For practical purposes a mere change in the mechanical structure may give an entirely different character to them, while their real calorific power is not altered. This is apparent in the coals, which are rendered almost worthless when reduced to dust, until in the patent fuels they are reconverted into solid form. Wood possesses very different values in solid sticks, in shavings, and in sawdust. In ordinary use other circumstances are to be taken into account, as the arrangements for utilizing the heat produced, so that there shall be the least amount lost; also the provisions for insuring perfect combustion of the fuel. The loss of heat resulting from imperfect arrangements in these respects alone has been estimated at full one half of all that generated. The chimney necessarily carries off a considerable portion, as there will be no draught, and consequently no continued supply of air to support the combustion, unless the column floating upward by its rarity produces a partial vacuum to be filled with fresh air passing through the fire.

The quantity of this admitted should be limited to a proper excess only of that absolutely required for the thorough combustion of the fuel, and this can be determined for each variety of fuel only by the experience and good judgment of the operator, the object in view being a uniform rate of combustion, more or less rapidly conducted, according to the fuel employed and the special purpose to which it is applied. The quantities necessary for complete combustion of one pound of the different fuels are given in the following table, the temperature of the air being: 66.2° F. and its weight 0.075 lb.:

-The qualities of the American coals have been investigated by Prof. W. R. Johnson, who was commissioned by the United States government for this purpose, and whose report was addressed to the navy department in 1844 (Senate Document Xo. 386). The results are presented in a condensed form by Prof. Johnson in the American edition of Rnapp's Chemical Technology," the table below being arranged from the more detailed tables contained in his report. It contains 25 varieties of coals, five from each one of five different classes, succeeded by a summary of the results, presenting a general scale of relative values made up from the averages of the classes. From this it appears that in evaporative power under equal weights the Cumberland class surpasses the anthracite by about 2.3 per cent., and under equal bulks by 1.4 per cent. From single experiments, however, the most water evaporated was with anthracite. The anthracites also surpass the foreign bituminous coals 20 per cent. when we compare equal weights, and 26 per cent. by equal bulks.

In freedom from clinker the anthracites stand preeminent; in rapid production of steam when once in action, the Pennsvl-vania bituminous coals are somewhat superior to all others; and for rapidly getting up steam the foreign bituminous coals are most effective. Column A gives the relative evaporative power of equal weights of coal; B, comparative power of equal bulks of coal; C, relative freedom from tendency to clinker; D, rapidity of action in evaporating water; E, facility of ignition, or readiness with which steam is got up; F, sum of the relative values in the preceding columns.

His operations were conducted upon a large scale, four trials being usually made in ascertaining the evaporative power of each coal, and each trial consuming from 800 to 1.200 lbs. The total number of trials was 144, in which 62 1/2 tons were consumed. The object was particularly to determine what coals were best adapted for steam navigation; and the points of special attention were essentially the same as those to which the attention of the commission afterward appointed by the British government was directed, viz.: 1, the capacity of the coals for raising steam quickly; 2, for raising it abundantly for the quantity consumed; 3, freedom from dense smoke in its combustion; 4, freedom from tendency to crumble in handling; 5, capacity, by reason of its density, of close stowage; and 6, freedom from sulphur. The names and the exact localities of the particular kinds of coal which were employed in these experiments are now in most instances lost; but their composition being preserved in the records of their analyses, the principles established are readily applied to other coals of similar composition.-For further information relating to the subject of fuel, see Anthracite, Charcoal, Coal, Coke, Gas, Peat, and Wood.