This is largely used, especially when mixed with lime to retard the setting, in the construction of walls of buildings, cellar foundations, and, in general, in masonry where the unit-stresses are so low that strength is a minor consideration, but where a lime mortar would not harden because it is to be under water or in a solid mass where the carbonic acid of the atmosphere could not penetrate to the interior. When natural cement is dumped loosely in a pile, the apparent volume is increased one-third or even one-half. This must be allowed for in mixing. A barrel averages 3.3 cubic feet. Therefore a 1:4 mortar of natural cement would require one barrel of cement to 13.2 cubic feet (about one-half a cubic yard) of sand. A bottomless box similar to that illustrated in Fig. 7, and with inside dimensions of 3 feet X 2 feet 6 inches X 1 foot 9 inches, contains 13.2 cubic feet. It is preferable to use even charges of one barrel of cement in mixing up a batch of mortar, rather than to dump it out and measure it loosely. If the size of the barrel varies from the average value given above, the size of the sand box should be varied accordingly. The barrels coming from any one cement mill may usually be considered as of uniform capacity. Since it is practically somewhat difficult to measure accurately the volume of a barrel, owing to its swelling form, it is best to fill a sample barrel with loose, dry sand, and then to measure the volume of that sand by emptying it into a rectangular box whose inside area, together with the height of sand in it, can be readily measured.