MINING CONDITIONS UNDER CITY OF SCRANTON, PA.
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5.	The items given in the columns under the coal double-bed headings are the “First crack,” indicating the pressure in pounds per square inch required to produce the first crack in the sample. In other words it is the pressure which would cause the coal of the same quality as the sample to begin squeezing. The items under “Maximum load” in each case indicate the pressure in pounds per square inch at at which the sample crushed. The items under the head of “No. of tests” indicate the number of tests taken from the respective beds in the several collieries, the average of these tests being represented by the figures in the table.
6.	The horizontal line headed “General average” contains the average of all the tests under the respective coal beds as indicated.
The next horizontal line under “General average” indicates the percentage of the maximum load which is represented by the pressure necessary to produce the first crack, or the squeezing pressure. And the second horizontal line under the general average shows the percentage which tests 2 and 4 bear to test 1 in each case.
The grand average, or net result, is contained under a separate heading, and indicates generally the average squeezing and crushing strength as shown by the samples tested. The tests from the several coal fields are tabulated separately, and clearly show the superior strength of the harder coals and the weakness of the softer.
7.	From an inspection of this table, the following results appear to apply, approxi-׳ mately:
That the squeezing strength of a mine pillar whose width is twice its height is about 3,000 pounds to the square inch, and the crushing strength about 6,000 pounds per square inch, or, approximately, twice as much. And in general, other things being equal, the crushing strength of mine pillars would vary inversely as the square root of the thickness of the bed.
The same general rule apparently holds true, also, for the squeezing strength in all cases where the height of the pillar is less than its width. In tall pillars having a height greater than their width, the squeezing strength apparently remains nearly constant, while the crushing strength continues to diminish with height according to the above rule.
Wm. Griffith, Chairman. Harry E. Yewens,
J. H. Fisher,	H. H. Stoek,
Morgan Davis, Jr.	J. T. Beard,
Coal-test Committee, Scranton Engineers' Club.