MERCANTILE STEAM TRANSPORT ECONOMY. 43? 



In reference to the Table of the performances of steamers, which I recently 

 gave to Mr. Atherton, it is necessary to remark that too much confidence is 

 not to be placed in it as an exact document. Though I aimed at the truth, 

 it is possible I may have erred in the speed which is generally on the Clyde 

 tried between the Clock and Cumbrae lighthouses, or 13f nautical miles — 

 too great a distance for maintaining a uniform speed, especially in new ves- 

 sels with strange firemen, &c. I believe the statement, however, to be nearly 

 true, and the study of it affords useful lessons. The last column shows the 

 efficiency of the vessels by both formulas; I adhere, however, to the mid- 

 section formula, as being the more correct. 



The * Vulcan's ' speed and power is deduced from a number of trials at a 

 measured statute mile on the Garelock. The ' Simoom s ' performances I 

 obtained from one of the Dockyards. 



The ' Bogota,' a common paddle-wheel steamer employed by the Pacific 

 Steam Navigation Company, and loaded very deeply at her trial, shows a 

 very inferior result to that of the screw-steamer 'Black Swan' (now 'Ganges'), 

 not deeply laden. Their displacements are nearly alike, and their speeds 

 about equal ; yet the paddle vessel (too deeply laden) requires about 60 per 

 cent, more power than the screw. 



The ' London ' and ' Lady Eglinton ' are two screw vessels near enough 

 alike to be comparable. Their screws are the same diameter, but the one is 

 more immersed than the other, which I imagine is sufficient to account for 

 at least part of the difference in the efficiency of the two vessels. 



The ' Edina ' was constructed by Messrs. Barclay and Curie, and her 

 engines by Inglis ; but I was kindly invited to the trial, and got the particu- 

 lars of displacement, power, and midship section from the constructors. The 

 trials of the screw -steamer ' Lancefield ' are not so satisfactory as could be 

 desired, there being a little uncertainty as to speed. At the first trial the 

 screw was not immersed : the result shows a very low coefficient. The speed 

 at the other trials is uncertain, as it was taken at sea, and not in the usual 

 way for such calculations. 



I was unfortunate in not getting the particulars of the power and speed of 

 the ' Persia ' before she left the Clyde, so as to add her performances to the 

 Table. 



Letter by Mr. Atherton on Mr. J. R. Napier's paper. 

 To the President of Section G. on Mechanical Science. 



Sir, — With reference to Mr. James R. Napier's remarks on my paper, 

 " Mercantile Steam Transport Economy," I beg to submit the following ob- 

 servations. Mr. J. R. Napier concurs with me as to the indefiniteness of the 

 term "nominal horse-power," as at present applied in marine engineering 

 practice, and in the desirableness of having the unit of power, denoted horse- 

 power, specifically defined ; and he prefers that the measure originally pro- 

 posed and acted upon by Watt, viz. 33,000 lbs. weight raised 1 foot high 

 per minute, be now adopted as the statute unit of horse-power. 



On this point I have merely to remark, that scientifically it is a matter of 

 indifference what may be the statute measure of the unit, provided it be 

 specific. In my ' Essay on Steam-ship Capability,' I based my calculations 

 and tables on 132,000 lbs. raised 1 foot high per minute, because that was the 

 average performance, per nominal horse-power, of the ten mail packets then 

 employed in Her Majesty's Service. In my paper on " Mercantile Steam 

 Transport Economy," I have suggested that 100,000 lbs., raised 1 foot high 

 per minute, be adopted as the statute unit of horse-power, because that is, I 



