MESSES EDITORSIn the SCIENTIFIC AMERICAN of March 6, page 208, Messrs Hamblin Heath asked for information regarding what " a low pressure condensing steam engine can do, or had done in grinding grain, with the amount of fuel consumed," a very pertinent and important question The late John Farey, Esq, used to say that a bushel of wheat had never been ground with less fuel than was consumed by Bolton Watt's steam engines seventy years ago "A bushel of wheat ground in one hour with eight pounds of coal is one horse power" The horse power may also be represented by grinding one pound of wheat, or raising 33,000 pounds one foot, or evaporating 1158 pounds of water, or burning '1343 of a pound of coal in one minute On pages 51516 of Mr Farey's work on the steam engine, it is stated that 8J pounds of coal sufficed to grind a bushel of wheat, and dress some flour, and that 806 pounds of coal evaporated 69477 pounds of water or 8*62 times its own weighttime, one hour The steam engine did not work with expansion, therefore allowance must be made for this The whole mechanical power contained in one pound of water is represented by its pressure and expansion In the case in question, the pressure was 12*572 pounds per square inch, or 29*05 pounds on a base of water one foot high and weighing one pound; and its expansion from water to steam of that elasticity, 1,955 times, the product of which is 56,778 pounds, from which, by deducting onetwelfth for back pressure, we have 52,047 pounds or 1*577 horsepower (52047333,000) Thus then, of the whole laboring force, only twothirds of it was effective Anything which can beat that, without expansion, is a clear gain The power is in the steam, not in the engine, therefore when a pound of water evaporated into steam does not come up to the standard above, the engine or transmitter of power is at fault One pound of water evaporated under a pressure of 90 pounds per square inch, or 208 pounds on a water base of one foot high to the pound, expands 321 times, and its mechanical power is equal to lifting 66*776 pounds one foot high, but from this we must deduct the back pressure of the atmosphere and the obstruction from the exhaust port and pipe, which if we call onefourth, we have a total working power of about If horse power as the utmost attainable Yours respectfully, THOMAS PROSSER New York, April, 1858 The following is another letter on the same subject : MESSRS EDITORSI have a " Corliss engine," four feet stroke, fourteen inches cylinder, and have kept a correct account of fuel for the last year ending Dec 31, which I consider the only correct way to get at the cost Have run 292 days of ten hours each ; used thirtyeight horse power per day, without allowing anything for shafting and small machinery, which would add three horse power certain per daythe cost of fuel per day is just $5 77 That is a trifle over 15 cents per horse power per day The work done was cutting and grinding dyewoods, and grinding corn with an "Old's mill" I have calculated the power of an " Old's mill" at about threequarters the power required to do the same work on the oldfashioned grist mill The engine has run thirtythree months, and three dollars will pay all the expense of repairs, and that has been in broken bolts, done through the carelessness of the engineer The two boilers used are the common cylinder kind, thirty feet long, thirty inches in diameter, and I am satisfied they are the cheapest boilers in use when properly made, and as for safety no one can dispute that point I will state that the steam carried is from sixty to eighty pounds per square inch pressure by Ashcroft's steam gage WM BRIDEN Providence, R I, April, 1858