumption of coal I estimated would be 60 tons per voyage. 



This appeared to be the best result that could be expected 

 under the circumstances, and the scheme was eventually 

 approved. 



I will now shortly explain the alterations that were neces- 

 sary to carry out the idea. The method of compounding 

 those engines, then, was as follows : — 



To remove the old starboard cylinder, and in its place to fit 

 a new high-pressure cylinder, 40 inches diameter x 72 inches 

 stroke, with a receiver cast round it in the form of a jacket, 

 making the outside of the receiver about the same size as the 

 outside of the prosivnt old cylinder. To ha,ve two slide jackets 

 fitted as usual, but on the wing sides of the cylinder. To 

 admit the steam from the boilers by a steam trunnion at the 

 wing side as at present, and to exhaust it into the receiver 

 surronndiTigthe high-pressure cylinder and through its mid- 

 ship trunnion into a pipe or passage to be formed in the casting 

 of the old condenser between the two air pumps. To reverse 



