-------+-------------------------------------------------------------- | Effective Size in Millimeters 10 per cent finer than: _h_/_l_+------+------+-------+--------+--------+--------+-------+----- | 0.10 | 0.20 | 0.30 | 0.35 | 0.40 | 0.50 | 1.00 | 3.00 -------+------+------+-------+--------+--------+--------+-------+----- | | | Million Gallons per Acre daily. | | .001 | .01 | .04 | .10 | .13 | .17 | .27 | 1.07 | 9.63 .005 | .05 | .21 | .48 | .65 | .85 | 1.34 | 5.35 |48.15 .010 | .11 | .43 | .96 | 1.31 | 1.71 | 2.67 | 10.70 |96.30 .050 | .54 | 2.14 | 4.82 | 6.55 | 8.55 | 13.40 | 53.50 | .100 | 1.07 | 4.28 | 9.63 | 13.10 | 17.10 | 26.70 |107.00 | 1.000 |10.70 |42.80 | 96.30 | 131.00 | 171.00 | 267.00 | | -------+------+------+-------+--------+--------+--------+-------+----- The above table is computed with the value _c_ taken as 1000, this being approximately the values deduced from the earliest experiments. More recent and extended data have shown that the value of _c_ is not entirely constant, but depends upon the uniformity coefficient, upon the shape of the sand grains, upon their chemical composition, and upon the cleanliness and closeness of packing of the sand. The value may be as high as 1200 for very uniform, and perfectly clean sand, and maybe as low as 400 for very closely packed sands containing a good deal of alumina or iron, and especially if they are not quite clean. The friction is usually less in new sand than in sand which has been in use for some years. In making computations of the frictional resistance of filters, the average value of _c_ may be taken at from 700 to 1000 for new sand, and from 500 to 700 for sand which has been in use for a number of years. The value of _c_ decreases as the uniformity coefficient increases. With ordinary filter sands with uniformity coefficients of 3 or less the differences are not great. With mixed sands having much higher uniformity coefficients, lower and less constant values of _c_ are obtained, and the arrangement of the particles becomes a controlling factor in the increase in friction. The friction of the surface layer of a filter is often greater than that of all the sand below the surface. It must be separately computed and added to the resistances computed by the formula, as it depends largely upon other conditions than those controlling the resistance of the sand. While the value of _c_ is thus not entirely constant, it can be estimated with approximate accuracy for various conditions, from a knowledge of the composition, condition, and cleanliness of the sand, and closeness of packing.