238 report — 1859. 



There ave two modes by which these results may be tested, the one analytical, the 

 other synthetical. 



I will begin with the first, and employ the other in elucidation. The effective power, 

 or total resistance, from the actual power of 54"59 horse-power, or 1801470 lbs. at 

 six knots, is 2961*67 lbs., which is thus distributed : — 



lbs. 



Specific resistance 1896 



Equivalent of slip 315*67 



Resultant of absorbed power 750 



Total 296167 



I have in this analysis, as in my former paper, classed under the general term 

 "equivalent of slip," two distinct elements, which I must now separate. 



The first is that portion of the effective power which overcomes the resistance of 

 the water to the rotation of the blades of the screw, and which is sometimes called 

 " lateral slip." The second is that portion of the effective power which is employed 

 in pushing back the water to obtain a fulcrum. 



lbs. 



The first is 204 



The second is 11T67 



Making together the 315-67 



The absorbed power is composed of — 1st, the friction of moving the machinery ; 

 2nd, the additional friction of the load; 3rd, the back pressure (as we are dealing 

 with a non-condensing engine) from the blast-pipes. 



lbs. 



For the first we have 136*92 



For the second 189*60 



For the third 423-48 



Making the total of 750 



In order to give a clearer view of these elements, I will reverse them, and show the 

 corresponding pressure upon the piston : — 



Pressure on piston, 



lbs. lbs. per square inch. 



Back-pressure 423*48 4 - 64 



Additional friction of load 189*60 207 



Moving friction 13692 1-50 



Resistance of water to rotation of blades 204 - 00 2*23 



Slip in terms of effective power 111-67 1'22 



Specific resistance of vessel 1896-00 2078 



Making the totals of 2961*67 and 32-44 



The pressure on the piston by the diagrams is 32 - 07 lbs., showing a difference of 

 0'37, which may be considered near enough. 



Now, if it be said that this is an arbitrary classification not resting upon known 

 data, I reply, not altogether so. I have lying before me diagrams of back-pressure 

 from 3'45 lbs. per square inch on the piston, moving at 236 feet per minute, with a 

 mean pressure of 6756 lbs. to 12*3 lbs. per square inch on the piston, moving at 569 

 feet per minute, with a mean pressure of 65 lbs., the diameters of blast-pipes varying 

 from 4| to 4 J inches, and the relations of steam ports from 14 X 1 J inches to 12| X 1 \ 

 inches of area. 



The 'Erminia's' blast-pipes were 4 inches in diameter, the steam ports 12 X \\, 

 and the speed of piston 1 57 feet per minute, with a pressure of 32 - 07 lbs. per square 

 inch in the cylinder. 



The back-pressure of 4-64 lbs. which results from the analysis is therefore pro- 

 bable, and consistent with experience. 



The next element of additional friction arising from the load, viz. 2 - 07 lbs. per 

 square inch, is calculated upon the specific resistance of 1896 lbs., and may be dis- 



