By Mr. ANDREW CARNEGIE, New York.

In these days of depression in manufacturing, the world over, it is specially cheering to be able to dwell upon something of a pleasant character. Listen, therefore, while I tell you about the natural gas fuel which we have recently discovered in the Pittsburg district. That Pittsburg should have been still further favored in the matter of fuel seems rather unfair, for she has long been noted for the cheapest fuel in the world. The actual cost of coal, to such as mine their own, has been between 4s. and 5s. per ton; while slack, which has always been very largely used for making gas in Siemens furnaces and under boilers, has ranged from 2s. to 2s. 6d. per ton. Some mills situated near the mines or upon the rivers for many years received slack coal at a cost not exceeding 1s. 6d. per ton. It is this cheap fuel which natural gas has come to supplant. It is now many years since the pumping engines at oil wells were first run by gas, obtained in small quantities from many of the holes which failed to yield oil.

In several cases immense gas wells were found near the oil district; but some years elapsed before there occurred to any one the idea of piping it to the nearest manufacturing establishments, which were those about Pittsburg. Several years ago the product of several gas wells in the Butler region was piped to two mills at Sharpsburg, five miles from the city of Pittsburg, and there used as fuel, but not with such triumphant success as to attract much attention to the experiment. Failures of supply, faults in the tubing, and imperfect appliances for use at the mills combined to make the new fuel troublesome. Seven years ago a company drilled for oil at Murraysville, about eighteen miles from Pittsburg. A depth of 1,320 feet had been reached when the drills were thrown high in the air, and the derrick broken to pieces and scattered around by a tremendous explosion of gas. The roar of escaping gas was heard in Munroville, five miles distant. After four pipes, each two inches in diameter, had been laid from the mouth of the well and the flow directed through them, the gas was ignited, and the whole district for miles round was lighted up. This valuable fuel, although within nine miles of our steel-rail mills at Pittsburg, was permitted to waste for five years.

It may well be asked why we did not at once secure the property and utilize this fuel; but the business of conducting it to the mills and there using it was not well understood until recently. Besides this, the cost of a line was then more than double what it is now; we then estimated that £140,000 would be required to introduce the new fuel. The cost to-day does not exceed £1,500 per mile. As our coal was not costing us more than 3s. per ton of finished rails, the inducement was not in our opinion great enough to justify the expenditure of so much capital and taking the risk of failure of the supply. Two years ago men who had more knowledge of the oil-wells than ourselves had sufficient faith in the continuity of the gas supply to offer to furnish us with gas for a sum per year equal to that hitherto annually paid for coal until the amount expended by them on piping had been repaid, and afterward at half that sum. It took us about eighteen months to recoup the gas company, and we are now working under the permanent arrangement of one-half the previous cost of fuel on cars at work. Since our success in the use of this new natural fuel at the rail mills, parties still bolder have invested in lines of piping to the city of Pittsburg, fifteen to eighteen miles from the wells.

The territory underlain with this natural gas has not yet been clearly defined. At the principal field, that of Murraysville (from which most of the gas is obtained to-day), I found, upon my visit to that interesting region last autumn, that nine wells had been sunk, and were yielding gas in large quantities. One of these was estimated as yielding 30,000,000 cubic feet in 24 hours. This district lies to the northeast of Pittsburg, running southward from it toward the Pennsylvania Railroad. Gas has been found upon a belt averaging about half a mile in width for a distance of between four and five miles. Beyond that again we reach a point where salt water flows into the wells and drowns the gas. Several wells have been bored upon this belt near the Pennsylvania Railroad, and have been found useless from this cause. Geologists tell us that in this region a depression of 600 feet occurs in the strata, but how far the fault extends has not yet been ascertained. Wells will no doubt soon be sunk southward of the Pennsylvania Railroad upon this half-mile belt. Swinging round toward the southwest, and about twenty miles from the city, we reach the gas fields of Washington county.

The wells so far struck do not appear to be as strong as those of the Murraysville district, but it is possible that wells equally productive may be found there hereafter. There are now four wells yielding gas in the district, and others are being drilled. Passing still further to the west, we reach another gas territory, from which manufacturing works in Beaver Falls and Rochester, some twenty-eight miles west of Pittsburg, receive their supply. Proceeding with the circle we are drawing in imagination around Pittsburg, we pass from the west to the southwest without finding gas in any considerable quantity, until we reach the Butler gas field, equidistant from Pittsburg on the northwest, with Washington county wells on the southwest. Proceeding now from the Butler field to the Allegheny River, we reach the Tarentum district, still about twenty miles from Pittsburg, which is supplying a considerable portion of the gas used. Drawing thus a circle around Pittsburg, with a radius of fifteen to twenty miles, we find four distinct gas-producing districts.

In the city of Pittsburg itself several wells have been bored; but the fault before mentioned seems to extend toward the center of the circle, as salt water has rushed in and rendered these wells wholly unproductive, though gas was found in all of them.