The weaver passes the shuttle， with its woof， from right to left between these two sets of threads-that is， over one set and under the others. Then the heddle is made to change the positions of these alternate threads， lowering one set and raising the others， after which the shuttle is returnedbetween the two as before， and so the work goes on.
There is but one thread of the woof or weft， and it is carried from side to side by the shuttle. The fact of its passing each time between the alternate sets of warpthreads forms the selvage at the sides of the cloth. It is important to remember that this is the mode of working the simple hand-loom. All our woven goods are now made in enormous， steam power-looms worked by machinery too intricate for us to consider here. But if we can follow the working of the hand-loom， we shall be able to understand the principle of even the ponderous machinery of our great cotton mills.
Lesson 57
第57章The Effects of the Differences in Specific Heat"Our last lesson on heat showed us that different bodies have different capacities for taking in and holding heat，" began Mr. Wilson， "and we learned to speak of this as the specific heat of such bodies. We are now going to learn some of the effects of these differences.
"Let us begin again with a simple experiment. Here are two small test-tubes， one containing water， the other an equal quantity of mercury， both being at the same temperature， as may be seen by the small thermometer standing in each. I will plunge both of them into the kettle of water whichis boiling over the spirit-lamp. The thermometers in the tubes will indicate their rise in temperature. As we watch， the mercury in the one tube is seen to reach 212°F (the temperature of the boiling water) in about half the time that the water in the other tube takes.
"We will now remove both tubes from the heat， and leave them to cool down from their present temperature (212°). The mercury cools twice as rapidly through the same number of degrees as the water does.
"Hence we see that water takes twice as long as mercury to reach a certain temperature， and is twice as long incooling. The same thing would happen with the lead and iron balls， which we used in our former lesson. The lead cannot take up so much heat as the iron. It would therefore reach the temperature of the boiling water in much less time than the iron. As， moreover， it cannot hold so much heat as the iron， it has less to part with when it cools， and， consequently， the cooling goes on more rapidly than it would in the iron.
"We might expose a variety of substances to the same source of heat-say by placing them in front of the same fire-and we should find that some would rise in temperature more slowly than others. If we removed them in their heated state， at the same moment， into a cold room， those which had grown hot slowly would also cool slowly， and vice versa， because those which have a greater capacity for holding heat than others， have more to take up in heating， and more to part with in cooling， and must therefore take longer.
"Water， as we have seen， has a high specific heat. That is to say， in rising through any number of degrees in temperature， it necessarily takes in an immense quantity of heat， and hence it takes longer to get hot and longer to grow cool than other bodies.
"Think of the oceans， rivers， and lakes all over the world， with their countless multitudes of living inhabitants. It is this high specific heat of water which makes rapid elevation or depression of temperature in them impossible.
"Picture， on the other hand， an ocean of mercury， under the rays of a tropical sun. Even if it were otherwise habitable， the creatures in it could never live through such sudden changes in temperature as would certainly take place. Such a liquid would be quite uninhabitable.
"During the cold season of the year， on the other hand， the sea and all great bodies of water cool. But in cooling they have a great quantity of heat to give out for every degree they lose in temperature. Hence they cool very slowly， and while cooling， the heat which they give out tends to equalize the temperature of the adjoining land.
"Mercury has a low specific heat. A comparatively small quantity of heat is sufficient to raise its temperature through any given number of degrees. Hence it rises and falls in temperature very rapidly. It is this which makes mercury so peculiarly useful for filling thermometers."
