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 2387 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 series, 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." 11* This article has elicited criticism and additional facts from others, equally paractical, through the same useful channel.

This subject is also treated on at some length, in Dr. Ure's Dictionary of Mines, &c. The method of working the thick coal veins of France will be found in the present volume, at page 332, showing the practice adopted in the mine of Blanzy. The mode of working the main or ten yard coal at Dudley, in England, is also described at page 286 and illustrated by fig. 20.

At page 420, we have supplied a brief account of an extremely interesting coal mine, that of Anzin, in the coal basin of Valenciennes. The coal measures are here, as at Mons in Belgium, 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.

In order to show more distinctly the position and arrangement for the ventilating fires at the bottom of the shafts, we introduce the following enlarged figure of those employed in the mines of Anzin.

Diagram showing the arrangement of a ventilating furnace, "foyer d'aerage."

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.

Coal Formation.

Chalk Formation.

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.

Shaft of the Coal Mine of Anzin in France.

a b

Coal Measures.

"Morts-terrains" or dead lands.