1078 Transactions of the American Institute. 



An inquiry was here made as to the amount of damage caused ta 

 furniture, clotliing, etc., by the dust in the city of New York. No 

 direct reply was given to this question, but it was stated that Mr. 

 Edwin Chadwick estimated that the dust of London entailed upon 

 the inhabitants an amount of extra washing which was not less than 

 £1,000,000 per annum. 



Dr. J. Y. C. Smith hoj)ed that the time would come when steam 

 carriages and good roads would effectually abate this nuisance. 



The interesting article was read by the author — 



On the Computing and Compaking of Indicatok Diagrams. 



G. H. Babcock. — It is a fact well known to engineers, that by none of 

 the ordinary methods of expressing the comparative merits of different 

 engines, can a satisfactory conclusion be reached as to the value of any 

 pecuhar construction, unless all the conditions and circumstg,nces under 

 which the comparison is made are precisely similar. The amount of coal 

 burnt per hour per horse-power is a common criterion, but so many 

 other conditions than that of the construction of the engines enter 

 into and effect the result, such as the evaporative efficiency of the 

 boiler, the quality of the fuel, the load, the amount of protection 

 to the boilers and pipes, from radiation, etc., that it affords no just 

 criterion by which to judge the perfection of the engine alone, unless 

 these several conditions are all equal in the classes in which a com- 

 parison is desired. 



The amount of water required per hour per horse-power gives a 

 much nearer approximation to the relative value of different engines ; 

 but in this comparison, also, it is necessary that the load relative to 

 the capacity of the engine, the quality of the steam, whether dry or 

 wet, and all the exterior conditions should be precisely similar, to 

 render the comparison just. But even when the exterior conditions 

 are similar neither of these tests give any result by which can be 

 determined, save by inference, the relative advantages of different 

 mechanical arrangements or proportion of parts. It being almost 

 impossible, except by special construction for the purpose of experiment 

 to lind two engines under circumstances where the sole difference is 

 in some specific point of arrangement or proportion, the precise effect 

 of that difference cannot be determined by the consumption of fuel 

 or water. 



The use of the indicator is the most satisfactory way yet devised 

 for ascertaining the relative efficiency of various constructions and 



