General View or Table of the relative density of different species of Coal and Anthracite, at various parts of the World.

In a previous page we took occasion, by means of the requisite tables, to exhibit the relative amounts of carbon and volatile matters which exist in the principal bituminous coals of England, Scotland, Wales, Belgium, France, and the United States of America; and hence to show their comparative adaptation to the manufacture of iron. The following table offers another method of making useful comparisons of the bituminous and non-bituminous combustibles in various parts of the world, by means of their respective specific gravities and weights per solid cubic yard, in pounds, avoirdupois.

From the foregoing table several useful facts are made apparent. The first is, the greater weight of the American anthracites than those of Europe; second, that the bituminous coals very closely coincide in both quarters of the globe.

We have neither added the weight of the intermediate species of coal to the bituminous column nor to the anthracites; because, in either case, it would have unduly affected the true character of the averages. Third, as regards the table of American anthracites-and it may be correct also to include that of the bituminous coals-it will be seen, with the assistance of a map, that their specific gravity

increases as we advance from west to east: confirming also the fact, noted elsewhere, that the weight of the combustible decreases in proportion to the amount of bitumen with which it may be charged.

The Anthracites of Pennsylvania, commonly distinguished as White Ash, or Red Ash Coals, and selected according to their respective qualities.-Whilst treating on the comparative value of the varieties of Pennsylvania anthracite, as applied to iron making, we have said but little in relation to their relative values for domestic use. It seems established that, for closed furnaces, for warming houses, the white ash variety, being the most compact, dense, and slow burning, is more durable, and consequently, more preferable than the softer red ash coal. In open grates, for warming apartments, the latter is decidedly preferred. We have observed a recent statement of the result of an experiment, in relation to this point, which, as regards the warming of apartments, seems tolerably decisive.

A very important and interesting experiment was recently made. for the purpose of testing the comparative value of the red and white ash coals for domestic purposes. Two rooms of nearly the same size, and having the same temperature, were selected to ascertain how many pounds of each kind would be required to heat them to a temperature of 65 degrees, during a period of 15 hours, when the temperature out of doors at 9 A. M. was at ten degrees below the freezing point. Two days were occupied in the trial, so that the red and white ash coals might be used in alternate rooms. Fires were made at 9 A. M. and continued until 12 P.M. Two thermometers (one in each room) were suspended at the greatest distance from the grates, and the temperature was carefully registered every hour. The result was as follows:

Thirty-one pounds each day of the Schuylkill red ash coal gave a mean temperature of 64 degrees; and thirty-seven pounds each day of the white ash, taken from a vein of high repute in the Lehigh region, gave a mean temperature of 63 degrees. Making 2000 pounds of the red ash to be equal to 2,387 pounds of the white; or, red ash coal at $5.50 per ton, to be equal to white ash at $4.61. This settles the question on the score of ECONOMY.

DEPTHS OF COAL MINES.

The following statement has been prepared from a much more extensive sèries, in order to exhibit the minimum, the maximum, and the average depths beneath the surface at which beds of coal are at this time productively worked, in the principal mining regions of the world.

SYSTEMS FOR WORKING. COAL MINES.

We have devoted but small space to this subject; not that we are insensible of its extreme importance, but because it was somewhat out of the scope we had assigned to the present volume, and also because this knowledge may be separately obtained through the medium of numerous publications by experienced persons; conveying that precise description of information, for the benefit of those who are practically engaged in this service, or are interested in this description of property. It would be invidious, perhaps, to make mention of some of these, without including all.

The Parliamentary Reports embody a great amount of practical information on the methods employed in excavating coal mines. The pages of the Mining Journal, during many years, have been rich in valuable details of the same kind: and among the most recent of its articles is one from Mr. Dunn, "on the various systems practised in the conducting of coal mines, and of the methods employed in counteracting the effects of inflammable air.”* This article has elicited criticism and additional facts from others, equally practical, through the same useful channel. This subject is also treated on at some length, in Dr. Ure's Dictionary of Mines, &c. The coal measures of Anzin in the coal basin of Valenciennes, as at Mons, in Belgium, are covered by an enormous thickness of horizontal cretaceous and tertiary strata, through which it is necessary to penetrate. These overlying beds are called by the French miners "morts-terrains," or dead lands, and being highly charged with springs of water, require great skill and enormous expense in sinking the shafts through, until they reach the inclined coal seams, at the depth of from two hundred and twenty to eight hundred feet beneath the surface.

The annexed figure affords a remarkably instructive view of these circumstances, both in a mining and geological sense; showing the revétement or impervious lining of the shaft, through the "dead formations;" the mode of ascent and descent provided for the miners, and the position of the ventilating fire, near the bottom of the vertical shaft.

* Mining Journal, March 21 and 28, 1846. The reader will derive much interesting information, respecting coal mining operations, from the lectures of Professor Anstead, as reported in the London Mining Journal, 1847-8.