Fig. 72. Boomerang
George asked his father to describe one and to explain its uses. Mr. Gregg told the boys that a boomerang, as used by the aborigines of Australia for a weapon or missile of war or in the chase, consisted of a flat piece of hard wood bent or curved in its own plane, and from 16 inches to 2 feet long. Generally, but not always, it is flatter on one side than on the other. In some cases the curve from end to end is nearly an arc of a circle; in others it is rather an obtuse angle than a curve, and in a few specimens there is a reverse curve toward each end. In the hand of a skilful thrower, the boomerang can be projected to a great distance, and made to ricochet almost at will. It can be thrown in a curved path, somewhat as a ball can be "screwed" or "twisted," and it can be made to return to the thrower, striking the ground behind him. It is capable of inflicting serious wounds.
"It is very good of you, father," said Fred, "to tell and show us all these things; I'd like very much to have a very common, every-day matter explained: the theory of the pump." The following questions also were asked by one or another on the same line: What is the greatest distance or height a pump of any type can be placed away from the water? Is there any limit to the length of the delivery pipe to the tank? What is the difference between a lift and a plunger or force pump? Is it the sucker of the pump that draws the water up, or does it flow because the air being drawn out of the pump barrel and forced on the water outside, causes it to flow into the pump?
Mr. Gregg started in at once to give them the facts desired: "Theoretically, the greatest height a pump can be fixed above the water level depends on certain conditions: the atmospheric temperature, and the altitude the pump is to be fixed above the sea-water level. The higher the temperature, and the greater the altitude, the less distance the height of the pump can be above the water. The height to which water can be drawn from the source to the top of the bucket, or under side of a piston or plunger, when at the top of the stroke, or what is termed the 'height of suction,' cannot reach more than about 33 feet when the pump is at the sea level. If a tube about 34 feet long is immersed in a well, and the air is extracted by means of an air pump at the upper so that a vacuum is formed, the water will not rise in the tube until the air is expelled, when it will not rise more than 33 feet, even though there is a complete vacuum formed in the upper end of the tube. The reason why the water will not rise in the tube higher than this, is that the height of the water counterbalances the pressure of the atmosphere. This height is the theoretically greatest height that water will rise in a suction pipe. For the pump to discharge water, it is necessary for the water to be in motion, and to set and keep it in motion a portion of the water will rise, due to the atmospheric pressure. The shorter the suction pipe, the more certain the pump is of being completely filled at every stroke of the pump handle.
"The action of the pump is as follows: The bucket on moving upward attracts the air, so that the atmospheric pressure on the surface of the water in the well causes the water to follow the bucket up the suction pipe, through the suction valve, into the working barrel. On the return stroke, the suction valve will close, the valve in the bucket will open, and the water which before was under the bucket will pass through it to the top side. When the bucket is again raised, the water will be lifted through the delivery valve into the delivery pipe. There is practically no limit to the height of lift, which may be any height consistent with the strength of the pump and the available power. The ordinary pump used for raising water to the level of the top of the bucket, is termed a lift pump; for raising water above this, a force pump or a plunger pump must be used, when the water is displaced by a solid plunger on its downward stroke, when the quantity of water raised will be equal to the volume of the plunger. This system may be repeated when water is to be lifted more than ordinary heights."
X
WALL MAKING AND PLUMBING
A few evenings later, Mr. Gregg and his little family were gathered together on the river's bank, watching the movements of a number of pleasure boats and launches, when a good-sized tugboat came along and made quite a "wash" as she steamed past the Gregg domain. Mr. Gregg noticed that this had actually carried down a portion of the bank near the new pier, and he called Fred's attention to it. The two, followed by George, walked to the pier, and, to their alarm, found that quite a piece of the bank had been carried away by the current, the tides, and the frequent wash of passing steamers.