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The Roman Empire’s 53,000-mile network of roadways was a marvel of ancient engineering. In the early stages of construction, the network was quite modest, connecting Rome with surrounding population centers. By the middle of the fourth century BC, Rome recognized that efficient and well-maintained roads provided them with a military advantage over their rivals. They lengthened their highways and advanced into new territories. Viaemilitares, or “military roads” expedited the movement of troops marching from Rome to other positions within the large empire. Military successes, due in part to the network of roads, expanded the empire. As a result of the empire’s growth, Roman roads began to serve new purposes, accommodating civilians who found the routes useful for trade and communication.

The development of the empire also led to the standardization of Roman roads. As early as 450 BC, legislation detailed that roads should be eight feet wide in straight sections and sixteen feet wide in curved sections. Later, major thoroughfares received official milestone markers, set at intervals of 1,000 paces. These milestones recorded the distance to the miliarium aureum, the “golden milestone” in Rome, which was the point of convergence of all the roads running out of the city. They often included additional information, such as the distance to local towns or the names of authorities responsible for construction and maintenance.


Just as humans periodically wash themselves for hygienic purposes, animals, from reptiles to mammals, engage in similar grooming activities. Many times, animals will groom themselves, which is known as personal grooming or auto-grooming. During personal grooming, an animal will attempt to remove foreign objects like dirt, dead skin, and parasitic insects from their bodies, and doing so helps the animal remain healthy.

Many animal species also participate in something called allogrooming, where one individual undertakes the grooming of another. Studies have shown that the items removed from an animal’s body by a social groomer are identical to those that would be removed if the animal were grooming itself. That is, allogrooming appears to function in a similar way to personal grooming, keeping the animal healthy.

However, scientists have learned that social grooming serves an additional purpose. In some animals, particularly primates, allogrooming is part of the social foundation of the group system. For example, the performance of allogrooming can reinforce an individual’s status in relation to others in the group, as the dominant male or female of a population may be more likely to receive grooming from subordinate members. Also, allogrooming helps strengthen bonds of trust between individuals and reduce tension in the group after an unexpected event. The pleasurable sensations experienced during grooming encourage feelings of calm and relaxation.


While traveling through the atmosphere, light bumps into molecules of nitrogen and oxygen. When white sunlight collides with these gas molecules, it may get partially absorbed. After a while, the absorbed light is radiated in a different direction by the molecule. This process affects the different colors of light in a different manner. All of the colors can be absorbed and dispersed irrespective of their wavelength, but the atmosphere absorbs shorter wavelengths more. This phenomenon, known as Rayleigh scattering, was named after the English physicist Lord John Rayleigh, who first outlined it in the 1870s. Rayleigh scattering is responsible for the blue coloring of the sky; the elements in the atmosphere absorb and radiate blue light more easily because of its shorter wavelength. As a consequence, the sky is scattered with blue. This blue light can be picked up by the human eye whichever direction the observer looks.

In contrast, sunsets are red. This occurs because when the sun is near or below the horizon, its light has traveled a longer distance through the atmosphere. Due to the distance being more than 30 times longer than when the sun is positioned directly overhead, the majority of the short-wavelength colors have already been scattered by the time the light comes into contact with the human eye. Only colors with longer wavelengths remain to be observed, thus giving sunsets their characteristic appearance. When the sun has gone down, there is no white light available, and this is why the sky is black at night.


Volcanoes erupt when pressure from the molten rock, or magma, inside becomes too intense and forces its way to the surface. As soon as this pressure becomes too much, the magma leaks out to the surface in order to erupt. Volcanic eruptions are often devastating to the land and people living in the surrounding environment. Therefore, it is vitally important to be able to predict an eruption.

Unfortunately, it is extremely difficult to accurately predict when an eruption will occur. However, as technology becomes more advanced, geologists are becoming increasingly skilled at spotting the warning signs of an eruption and predicting when one will occur. The most important indicators of an eruption are seismicity, gas emissions, and ground deformation.

Seismicity refers to the activity of earthquakes in a given area. An earthquake in the area of a volcano can signal an eruption because it indicates that magma is moving below the surface. Before an eruption, magma moves into the area beneath the volcano and collects in a magma chamber, which causes the volcano’s slopes to swell slightly. As it moves closer to the surface, the magma’s movement produces small earthquakes. These earthquakes can be detected and recorded by a seismometer, a machine which records motion. This information can be sent for immediate interpretation to an observatory.

Another warning sign of an eruption is gas emission. Gases released near the volcano can be measured for changes in quantity and makeup. Large amounts of sulfur dioxide indicate an increase of magma gathering near the surface. On May 13, 1991, for example, 500 tons of sulfur dioxide was released from Mt. Pinatubo in the Philippines. Two weeks later, sulfur dioxide emissions had increased to 5,000tons, ten times the earlier amount. Mt. Pinatubo then erupted on June 15, 1991. Correlation spectrometers (COSPECs) are used to measure amounts of sulfur dioxide and were used successfully to forecast this eruption.

Ground deformation is also an important sign in predicting an eruption. Changes in surface shape may indicate that magma is rising beneath a volcano. Measuring small changes on the summit and slopes of a volcano is therefore one of the most important methods in forecasting an eruption. These changes are measured using a tiltmeter. This instrument measures very small changes from the horizontal level and was used in 1980 to detect a bulge on the surface of Mt. St. Helens in the United States. The authorities were able to move people out of the area before it exploded two months later.


Unofficial and unregulated economic activities occur all over the world, in spite of state authorities’ efforts to control their economies. Such activities make up what is referred to as the informal economy, where the exchange of goods and services is untaxed and unmonitored. Workers in the informal economy come from all kinds of trades. Examples include occasional workers, from street vendors and garbage recyclers to unpaid family workers, and informally employed workers in formal and informal enterprises.

Economists theorize that causes of the informal economy are diverse. Unfortunately, some people are forced into informal work by their circumstances. This may happen when there are too few jobs for the labor supply. It may also be caused by insufficient wages, which force people to find part-time work that can provide extra money without having to be officially recorded.

However, necessity is not the only reason people turn to the informal economy. Some workers voluntarily participate. They may be enticed to join it by the potential for certain benefits. The informal economy enables some people to cut costs, for example by escaping taxation or working on illegally occupied property. Another theoretical benefit of the informal economy is flexibility and convenience. Informal workers may be able to adjust their schedules and work from home.

Of course, such benefits of the informal economy are in some cases outweighed by its problems. Workers in the informal economy have less access to social services than those in the formal economy. They do not receive the benefits of insurance and pension systems offered by the government or formal enterprises. In addition, compared to those in the formal economy, they have fewer rights as employees. Average incomes are lower, equipment can be old and dangerous, and exploitation is more common.


From 1830 onward, many groups in the U.S. supported the demands of the Free Soil Party to provide free homesteads for citizens. Labeled the most important act for people’s welfare ever passed in the U.S., the Homestead Act had the goal of shaping the American West by populating it with farmers. On May 20, 1862, President Abraham Lincoln finally signed the act and the law came into effect on January 1, 1863.

The Homestead law allowed any American citizen or intended citizen, who was either the head of a family or at least 21 years old, to claim 160 acres of surveyed government land if they fulfilled some straightforward conditions. Claimants had to improve the plot of land by growing crops and maintain it, as well as living on it for five 10 years. After five years had gone by, the original claimant was entitled to the property, free of charge, except for a small registration fee. People from all walks of life settled on land in the vast American West. These included newly arrived immigrants, landless farmers from the East, single women, and former slaves.

Although the door to settlement in the West had been opened, it did not come without a number of limitations. Most of the time, available land for homesteading was located in the Great Plains, where there was insufficient rainfall, with drought a common occurrence. It was not well suited to small farming; instead, it was better adapted to cattle ranching and mining that needed much more than the 160 acres legally allowed. To make matters worse, many of the settlers were too poor to be able to afford the necessary tools, seeds, and livestock.

Despite the hardships faced by settlers, by 1900 approximately 600,000 farmers had successfully secured ownership of land covering about 80 million acres. By helping to create more than 372,000 farms, the Homestead Act played a large part in American prosperity during the 20th century.

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