The results are shown in the tables, and plotted in the curves, the ordinates being cumulative percentages, i. e., the total percentage unable to pass a given sieve, and the abscissæ being proportional

SCREEN-CLASSIFICATION OF PREPARED GALENA CRUSHED IN JAW CRUSHER.

Weight taken in pounds. 151 149 147 148 151 149 147 148

to the logarithms of the sieve apertures. The reason logarithmic proportion was preferred to direct proportion, is the difficulty of representing on a diagram, a range of .16 mm .to 44 mm. Further,

SCREEN-CLASSIFICATION OF PREPARED GALENA CRUSHED IN ROLLS.

logarithmic plotting prevents the lower part of the curves from being unduly drawn out, and this is the least important region, since it represents the oversize, or abnormally flat pieces, whose distribution is largely accidental, though the amount is fairly uniform. In the upper parts of the curves, several cases of change of curvature are made visible by the large scale employed. These are not due to accidental causes, as are those at the lower end, but to defective analysis, which is, indeed, inevitable, since the amount left on any screen in the small sizes is, to some extent a compromise, particularly in the case of the galena; for to shake long enough to get everything possible through involves production of undersize by

SCREEN-CLASSIFICATION OF PREPARED GALENA CRUSHED IN GYRATORY CRUSHER.

*The tests asterisked were made after changing the gyratory cone.

attrition. It is, however, worthy of note that these experimental errors are almost of the same order of magnitude as the changes with varying speed. It should be remarked that there is a discontinuity in the results of the gyratory crushing of galena, owing to the changing of a cone, so that instead of a series of four curves we have two groups of two.

The material was prepared as follows: The ore having been spalled small enough to be taken by the jaw crusher was piled in a heap, and four barrels were filled by putting a shovelful into each in turn, none of the material being fine enough to pass a 21⁄2" ring and the great majority very much coarser. The material from the jaw crusher lying between 32 and 21.4 mm. was used in the gyra

tory, and the material obtained between 21.4 and 5.2 mm. used in the rolls, the object of this feeding of sized material being to make, so far as possible, the speed the only variable in the experiments. With the galena the sizing was confined within even closer limits, 63 to 32 mm., 32 to 21.4 mm., and 21.4 to 15.2 mm. for the three machines respectively.

The results obtained seem to admit of only one interpretation; that while decrease of speed tends to decrease the production of undersized material, it only does so in a degree so small that in general it is altogether disproportionate to the disadvantage of decreased capacity. Moreover the range both in speeds and in tenacity of material were sufficiently great to render it probable that similar results would be obtained under almost all conditions.

INTERNAL STRESSES IN MASONRY DAMS.
By S. D. BLEICH.

In the usual design of masonry dams and retaining walls, the only internal stresses which are considered are the direct compressive stresses on horizontal sections. The effect of shear from the horizontal forces acting against the wall is taken into account in a very elementary and approximate manner. It is thought, that if the following three conditions of stability are satisfied, the wall is safe. These are:

1. The maximum compressive stress due to the vertical forces must not exceed the safe limit for the material composing the wall. 2. No tension shall exist at any point of a section. This requires that the line of pressure must not pass outside the middle third.

3. The resistance to sliding at any horizontal section shall be greater than the total horizontal force at this section. This requires that the line of pressure must never make with the normal at any horizontal joint an angle greater than the angle of repose. A fourth condition of stability is frequently stated, that the wall shall be safe against overturning. But when condition (2) is satisfied, there can possibly be no overturning.

Many elaborate analyses have been made for walls of straight and curved profiles. But all of them with the exception of some

VOL. XXVII.-3.