there shall be used not less than 420 lbs. of cement per cubic yard of finished concrete, excluding, of course, the space occupied by any one-man stones that it may contain.

Using a Mixture of Gravel and Sand Without Screening.

If there be available and located conveniently for the work a large body of clean, mixed gravel and sand, it may prove economical to use it without screening in one of two ways, viz.:

First. By constantly making tests and finding what amount of sand or of gravel should be added to the natural mixture in order to secure the correct proportions, sifting out a supply of the material required, storing it close at hand for use, and combining it with the said natural mixture; and

Second. By using the natural mixture directly as it comes from the pit.

In either case the rule previously given for a volume of cement equal to at least one and one-tenth times that of the voids should be followed; and as a matter of precaution, the amount per cubic yard of finished concrete (excluding all embedded one-man stones) should be 460 lbs. for the first-mentioned of these two methods and 500 lbs. for the second. This is in accordance with the author's standard specifications.

Whether it is most economic to separate all the ingredients of the natural mixture and re-mix in the desired proportions, to add sand or gravel to the natural mixture, or to use the pit run, can only be determined for each case as it arises by some very careful computing, based upon the governing prices of labor and materials. In fact it might be necessary to make some actual experiments. If labor were very high and the price of cement delivered at site were reasonably low, it would probably be most economical to use 80 lbs. extra of cement per cubic yard of concrete and employ the pit run; but if cement were very expensive and labor cheap, it would be most economical to sift the natural mixture and remix.

Whether adding either sand or gravel to the natural mixture, in order to bring it to best proportions is economical, will depend upon whether the screening out and incorporating of the additional material and the occasional testing of the pit run will cost less than the 40 lbs. of cement saved per cubic yard of finished concrete.

Waterproofing

In certain places it is necessary that the construction should be absolutely waterproof ; and whether accomplishing this is to be done by adding some foreign ingredient to the cement or by inserting a layer of burlap or other similar material covered with pitch or asphaltum applied hot is a problem in economics that has to be solved for each case by some close figuring. There are on the market several patented materials to add to concrete for waterproofing, the manufacturer of each of which claims it to be the best possible  for  the  purpose;  but,  as  far  as  the  author  knows,  there