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Before the mid-nineteenth century, people in the United States ate most foods only in season. Drying, smoking, and salting could preserve meat for a short time, but the availability of fresh meat, like that of fresh milk, was very limited; there was no way to prevent spoilage. But in 1810 a French inventor named Nicolas Appert developed the  cooking-and-sealing process of canning. And in the 1850's an American named Gail Borden developed a means of condensing and preserving milk. Canned goods and condensed milk became more common during the 1860's, but supplies remained low because cans had to be made by hand. By 1880, however, inventors had fashioned stamping and soldering machines that mass-produced cans from tinplate. Suddenly all  kinds of food could be preserved and bought at all times of the year.

Other trends and inventions had also helped make it possible for Americans to vary their daily diets. Growing urban populations created demand that encouraged fruit and vegetable farmers to raise more produce. Railroad refrigerator cars enabled growers and meat packers to ship perishables great distances and to preserve them for longer  periods. Thus, by the 1890's, northern city dwellers could enjoy southern and western strawberries, grapes, and tomatoes, previously available for a month at most, for up to six months of the year. In addition, increased use of iceboxes enabled families to store perishables. An easy means of producing ice commercially had been invented in the 1870's, and by 1900 the nation had more than two thousand commercial ice plants, most of which made home deliveries. The icebox became a fixture in most homes and remained so until the mechanized refrigerator replaced it in the 1920's and 1930's.

Almost everyone now had a more diversified diet. Some people continued to eat mainly foods that were heavy in starches or carbohydrates, and not everyone could afford meat. Nevertheless, many families could take advantage of previously  unavailable fruits, vegetables, and dairy products to achieve more varied fare.

John James Audubon, nineteenth-century artist and naturalist, is known as one of the foremost authorities on North American birds. Born in Les Cayes, Haiti, in 1785, Audubon was raised in France and studied art under French artist Jacques-Louis David. After settling on his father's Line Pennsylvania estate at the age of eighteen, he first began to study and paint birds.

In his young adulthood, Audubon undertook numerous enterprises, generally without a tremendous amount of success; at various times during his life he was involved in a mercantile business, a lumber and grist mill, a taxidermy business, and a school. His general mode of operating a business was to leave it either unattended or in the hands of a partner and take off on excursions through the wilds to paint the natural life that he saw. His business career came to end in 1819 when he was jailed for debt and forced to file for bankruptcy.

It was at that time that Audubon began seriously to pursue the dream of publishing a collection of his paintings of birds. For the next six years he painted birds in their natural habitats while his wife worked as a teacher to support the family. His Birds of America, which included engravings of 435 of his colorful and lifelike water colors, was published in parts during the period from 1826 to 1838 in England. After the success of the English editions, American editions of his work were published in 1839, and his fame and fortune were ensured.

The changing profile of a city in the United States is apparent in the shifting definitions used by the United States Bureau of the Census. In 1870 the census officially distinguished the nation's “urban” from its “rural” population for the first  time. “Urban population” was defined as persons living in towns of 8,000 inhabitants  or more. But after 1900 it meant persons living in incorporated places having 2,500 or more inhabitants.

Then, in 1950 the Census Bureau radically changed its definition of “urban” to take account of the new vagueness of city boundaries. In addition to persons living in incorporated units of 2,500 or more, the census now included those who lived in  unincorporated units of that size, and also all persons living in the densely settled urban fringe, including both incorporated and unincorporated areas located around cities of 50,000 inhabitants or more. Each such unit, conceived as an integrated economic and social unit with a large population nucleus, was named a Standard Metropolitan Statistical Area (SMSA).

 Each SMSA would contain at least one central city with 50,000 inhabitants or more or  two cities having shared boundaries and constituting, for general economic and social purposes, a single community with a combined population of at least 50,000, the smaller of which must have a population of at least 15,000. Such an area included the county in which the central city is located, and adjacent counties that are found to  be metropolitan in character and economically and socially integrated with the county of the central city. By 1970, about two-thirds of the population of the United States was living in these urbanized areas, and of that figure more than half were living outside the central cities.

While the Census Bureau and the United States government used the term SMSA,  by 1969 there were 233 of them, social scientists were also using new terms to describe the elusive, vaguely defined areas reaching out from what used to be simple “towns” and “cities”. A host of terms came into use: “metropolitan regions,” “polynucleated population groups,” “conurbations,” “metropolitan clusters,” “megalopolises,” and so on.

Schizophrenia is often confused with multiple personality disorder yet is quite distinct from it. Schizophrenia is one of the more common mental disorders, considerably more common than multiple personality disorder. The term "schizophrenia” is composed of roots which mean "a splitting Line of the mind," but it does not refer to a division into separate and distinct personalities, as occurs in multiple personality disorder. Instead, schizophrenic behavior is generally characterized by illogical thought patterns and withdrawal from reality. Schizophrenics often live in a fantasy world where they hear voices that others cannot hear, often voices of famous people. Schizophrenics tend to withdraw from families and friends and communicate mainly with the “voices” that they hear in their minds.

It is common for the symptoms of schizophrenia to develop during the late teen years or early twenties, but the causes of schizophrenia are not well understood. It is believed that heredity may play a part in the onset of schizophrenia. In addition, abnormal brain chemistry also seems to have a role; certain brain chemicals, called neurotransmitters, have been found to be at abnormal levels in some schizophrenics.

The hard, rigid plates that form the outermost portion of the Earth are about 100 kilometers thick. These plates include both the Earth's crust and the upper mantle.

The rocks of the crust are composed mostly of minerals with light elements, like aluminum and sodium, while the mantle contains some heavier elements, like iron and  magnesium. Together, the crust and upper mantle that form the surface plates are called the lithosphere. This rigid layer floats on the denser material of the lower mantle the way a wooden raft floats on a pond. The plates are supported by a weak, plastic layer of the lower mantle called the asthenosphere. Also like a raft on a pond, the lithospheric plates are carried along by slow currents in this more fluid layer beneath  them.

With an understanding of plate tectonics, geologists have put together a new history for the Earth's surface. About 200 million years ago, the plates at the Earth's surface formed a “supercontinent” called Pangaea. When this supercontinent started to tear apart because of plate movement, Pangaea first broke into two large continental masses  with a newly formed sea that grew between the land areas as the depression filled with water. The southern one — which included the modern continents of South America, Africa, Australia, and Antarctica — is called Gondwanaland. The northern one — with North America, Europe, and Asia — is called Laurasia. North America tore away from Europe about 180 million years ago, forming the northern Atlantic Ocean.

 Some of the lithospheric plates carry ocean floor and others carry land masses or a combination of the two types. The movement of the lithospheric plates is responsible for earthquakes, volcanoes, and the Earth's largest mountain ranges. Current understanding of the interaction between different plates explains why these occur where they do. For example, the edge of the Pacific Ocean has been called the “Ring  of Fire” because so many volcanic eruptions and earthquakes happen there. Before the 1960's, geologists could not explain why active volcanoes and strong earthquakes were concentrated in that region. The theory of plate tectonics gave them an answer.