The first maps of Venus were made using radar beams transmitted from Earth. Radar was the only way to map its surface, because the clouds on Venus are so thick that the surface cannot be seen through them. The results of these early attempts at mapping were relatively crude and difficult to interpret, although the regions known as Alpha and Beta Regiones were discovered.

The first direct view of the surface came from probes, which were landed on Venus in 1975 by the spacecraft Venera 9 and 10, and showed a dry rock-strewn surface. Maps of the surface improved dramatically in 1978, when the Pioneer-Venus 1 spacecraft went into orbit around Venus, equipped with a radar altimeter. It showed huge rolling plains stretching right around the planet, some lowland areas, and two highland regions called Ishtar and Aphrodite. The peaks of the highest mountains, Maxwell Montes, in the eastern part of Ishtar, were found to be 12,000 meters above the general surface level, so they are appreciably higher than the Himalayas. Aphrodite, which is larger than Ishtar, has a vast rift valley at its eastern end nearly 3,000 meters deep, 2,200 kilometers long, and 280 kilometers wide. Two shield volcanoes, broad volcanoes formed of successive outpourings of lava, which are much larger than any found on Earth, were also found isolated from the two upland areas. Six years later a great many impact craters and small volcanoes were found by Venera 15 and 16 orbiters.

Later yet, the Magellan spacecraft entered orbit around Venus in August 1990, and over the next two years completed a detailed radar mapping of the surface. It found that the surface is mostly volcanic, with large lava-flooded plains and thousands of volcanoes. There are also signs of tectonic activity, which has caused, for example, multiple faulting and deep fractures. There are a number of rift valleys, some of which have been partly flooded by molten lava, and a number of impact craters, the density of which has enabled the ages of various areas to be estimated. The absence of impact craters in an area suggests an age of no more than a few tens of millions of years.

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We human beings have developed special technologies, such as air conditioning and oil burners, to protect ourselves from discomfort and death by extremes of heat or cold. Our bodies have also evolved strategies for maintaining a fairly constant body temperature despite large fluctuations in the environmental temperature. One such strategy involves the unconscious brain-based rhythms that time when we sleep and wake. The timing of sleep is controlled by a brain clock designed for twin strategies: survival and energy conservation.

In the mid-nineteenth century, German physiologists recognized the link between sleep and body temperature. They established the clinically important fact that human beings normally experience daily fluctuations in core body temperature of about 1.5 degrees Fahrenheit, with a peak in the late morning or early afternoon and a trough at night—coinciding with sleep. It has since been well documented that this rhythmic change is not a reflex response to the more extreme daily fluctuations of ambient light and temperature. It is, rather, a rhythm generated by the brain, which anticipates the ambient variability and fits behavior to it.

Why does body temperature vary? And why do people sleep when body temperature is low? The answers lie in the connection between body temperature and metabolism. Our bodies constantly generate heat as well as obtain biochemical building blocks for our tissues. To sustain metabolic activity, we must obtain energy by consuming food.

Within the same organism, a high level of metabolic activity will coincide with a high body temperature. Because sleep is a state of lower metabolic activity, with a lower body temperature, it is energy conservative. Even if we cannot seek food during sleep, we do not use up much energy, because our muscles become inactive. And interestingly, we do not normally become hungry while asleep. Removing individuals temporarily from the effort to find nourishment is one of the main contributions sleep makes to the survival of mammalian species. By decreasing the amount of food that needs to be consumed, sleep makes it possible for animals to compete more successfully for limited food supplies.

The flute is one of the most ancient of musical instruments, though its modern form is one of the most technically sophisticated instrument designs. In its long history the flute has been made of almost every imaginable material, from the most precious, such as gold or ivory, to the very unassuming, such as wood or hard rubber. Because it can be easily turned and drilled, wood has a long and distinguished tradition in flutes, and its mellow tone still gives it a place in flute making. At various times in the past, musicians and instrument makers have favored different woods. In the eighteenth century, for example, fruit woods such as pear and cherry were popular, along with boxwood. Later, ebony and cocuswood—exotic, dense wood products of the tropics—were widely adopted. Often trimmed with gold or silver, wooden flutes were prizedexamples of craftsmanship. More showy, but less practical, were flutes made of ivory or even glass. These were valued more as show pieces or curiosities than as serious instruments.

The metal flute is a product of nineteenth-century Europe, when techniques and tools combined with new musical standards to encourage the use of silver and, occasionally, gold. The gold instrument of Mozart's The Magic Flute would have been unusual indeed when that opera was written at the end of the eighteenth century, but a hundred years later itseemed much more natural, especially for musicians seeking a "warmer" tone than that of the more common silver. Platinum and gold together make an even more spectacular flute, though the musical advantages are less clear. By the end of the nineteenth century, experiments on even more novel materials had made flutes of hard rubber practical, particularly when metal was used for certain parts. Such flutes were intended to offer the student or amateur an inexpensive yet suitable alternative to the precious metals and woods.

Ice, unlike most solids, is slippery. That is why it is possible to skate on ice but not on solids such as marble or steel. Why is ice slippery? Incredible as it may seem, physicists have been perplexed by the slipperiness of ice and have not found a fully acceptable explanation of it.

One idea is based on the fact that whether a substance exists as a solid or liquid depends on the pressure exerted on it. Increasing the pressure lowers the freezing point of water, so it is a liquid even at temperatures below 0 degrees centigrade. Thus, when a skater moves on ice, the pressure from the skater's weight on the blade of the skate causes the ice immediately under it to melt. The skater slides on this tiny patch of water, which is continuously created as the skate moves forward.

The problem with this idea is that the pressure from even a heavy skater would change the freezing point of water just a tiny fraction of one degree. However, the temperature of ice is often well below the freezing point. On a cold day, the ice might be 10 or 20 degrees below freezing. So the minuscule effect of the skater's pressure would not be sufficient to cause the ice to melt. Yet, of course, it is possible to skate in very cold weather.

Another idea is that the slipperiness of ice is due to friction between the skate and the ice. Friction creates heat, and experiments have shown that the frictional heating caused by an ice skate moving across the ice is enough to create a thin, watery layer on top of the ice. The skater glides on this thin layer of melted ice.

The problem is that frictional heating occurs only when one thing is moving against another, but ice is slippery even when there is no motion. If you are standing on ice and tilt a little to one side, then, even though your skates are not moving, they will slip out from under you. So the ice is slippery even before there is any motion that melts it through friction.

Around 1825, a major migration westward toward Michigan and other midwestern regions started from the northeastern population centers of the United States. New York and New England farmers, exhausted from working small, rocky, infertile plots, were drawn by word of a rich new frontier. Midwestern regions such as Michigan, which had so recently been frontier, were linked almost overnight to the centers of American commerce. In 1825 the Erie Canal, constructed between Lake Erie and the Hudson River in the state of New York, was completed. It connected the east with what Americans rather innocently thought was the west and caused a dramatic drop in shipping rates. Before the canal, it had cost almost 20 cents a mile to ship a ton of freight overland from Buffalo, New York, to New York City; with the new waterway it was soon less than a penny a mile. The midwestern farm, with access to Buffalo by way of Lake Erie, now served the eastern consumer; the Michigan mine supplied the eastern manufacturer. The effect on Detroit, Michigan, was stunning. It became the hub of a region selling tobacco and flour and salted fish to people some 700 miles away. The effect on New York City, joined to the canal by the Hudson River, was equally important. It soon surpassed Boston, Massachusetts, as a center of commerce.

In the 1830s, there were 599,000 new immigrants to the United States, in the 1840s 1.7 million, and in the 1850s 2.6 million. Detroit was one focus of the influx; its population, 9,000 in 1820, would be well over 200,000 by the end of the century. The city was well placed to benefit from the continuing westward expansion. The surrounding area was rich in timber to satisfy the demand for wood to build horse-drawn carriages, railroad cars, and railroad ties for the expanding network of track. The mines to the north produced iron and copper for building a variety of machines.

Michigan's early industries, those that developed in the nineteenth century, drew people with particular skills that evolved into the skills required in the early twentieth century. The carriage makers, for example, became the automakers; the stove makers became the metalworkers in the automotive body and engine shops; the leather workers who did the upholstery for carriages eventually made seat covers for cars.