As cement is now so frequently sold in sacks of one-fourth barrel each, in which the cement is not so compact as in a barrel, we have assumed the contents of a barrel to be 3.45 cu. ft. for Portland, and 3.75 cu. ft. for natural, which are somewhat higher than the mean actual capacities of stave barrels as shown by tests. At three hundred eighty pounds and two hundred eighty pounds net weight respectively for Portland and natural, this is equivalent to one hundred ten pounds per cubic foot and seventy-five pounds per cubic foot packed. If we also assume that loose cement weighs eighty-five pounds per cubic foot for Portland and sixty pounds per cubic foot for natural; and that loose, dry sand weighs one hundred pounds per cubic foot, while loose, damp sand weighs eighty pounds per cubic foot, we may obtain the following comparisons, Table 59.

275. It is evident that in all specifications and in reports of tests, as well as in the use of cement, the method of stating proportions should be made clear, and in interpreting the results of tests this must be borne in mind. For instance, in tests to compare the value of limestone screenings with quartz sand, proportions by weight will favor the quartz, while proportions by volume will favor the screenings, since the latter are lighter.