演讲1：

　　（举例略）

　　Today, we’re going to take a look at the development of the skyscraper. We’ll start with some buildings in Chicago. One of the circumstances that let Chicago to become home to some of the buildings now considered the prototypes for later skyscrapers was this: in 1871, there was a great fire that destroyed much of the city. It was that tragic fire that cleared the way for a new kind of city that used the new building techniques and new materials developed during the 1800’s. One of these new structures was the Home Insurance Building, completed in 1885. It was ten stories high. Now granted, that’s no higher than some of the early skyscrapers in New York City. What makes Chicago’s Home Insurance Building important is that it had true skyscraper construction with an internal metal skeleton that carried the weight of the brick exterior. This metal-support system, along with the early development of the elevator, were the two innovations that made the later very tall industrial buildings possible. Chicago’s Reliance Building was another important building in the development of the skyscraper. It showed the architects’ understanding of the possibilities of metal-frame construction. By eliminating walls and opening up the sides as a glass box. It was the first expression of the skyscraper as a glass-shelf framed in a metal grid.

　　演讲2：

　　It was an Italian inventor who created the first wireless device for sending out radio signals in 1895. But not until the American inventor Lee De Forest built the first amplifying vacuum tube in 1906 did we get the first radio as we know it. And the first actual radio broadcast was made on Christmas Eve of 1906. That’s when someone working from an experimental station in Brand Rock, Massachusetts, arranged the program of two short musical selections of poem and brief holiday greeting. The broadcast was heard by wireless operators on ships with a radio through several hundred miles. The following year, De Forest began regular radio broadcasts in New York. These programs were similar to much of what we hear on the radio today in that De Forest played only music. But because there was still no home radio receivers, De Forest’s audience consisted of only wireless operators on ships in New York harbor. There is no doubt that radio broadcasting was quite a novelty in those days. But it took a while to catch on commercially. Why? Hmm, for the simple fact that only a few people, in fact, only those who tinkered with wireless telegraphs as a hobby owned receivers. It wasn’t until the 1920’s that someone envisioned mass appeal for radio. This was radio pioneer, David Sarnoff who predicted that one day there would be a radio receiver in every home.

　　演讲3：

　　The origin of earth’s moon, the largest moon in the solar system, is still something of a mystery. There are some theories about its origin, however. Now, keep in mind that a theory of the moon’s origin has to be consistent with two important facts. The first fact is that the earth contains a lot of iron, most of it has an iron core. But the moon contains practically no iron. The second fact is that, other than the difference in iron content, the moon and earth are composed of essentially the same minerals, a similarity not shared with any other planet or moon in our solar system. One of the earliest theories of the moon’s origin, I call it the Capture Theory, proposes that the moon was somehow captured by earth’s gravitational force. This theory is improbable, however, because it assumes that the moon and earth formed in different parts of the solar system. If this were true, you would expect the moon’s composition to be much different from earth’s composition, just as all the other planets in the solar system are so different from earth. A second theory of the moon’s origin is more promising. It is sometimes referred to as the Mars Theory because according to this theory, when earth was still molten, it was struck by a planet about the size of Mars. The impact caused the cores of the two planets to melt together and chunks of earth’s crust to be thrown out into space. These chunks came together to form the moon. Now remember, earth’s crust is low in iron because the iron is in earth’s core but high in various other minerals. This then accounts for why there is little iron but lots of other minerals on the moon

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