"Produce J H through the wall toward the water side. Find centre of gravity of wall in K and the weight of the wall, which in this illustration equals—
(AB + CD)/2 × AC × 140 lb. = (2 + 3)/2 × 8/1 × 140 lb. = 2,800 lb.
"From where J H produced meets a vertical line drawn through the centre of gravity, K, in L set of L N equal to 2,800 lb.; make L M equal to J H; complete parallelogram L M O N, when L O equals resultant of earth and wall.
"The magnitude and direction of P R can be found as in the first part of this article. Produce R P through the wall, and from where it cuts the resultant L O in S make S T equal R P. Let the diagonal L O now be produced so as to make S V equal to L O. Complete the parallelogram S T U V, when the resultant S U equals the combined resultant of earth, water, and wall, and as it passes within the middle third it can be considered safe.
"Now, boys," said Mr. Gregg, "I have not only told you how to build a retaining wall, I have also told you how to make all the necessary calculations for designing it, as the same figuring and diagraming, on this principle, will answer for any sea wall requiring like conditions.
"I know you both understand figures and geometry enough to make such calculations, if you are ever called upon to do so."
The next morning, before the boys had finished their breakfast, Nick was on hand ready to go to work, equipped with a pair of hip rubber boots which would enable him to wade in water two feet deep and remain dry.
Fred and George were soon ready and Mr. Gregg went out to tell them the proper way to commence. The foundation was the first consideration, so an examination of the site and was made, the length of the proposed walls measured off. While waiting for the tide to ebb to its lowest point, Nick and the boys busied themselves gathering up stones for the foundation and wheeling them to the point nearest where they were to be used.
After gathering all the stones thought necessary, the question of making the moulds for the concrete blocks was considered, and, as the greatest bulk of the blocks would be simply blocks with square ends and square faces, the moulds for these would be a box having inside dimensions of 12 inches deep, 12 inches wide, and 24 inches long. These dimensions would then allow of blocks being made in the moulds that will contain exactly 2 cubic feet. The mixed concrete was dropped gently in the mould and lightly tamped so as to make it solid. The mixture consisted of not less than 3 of cement, 5 of sand, and 7 of very fine gravel or broken stone, no piece being larger than a white bean. It was mixed in the same manner and in accordance with the rules given for making concrete for the sidewalk in Chapter I.
The mould should rest on a smooth block of stone, wood, or other suitable material, while being filled and tamped, and when full the surplus should be levelled off, by a straight-edge—wood or iron—drawn over the top of the mould, until all the surplus is removed. The mould is then allowed to stand a little while until the concrete "sets" fairly hard, when the mould may be removed. To make it easy to take the block out of the mould, the inside should be well sprinkled with neat cement before the concrete is put in, and the box itself might be made slightly tapering to permit the block to move out easy. This method, however, is not to be recommended, as the blocks do not fit so well in a wall as when left perfectly square. There are a number of devices for making moulds so that delivery of blocks may be easy. One of the best is to hinge one corner of the mould with heavy hinges, while the opposite diagonal corner is left loose but held in place by a strong hasp and staple. When the box or mould is full and the block ready to remove, the hasp is loosened, the mould opens across at the two corners and frees the block. Should there be any holes or defects on the face of the blocks, they can be filled with cement mortar made with 2 of cement and 3 of clean sand. Blocks of this size should season not less than 4 or 5 days, to set hard before being used.