As respects coal, there is a considerable difference in the effects of the several "varieties. The caking or binding coal with which London is supplied from the great coal fields in Northumberland and Durham, under the general name of Newcastle coal, is much esteemed, from its affording a great heat, and burning with a lively flame; but those of Wall's End are regarded as superior to the latter for domestic use, as they burn with a whiter and more brilliant flame, and do not cake so hard in the grate. The Tanfield Moor coals are preferred for forges and furnaces, as they burn slowly, and afford a strong and long continued heat. From the experiments of Mr. Watt, it appears that a bushel of Newcastle coal, which weighs about 84 lbs. is competent to convert from 8 to 12 cubic feet of water into steam, from the mean temperature of the atmosphere and that a bushel of Swansea coal will produce the same effect. Dr. Black states, 7.91 lbs. of the best Newcastle coals will convert one cubic foot of water into steam capable of supporting the mean pressure of the atmosphere; and this statement appears perfectly to accord with the more extended experiments of Watt Smeaton makes it require 11.4 lbs. of coal to produce the same result in steam; but Smeaton has omitted to state the kind of coal.

If he employed the Staffordshire coal, there is no discrepancy, as will appear from the table of Mr. Tredgold's experiments and calculations, which we shall subsequently insert in this article. Mr. Tredgold found, that after the brickwork, etc. of the boiler of a steam-engine was warmed, a little less than 1 lb. of Wall's End coals would make a cubic foot of water boil, from the mean temperature of 52°. To produce the same effect with inferior coals a stronger draft and more time and attention arc necessary. Splint coal, or hard coal, called by Kirwan slaty cannel coal, is regarded as equally valuable for many purposes as the Newcastle caking coal. It does not produce so much flame nor so much smoke; it does not kindle so quickly, nor does it agglutinate, like caking coal. A large body of splint coal makes a strong and lasting fire. Cherry coal, or soft coal, readily catches fire, and burns with a clear yellow flame, giving out much heat, and the flame continues till nearly the whole coal is consumed. It burns away more rapidly than either caking or splint coal, and leaves a white ash; it is easily distinguished from caking coal by its not melting or becoming soft when heated; It makes a more agreeable fire, and does not require to be stirred.

It requires care and management in an open grate, even to burn the small fragments which are made in breaking up the pieces to a fit size for the fire: hence the small coals are often mixed with clay, and made into balls. When these balls are dry, Mr. Gray says, they make an excellent addition to the fuel for an open fire, producing a very durable heat. Mr. Watt calculated that 112 lbs. of these coals produced the same effect in raising steam as 84 lbs. of the Newcastle coal.

The following table, by Mr. Tredgold, shows the comparative and real effect of the principal varieties of solid fuel in converting water into steam.

Kind of fuel.

Fraction of a pound that will beat one cubic foot of water one degree of

Fahrenheit's scale.

Pounds of fuel that will convert one cubic foot of water into steam.

Newcastle, or caking coal

0.0075

8.40

Splint coal

0.0075

8.40

Staffordshire cherry coal.

0.0100

11.20

Wood (dry pine) . . .

0.0172

19.25

„ (dry beech) . .

0.0242

27.00

„ (dry oak) . . .

0.0265

30.00

Peat of good quality . .

0.0475

53.60

Charcoal

0.0095 , .

10.60

Coke

0.0069

7.70

Charred peat

0.0205

23.00

Mr. S. F. Gray is of opinion that fire-balls, of the size of goose eggs, composed of coal and charcoal in powder, mixed with a due proportion of wet clay, and well dried, would make a much more cleanly and in all respects a pleasanter fire, than can be made with crude coals, and not more expensive. He states, that in Flanders and Germany the practice of making equal weights of clay and coals together, and forming them into cakes, is common, and that the labour of the preparation is amply repaid by the improvement of the fuel, the coals thus mixed burning much longer, and giving more heat, than when they are burnt in their crude state; that although clay is an incombustible body, the fact is certain that coals so mixed afford more heat. For the purpose of lighting a fire speedily, Mr. Gray recommends the formation of " kindling balls," composed of equal parts of coal, charcoal, and clay, the two former reduced to a fine powder, well mixed and kneaded with clay moistened with water, and then formed into balls of the size of hens' eggs, and thoroughly dried, which, he says, may be used with great advantage, instead of wood.

These kindling balls, he further observes, may be made so inflammable as to take fire in an instant, and with the smallest spark, by dipping them in a solution of nitre, and then drying them again; if made of pure charcoal mixed with a solution of nitre, they would be still more inflammable. In situations where coals are scarce or dear we think that the mixtures recommended by Mr. Gray might be found convenient and economical; but when it is considered that the average price of coals in England is not more than a shilling for a hundred weight, we can hardly conceive it possible that the same weight of fire-balls, of the size of hens' eggs, could be manufacturer for the sum mentioned. It would appear, from Mr. Gray's remarks, that he was not aware that several patents had previously been taken out for the very objects mentioned by that gentleman; and although the advantages of them may not be very apparent in most situations, there are doubtless many localities where it may be otherwise; for the latter reason we shall, therefore, insert a brief notice of some of them. Mr. Sunderland's patent dated 1825, is for a fuel, in which gas-tar, clay, and refuse woody matter, are combined in various proportions, according to the degree of inflammability required.

One part of gas-tar, one of clay, and two parts of any convenient woody matter, such as saw-dust, tanners' spent bark, dyers' refuse wood, or peat, burn extremely well. If equal parts of the tar, clay, and saw-dust be employed, they make a composition which burns vividly and with a brilliant flame. The materials are, of Course, to be thoroughly mixed, made up into lumps, and dried either artificially or in the open air, preparatory to their being used as fuel. Messrs. Christie and Harper's patent, dated 1824, was for various mixtures of culm and stone coal (or anthracite) with bituminous or caking coal, depending upon the nature of the heat required; for the boiler furnaces of steam engines, where the bars are half an inch apart, the patentees state that one-fourth of the bituminous coal answers well for invigorating the other three-fourths. In 1800 Mr. Peter Devey had a patent for an improved artificial fuel, and the same gentleman, in 1821, had another patent for fuel balls, the particulars of which will be found in the specifications of their patents in the Inrolment offices in Chancery.