【正文】 US li:n。 the width of the stripes indicates the rate of the seafloor spreading.37 IcebergsIcebergs are among nature’s most spectacular creations, and yet most people have never seen one. A vague air of mystery envelops them. They e into being somewhere in faraway, frigid waters, amid thunderous noise and splashing turbulence, which in most cases no one hears or sees. They exist only a short time and then slowly waste away just as unnoticed.Objects of sheerest beauty they have been called. Appearing in an endless variety of shapes, they may be dazzlingly white, or they may be glassy blue, green or purple, tinted faintly of in darker hues. They are graceful, stately, inspiring in calm, sunlight seas.But they are also called frightening and dangerous, and that they are in the night, in the fog, and in storms. Even in clear weather one is wise to stay a safe distance away from them. Most of their bulk is hidden below the water, so their underwater parts may extend out far beyond the visible top. Also, they may roll over unexpectedly, churning the waters around them.Icebergs are parts of glaciers that break off, drift into the water, float about awhile, and finally melt. Icebergs afloat today are made of snowflakes that have fallen over long ages of time. They embody snows that drifted down hundreds, or many thousands, or in some cases maybe a million years ago. The snows fell in polar regions and on cold mountains, where they melted only a little or not at all, and so collected to great depths over the years and centuries.As each year’s snow accumulation lay on the surface, evaporation and melting caused the snowflakes slowly to lose their feathery points and bee tiny grains of ice. When new snow fell on top of the old, it too turned to icy grains. So blankets of snow and ice grains mounted layer upon layer and were of such great thickness that the weight of the upper layers pressed the lower ones. With time and pressure from above, the many small ice grains joined and changed to larger crystals, and eventually the deeper crystals merged into a solid mass of ice.38 TopazTopaz is a hard, transparent mineral. It is a pound of aluminum, silica, and fluorine. Gem topaz is valuable. Jewelers call this variety of the stone “precious topaz”. The bestknown precious topaz gems range in color from rich yellow to light brown or pinkish red. Topaz is one of the hardest gem minerals. In the mineral table of hardness, it has a rating of 8, which means that a knife cannot cut it, and that topaz will scratch quartz.The golden variety of precious topaz is quite unmon. Most of the world’s topaz is white or blue. The white and blue crystals of topaz are large, often weighing thousands of carats. For this reason, the value of topaz does not depend so much on its size as it does with diamonds and many other precious stones, where the value increases about four times with each doubling of weight. The value of a topaz is largely determined by its quality. But color is also important: blue topaz, for instance, is often irradiated to deepen and improve its color.Blue topaz is often sold as aquamarine and a variety of brown quartz is widely sold as topaz. The quartz is much less brilliant and more plentiful than true topaz. Most of it is variety of amethyst: that heat has turned brown.NOTE:topaz / 39。s and 196039。 she hooks up her telephone modern connections and does office work between calls to the doctor.These are powerful images, but they are a limited reflection of reality. Telemuting workers soon learn that it is almost impossible to concentrate on work and care for a young child at the same time. Before a certain age, young children cannot recognize, much less respect, the necessary boundaries between work and family. Additional child support is necessary if the parent is to get any work done.Management too must separate the myth from the reality. Although the media has paid a great deal of attention to telemuting in most cases it is the employee’s situation, not the availability of technology that precipitates a telemuting arrangement.That is partly why, despite the widespread press coverage, the number of panies with workathome programs or policy guidelines remains small.30 The origin of RefrigeratorsBy the midnineteenth century, the term “icebox” had entered the American language, but ice was still only beginning to affect the diet of ordinary citizens in the United States. The ice trade grew with the growth of cities. Ice was used in hotels, taverns, and hospitals, and by some forwardlooking city dealers in fresh meat, fresh fish, and butter. After the Civil War( 18611865),as ice was used to refrigerate freight cars, it also came into household use. Even before 1880,half of the ice sold in New York, Philadelphia, and Baltimore, and onethird of that sold in Boston and Chicago, went to families for their own use. This had bee possible because a new household convenience, the icebox, a precursor of the modern refrigerator, had been invented.Making an efficient icebox was not as easy as we might now suppose. In the early nineteenth century, the knowledge of the physics of heat, which was essential to a science of refrigeration, was rudimentary. The monsense notion that the best icebox was one that prevented the ice from melting was of course mistaken, for it was the melting of the ice that performed the cooling. Nevertheless, early efforts to economize ice included wrapping up the ice in blankets, which kept the ice from doing its job. Not until near the end of the nineteenth century did inventors achieve the delicate balance of insulation and circulation needed for an efficient icebox.But as early as 1803, and ingenious Maryland farmer, Thomas Moore, had been on the right track. He owned a farm about twenty miles outside the city of Washington, for which the village of Georgetown was the market cente