increased as much as five per cent. by overcrowding them with coal, or feeding coal faster than the rolls can take it.

It has also been proven that the amount of waste is less when the coal is not broken at once into small sizes, but is passed through two sets of rolls,-the second set being closer together than the first set,-but the saving effected in this way is small.

The size of the rolls has some influence on the amount of waste made, and large rolls are now preferred by many colliery managers.

The illustration (Page plate No. 47) shows a safety device by which breakage is prevented when the rolls draw in material too hard to crush without breaking either the teeth, rolls, or gearing.

The frame carrying the bearings rests against a shell of cast-iron inserted in a box. This shell is made thick enough to stand a thrust of not more than nine* tons (?); if subjected to a greater pressure it breaks and allows the rolls to slide further apart, relieving them of all strain..

The cast-iron shell is quickly replaced by removing the cap covering the box in which it is placed.

The illustration also shows a convenient form of gearing by which the distance between the teeth (the two rolls) can be varied at any time, even when the rolls are running full speed.

"Crusher" or "Steamboat rolls" are now usually made from 2' 9" to 3' 6" in diameter and about three feet long. These are also sometimes called "Prepared coal rolls," and the smaller rolls "Pony" or "Monkey" rolls,—but the largest rolls are generally known by the former names; the next size smaller are called "Prepared coal rolls," and rolls used for breaking down egg or large stove to smaller sizes are known as "Monkey rolls" or "Small monkey rolls."

Screens.

These are always cylindrical, and are inclined at a slight angle so that the coal will slowly travel from through the

*This figure may not be correct; I am not certain that I remember it correctly.-H. M. C.

screen from one end to the other. The inclination is usually about one inch in twelve. Increasing the pitch has the same effect on the screening as shortening the screen, but increases instead of diminishes its capacity. Hence, the screening capacity in large breakers is increased by increasing the pitch of the screens, but to secure thorough screening very long screens are necessary, while at breakers handling a small output the screening may be equally well done by short screens with a very small inclination.

The main screens are usually from twenty to thirty feet in length and are made to separate three or four sizes; they are also called stove screens.

The broken screens, the mud or dirt screens, and the chestnut or pea screens are commonly from eight to sixteen feet long, the broken screens are sometimes made twenty to twenty-four feet long.

In diameter they range from four to eight feet.

They are constructed of a number of cast or wrought-iron spiders set at intervals of from three to five feet on a wooden, a cast or wrought-iron shaft, and covered in between with wire or cast-iron gratings of the proper mesh.

The spiders naturally divide the screen into "segments," and each segment may have a mesh of different size, but it is generally found necessary to make at least two adjoining segments of the same size.

The segments at the upper end of the screen, or the end into which the coal is fed, carry the smallest mesh and those at the lower end are made of the largest mesh.

Cast-iron meshes are now greatly preferred to those woven of wire or small iron rods, except for the very smallest meshes. Small meshes made of cast-iron are very weak, and cannot stand the blows given with a hammer from time to time, to shake out the fine dirt with which they become clogged.

Phoenix columns are now used by some companies for screen shafts. Wooden shafts are too elastic, and when long they spring in the center and break the segments, (when made of cast-iron); cast-iron shafts are too heavy,

and wrought-iron pipe of large size is more expensive, and at the same time is not as stiff as a Phoenix column.

I do not know that any attempt has yet been made to run the screens on outside friction pulleys, but it seems probable that such a plan might give satisfactory results, and do away with these heavy shafts.

The peripheral speed at which screens are run varies from about one hundred and twenty to two hundred and twenty feet per minute, but we generally find them running at a speed of from one hundred and sixty to two hundred feet per minute.

Short screens are frequently made with an outside jacket carrying a finer mesh than the main body of the screen, and long screens are commonly jacketed at the upper end.

The outside jacket is commonly made of a very small mesh to separate the fine dirt from the chestnut, pea, and buckwheat sizes, or from the two latter sizes, so that when this coal goes to the chestnut or pea screen, a considerable quantity of dirt has already been separated from it. In other cases the jacket is made with a large mesh (23" or 13") to take out from the end of the jacket the egg or large stove coal, the main body of the screen separating out the broken. In this case all the smaller sizes pass through the jacket (mesh) to go to the stove or main screen.

Chestnut and pea screens are nearly always jacketed; the dirt passes through the outer mesh, the buckwheat passes through the lower segment of the jacket, the pea out of the end of the jacket, and the chestnut out of the end of the inside screen; or the buckwheat comes out at the end of the jacket, and the pea coal comes out at the end of the inside mesh.

Two methods of driving screens are in common use:

1. By bevel gear on the screen shaft, usually at the lower end of the screen, as shown by Atlas Sheet No. XVI; or,

2. By cog-gear on the periphery of the screen, at its upper end, as shown by Atlas Sheet No. XVII, and by the main screens at the Hollenback Breaker, Atlas Sheet No. XVI.

The slate picking segments have already been described.

They are usually narrower than the regular screen segments, and are useful only when a considerable portion of the refuse occurs in flat pieces, and the coal does not break in this way.

To insure proper screening, i. e., the removal of all the fine dirt, and satisfactory sizing of the coal, with a minimum amount of loss, it is absolutely necessary to feed the screens regularly.

When the screens are crowded with coal, we always find a considerable quantity of dirt remaining in the coal, and we also find chestnut and pea coal mixed in with the stove, stove mixed with the egg, etc.

The screen feeder shown by Atlas Sheet No. XVII, as used by the Lehigh Coal and Navigation company, removes this difficulty; but appliances of this kind have not been generally adopted. The illustration plainly shows its action, so that description of it is not necessary.

When the coal is wet and dirty, washing is necessary to remove the dirt that adheres to each fragment. This is accomplished by a series of small streams or jets of water falling upon the coal in the screens from a perforated pipe or trough above the screen. The coal coming down over the main screen bars is sometimes washed in the same way.

This washing is not only necessary because without it the coal would look very dirty and not find a ready sale, but also because the coal must be bright and clean to enable the slate pickers to readily distinguish the slate, bone, etc., from good coal.

It is not possible by screening to separate the coal into grades of even approximately uniform size, hence we find that in any one size of coal the largest pieces will weigh two, three, or four, (or more) times as much as the smallest fragments. This is because the coal breaks into pieces of almost every conceivable shape.

Thus in large stove" coal screened over a 13 mesh and through a two-inch mesh,-which is very close screening,a flat triangular piece of coal about 21" in its least width and say one half inch thick, is the smallest piece that will not pass through the 13" mesh; it will contain about 1 cubic