The cubic contents of cars commonly used ranges from seventy to one hundred and forty cubic feet, or in weight from one ton and three quarters to four tons. These are the extremes, a mine car of average size holds about one hundred to one hundred and ten cubic feet, or in weight from two and a half to three tons of coal when loaded full.

In the Lehigh district very large mine cars are more common than in the Wyoming Basin. This difference is caused by the prevalence of steep dips in the former district.

The Hazleton district can boast of some very large cars, as, for instance, those in use at the No. 6 colliery and other workings of Pardee & Co.

At the Lehigh Valley Coal company's collieries, near Shenandoah, cars of one hundred and twenty to one hundred and forty cubic feet capacity are used. At collieries Nos. 2 and 4 cars of one hundred and forty cubic feet, carrying from three and three quarters to four and a half tons are used; at the No. 3 colliery the cars hold one hundred and thirty-three cubic feet, and at the No. 1 colliery the cars carry about three and a quarter tons, holding one hundred and twenty cubic feet. These cars are estimated by Col. Brown to weigh, empty, about 4500 and4757 lbs. In shape and in the methods of construction the mine cars in use present an almost infinite variety. Some are constructed almost entirely of wood, others entirely of iron, but the most common plan is to use wood braced with iron tie rods or bolts for the frame, and to protect and stiffen the joints by angle iron and cast-iron shoes.

In shape they vary from the broad low car of some of the Wilkes-Barre mines to the high narrow car used in the Schuylkill region, and from the square upright pattern to the hopper-shaped style.

In thin and flat seams the broad, low car is preferred; the hopper-shaped car is best adapted to the narrower guages when it is desired to increase the capacity without increasing the length or height of the car

Large square cars of moderate height with a capacity of one hundred cubic feet or more, are slowly but surely replacing all other patterns at mines where the height of the

seam, etc., permits; and the present tendency is strongly in favor of inside wheels.

They are supplied with a swinging door hung on an iron rod which runs across the top of the car through both hinges. The doors are locked by a double iron latch catching in a lip or latch holder on each side of the car, as in Page Plates 26 and 29, by a single vertical bolt or swinging latch in the center of the car just over the draw-hook (Page Plates Nos. 27 and 28)sometimes operated by a lever at the side of the car, by a rocking shaft carrying lips which catch the bottom of the door, (see Plate No. 25,) or by a movable iron hook (latch) placed bottom side up (to work by gravity) on the side of the car. Other fastenings have been used, but the double iron latch is the style now generally preferred. Complete sheet iron lining is frequently used, but it is not always considered necessary, as the car frame will often wear out as fast as the lining. The floor of the car is lined with iron plates about one eighth of an inch thick, which not only protects the plank from damage, but is of considerable advantage in allowing the coal to quickly slide out when the car is dumped.

Cars constructed of boiler iron have been used at a number of collieries, especially in the Lackawanna district, but as they have not been adopted at recently opened collieries equipped with the most perfect mining plants, we may conclude there are important objections to their use that outweigh any apparent advantages they possess. We may indicate some of the disadvantages: 1, Cost; 2, Difficulty and expense of repairs; 3, Their rigidity increases the wear on the wheels, axles, and track; 4, Their weight,-for equal strength.

Wheels and axles.

Cast-iron wheels are almost always used. Cars have been built with wrought iron spoke wheels, (see Page Plate No. 26, Summit Hill car,) but the ordinary cast-iron wheel seems better suited to mine work, and is much cheaper.

The most important problem which must be solved before the best form of mine car can be determined involves the

relative merits of the inside (or fixed) and the outside (or loose) wheels.

Viewed from the stand-point of a mechanical engineer, there can be no doubt of the superiority of the inside, over the outside, or loose wheel. The inside wheel cars run smoothly, while nothing can be more uncertain than the irregular running of loose wheeled cars. In the language of a prominent engineer, "We cannot get any two loosewheeled cars to run alike."

On the other hand, the loose-wheeled cars are cheaper, they are more easily lubricated, broken wheels can be instantly replaced by new ones, and the axles are less likely to bend.

I think the advantages of each style may render it superior to the other under certain conditions. At collieries hauling the coal a long distance underground on good roads, or transporting it in mine cars a considerable distance to the breaker, the inside wheels are doubtless the better form; but at mines transporting the coal only a short distance and on very bad roads, or over frequent sharp curves, outside wheels may give better results.

Loose or free running wheels are generally lubricated from the outside or through a tap hole in the hub, but several attempts have been made to obtain a self-lubricating wheel, by making either the axle or wheel carry the oil. The objections to hollow axles have prevented the adoption of this class of inventions, but self-lubricating wheels with hollow hubs have been successfully used at several collieries.

The difficulty of producing a wheel with hollow hub free from flaws, the increased weight of such wheels, their tendency to unequal wear, greater liability to breakage, and increased cost, have in the past and probably will in the future, prevent the general adoption of this method of lubrication.

Cars with loose outside wheels and axles running in pedestals do not seem to have any marked advantage over those with the simple loose wheel and fixed axle.

Almost as many different forms of axles and boxes as of mine cars are in use in the anthracite region, and it does