It is worthy of notice here that many of the insect-feeding animals-mammals, batrachia, and reptiles-have tongues specially fitted to assist in securing their prey. The tongue is not only long and flexible, but is capable of being thrust for a considerable distance out of the mouth. As a further aid in capturing their insect prey, the tongue is generally covered with a slimy, gummy fluid, which holds the victims fast while they are being conveyed to the mouth.
Among the Cetacea (whale-like animals) the great Greenland whale presents a remarkable peculiarity. Its mouth is entirely destitute of teeth; but hanging from the roof of the mouth, downwards, are a number of transverse horny plates of whalebone. These plates act the part of atrap, in which to entangle the small creatures that form the natural food of this sea monster. Birds, as we know, are without teeth. Their bills serve instead of teeth. Fishes and reptiles have usually a large number of small pointed teeth, all directed backwards, for the purpose of seizing and holding their prey, which they swallow whole.
Lesson 57
The Force-Pump
"Did any of you see that great fire last week?" asked Mr. Wilson. "Ah! I thought so; several of you saw it, and a very grand and awful sight it was. Did you ever feel curious to know how the firemen send out the water from their hose in that one continuous stream?""Oh yes, sir," said Fred, "do tell us, please. It has often puzzled me.""Well," said Mr. Wilson, "it is the work of a pump, but this pump is like neither the suction-pump nor the lifting pump. It is called a force-pump. I will describe it to you.
"In this contrivance, the
suction-pipe and barrel are exactly the same as in the lifting pump, and the piston too is solid-that is, it has no valve. The great difference is that in the force-pump the pipe leading from the barrel opens into an air-tight chamber of great strength- the condensing chamber.
The entrance into it is guarded by a strong valve opening upwards-that is, away from the barrel. The discharge- pipe dips down into this chamber, and passes upwards into the air, but there is no valve in it.
"As in the case of the lifting pump, the piston, in descending, presses down on the water in the barrel and closes the suction-valve. At the same time it drives the water through the pipe into the condensing chamber, and this is repeated with each downward stroke of the piston.
"You must remember that this condensing chamber is not only air-tight, but is full of air. As the water rushes in, this air is very much compressed, and then commences a struggle between the compressed air and the waterwhich is still being forced in. The air presses down on the water, but the water is incompressible; it cannot be compressed into smaller bulk. It cannot return by the way it came, partly because of the valve, and partly because of the body of water rushing up behind it. There is one way ofescape-up the discharge-pipe; and up that pipe it rushes in a continuous stream.
"I have been describing to you an ordinary force-pump. The fire-engine is really a double force -pump ; that is all. There is a central condensing chamber with a force-pump on each side. The pipes from both pumps open into the one condensing chamber, and the delivery-hose passesout from it. A strong hose from the bottom of each pumpis attached to the water supply in the road, and, as the pumping goes on, the water is raised through these pipes into the barrels of both pumps, and passed from them into the common condensing chamber, whence it is sent out in a continuous stream through the delivery hose."Lesson 58
Ice
"We are going to have one or two further experiments with our phial of colored water today," said Mr. Wilson. "You remember I called it our water-thermometer.
