A portion of these blocks must have a bevel face on them to form the batter on the front of the wall. There must also be a proper proportion of them having their ends bevelled to the batter of the wall, to use as "headers." A header in brick, stone, or concrete, is a unit, or piece, that is laid in the wall with its ends showing through on the face, while a "stretcher" shows its whole length on the face of the wall. Other portions of brick or stone, when built in a wall, are called "closers."
The batter on the blocks is formed by making one side of the mould lower than the other. In this case, the difference in the width of the sides of the mould would be 11⁄2 inches; because the height of the wall being 8 feet, the blocks 1 foot thick, and the batter 1 foot, there would be a falling off on each block of 11⁄2 inches in order to have the top front of the wall 12 inches back from the bottom front. The ends of the header blocks may be battered by placing in the ends of the mould a piece of wood 12 inches wide, and the lower edge 11⁄2 inches thick, and the top edge planed to a thin wire edge. The end or section of the plank will then have the appearance of a wedge 12 inches long, 11⁄2 inches thick on one end, and tapered to nothing at the other end. When the block is taken from the mould, and the wedge piece removed, the block will show the same batter on its end as the stretchers do on their face, and they can be built in together without showing any difference in the slope, if the work is carefully done.
Nick, who had had some experience in this kind of work, found no difficulty in understanding the whole process.
At low tide he set to work to make a solid bed for the foundation, while the boys handed him the stone and the prepared mortar as he required it, so that before the tide rose one side of the stone foundation was ready to receive the concrete blocks. During the interim between tides, Nick and the boys made the moulds, prepared for mixing the concrete, and got old timbers and lumber for a temporary scaffolding. After the moulds were made and some concrete mixed, Nick began on the blocks. It was not long before he had a sample, which seemed all right, and before he stopped quite a number of them were ranged on boards "setting."
On the sixth day after it had been commenced, the job was entirely finished. The joints in the wall had been nicely "pointed" up with cement mortar by aid of a fine-pointed trowel. The back, or ground side of the wall was filled in with earth, and danger to the pier was entirely removed.
That night Mr. Gregg told the boys and Jessie—who had watched closely the growth of the wall—quite a lot about Portland cement and concrete, which interested them very much. Portland cement as we have it now was unknown a hundred years ago, but an Englishman invented the method of making it and properly proportioning the various materials used. Fifty years ago there was scarcely any made in this country, the little that was used being imported from England, and later from Belgium; but now more of it is made and used in the United States than anywhere else in the world. He pointed out that the building of the Panama Canal was made much easier and less costly because of cement, and that the largest dam ever built had just been suggested, to dam the Mississippi near Keokuk, Iowa. This would be over 5,800 feet long and nearly 40 feet high and from 25 to 35 feet thick. He told of the various big storage dams being built and contemplated by the United States, in Montana, Arkansas, Nebraska, Wyoming, New Mexico, Dakota, Texas, and many other places, at a cost of hundreds of millions of dollars—which never would have been attempted if concrete had not been available. He also made mention of the great wall that now protects Galveston from the ravages of the sea. It is not many years since Galveston was almost destroyed by tidal waves that caused an enormous loss of life, and destruction of property amounting to over $17,000,000. The wall was built to prevent a recurrence of similar disasters. It is 17,503 feet long, 17 feet high, and 16 feet thick at the base. Another recent work is the enormous dam built by English engineers across the river Nile at Assiout, about 250 miles above Cairo in Egypt, which increases the area of good land some 300,000 acres. Ancient Babylon is again to blossom and become a beautiful country to live in, for British engineers are laying out plans for building storage dams and irrigating canals in these now sandy and barren lands. All, or nearly all, of these works and proposed works would never have been thought of, if Portland cement had not been in existence.
Mr. Gregg, after finishing his talk on concrete, noticed that George had two fingers on his right hand tied up, and on inquiry was told that George had his fingers hurt by a concrete block falling on them just as the retaining wall was being finished. The father insisted on seeing the bruised fingers and found they were not badly hurt, though the skin in one place was broken. George explained that his mother had washed his hand, dressed the wound, and applied an antiseptic to it, so that it was all right now and did not pain him.
"You were wise to go to your mother and have your bruise attended to immediately, otherwise you might have had something serious happen to you, as lockjaw frequently comes from wounds of that kind, if deep enough and not attended to immediately. It is often said that lockjaw or tetanus is caused by a wound made by a rusty nail. It is certainly bad to be wounded with a rusty nail—or any other rusty iron—and tetanus may follow; but it does not follow because the nail is rusty, but because the tetanus microbe that may be on the nail, or on the skin when the wound is made, is carried into a favourable place for development.
"This tetanus microbe, which has a long name, is very plentiful and is scattered broadcast by every gust of wind. It is a microbe of dirt, and the ground and street abound with it. Its first home and breeding place is in the intestines of horses and other domestic animals, but its greatest danger to the human family is when it gets into the blood by way of a wound. Cleanliness, in this as in many other cases, is both a preventive and a cure."
"Father," said Jessie, "I saw a very funny thing to-day while watching Nick and the boys finish the wall. The train across the river came to a standstill for some reason or other, and, as I was watching it, I saw three puffs of steam go out of its boiler, and a short time after I heard three loud whistles. This seemed to me quite curious, but while I was thinking over it, there were three more jets of steam, followed by three more 'toots.' How was it that I saw the toots before I heard them?"