MESSRS. EDITORS—We have seen a number of statements in the SCIENTIFIC AMERICAN relating to improvements in steam engines to save fuel, but we do not recollect seeing any statements regarding the amount of work performed with a given amount of fuel, which would give those who are not acquainted with a steam engine, a better idea of its power and fuel-economizing qualities than merely to state the amount of fuel consumed per horse power. We have put into a grist mill which runs three pairs of stones (two of four, and one pair of three feet) an engine, the cylinder of which is 10 inch bore and 24 inch stroke, making 150 revolutions—or GOO feet velocity of piston —per minute. Tlio boiler of it is 20 feet long, 48 inches in diameter, has one 24 inch flue, and is set in a brick arch. On an average, it consumes one bushel of bituminous coal to grind ten bushels of wheat or rye. When the grain is dry, the engine will grind 200 bushels of wheat or rye with 20 bushels of coal, but when the grain is damp, it will not grind quite so much. We do not think this is so much work as can be performed with a boiler of superior construction, but there are grist mills running in Western Pennsylvania which consume 50 bushels of coal in grinding 200 bushels of grain. We would like to see some statements of what a low-pressure condensing steam engine can do, or has done, in grinding grain, with the amount of fuel consumed. HAMBLIN & Hl5ATH. West Greenville, Pa., February, 1858. [The above letter relates to a very important question. From the statements regarding the fuel consumed and work done by an engine, however, we cannot receive a proper idea of its power, but a very just idea of tbe duly it performs. For' example, if an engine grinds 200 bushels of grain in five hours, and consumes 20 bushels of coal to do so, its power will be twice that of an engine which consumes only half the amount of fuel, but takes ten hours to do the work. The power of engines is a different question from that of the duty of engines, which latter merely takes in two elements of calculation, namely, the amount of work done by a certain quantity of fuel. It is upon this principle that the Cornish engines are judged. Our correspondents are perfectly right, however, in considering the boiler as part of the engine—it is the principal part; and we think their communication should do good, in leading those to look into the subject who have steam engines performing so small an amount of duty as thoso referred to in Western Pennsylvania. Besides the construction of the engine, the arrangement of the gearing, shafting, &c, should always be given, because much power may be expended in overcoming unnecessary resistance, friction, &c, by a bad arrangement of the machinery.—EDS. A CHAIB was patented Feb. 16th, wliich will continually give currents of cool air on alternate sides of the operator, by the person simply moving the chair—a pleasant and easy motion—three inches from right to left and vice versa. The inventor is L. It. Breisach, of this city.