Note:  The original text reads ‘porus’] porous U tube, continuous from side to side of the valley, the outcrop on the surrounding hills forming the mouth of the tube.  The rain filtering down through the porous layer to the bottom of the basin forms there a subterranean pool, which, with the liquid or semi-liquid column pressing upon it, constitutes a sort of huge natural hydrostatic bellows.  Sometimes the pressure on the superincumbent crust is so great as to cause an upheaval or disturbance of the valley.  It is obvious, then, that when a hole is bored down through the upper impermeable layer to the surface of the lake, the water will be forced up by the natural law of water seeking its level to a height above the surface of the valley, greater or less, according to the elevation of the level in the feeding column, thus forming a natural mountain on precisely the same principle as that of most artificial fountains, where the water supply comes from a considerable height above the jet.

HOW MANY CUBIC FEET THERE ARE IN A TON OF COAL.—­There is a difference between a ton of hard coal and one of soft coal.  For that matter, coal from different mines, whether hard or soft, differs in weight, and consequently in cubic measure, according to quality.  Then there is a difference according to size.  To illustrate, careful measurements have been made of Wilkes-barre anthracite, a fine quality of hard coal, with the following results: 

Cubic-feet    Cubic feet
in ton of     in ton of
Size of coal.   2,240 lbs.    2,000 lbs.

Lump            33.2          28.8
Broken          33.9          30.3
Egg             34.5          30.8
Stone           34.8          31.1
Chestnut        35.7          31.9
Pea             36.7          32.8

For soft coal the following measures may be taken as nearly correct; it is simply impossible to determine any exact rule, even for bituminous coal of the same district:  Briar Hill coal, 44.8 cubic feet per ton of 2,240 pounds; Pittsburgh, 47.8; Wilmington, Ill., 47; Indiana block coal, 42 to 43 cubic feet.

The dimensions of the great wall of China and of what it is built.—­It runs from a point on the Gulf of Liantung, an arm of the Gulf of Pechili in Northeastern China, westerly to the Yellow River; thence makes a great bend to the south for nearly 100 miles, and then runs to the northwest for several hundred miles to the Desert of Gobi.  Its length is variously estimated to be from 1,250 to 1,500 miles.  For the most of this distance it runs through a mountainous country, keeping on the ridges, and winding over many of the highest peaks.  In some places it is only a formidable rampart, but most of the way it is composed of lofty walls of masonry and concrete, or impacted lime and clay, from 12 to 16 feet in thickness, and from 15 to 30 or 35 feet in height.  The top of this wall is paved for hundreds of miles, and crowned with crenallated battlements, and towers 30 to 40 feet high.  In numerous places the wall climbs such steep declivities that its top ascends from height to height in flights of granite steps.  An army could march on the top of the wall for weeks and even months, moving in some places ten men abreast.