Panama Canal (Spanish Canal de Panamá), canal joining the Atlantic and Pacific oceans across the Isthmus of Panama. Running from Cristóbal on Limón Bay, an arm of the Caribbean Sea, to Balboa, on the Gulf of Panama, the canal is slightly more than 64 km (40 mi) long, not including the dredged approach channels at either end. The minimum depth is 12.5 m (41 ft), and the minimum width is 91.5 m (300 ft). The construction of the Panama Canal ranks as one of the greatest engineering works of all time.

Location and Structure 
The approach to the canal from the Atlantic is along 7.2 km (4.5 mi) of dredged channel. The canal then proceeds for 11.1 km (6.9 mi), veering slightly westward before reaching the Gatun Locks. Ships are lifted 25.9 m (85 ft) by these three locks, to the level of Gatun Lake. The lake was formed as a result of the damming of the Chagres River by the Gatun Dam, which adjoins the locks. The Gatun Locks open directly into one another and are double, as are the other locks, so that one ship can be raised while another is being lowered. All the lock chambers on the Panama Canal have a length of 305 m (1000 ft) and a width of 33.5 m (110 ft).
From the Gatun Locks the canal passes through Gatun Lake in a southern and southeastern direction to the mouth of Gaillard Cut (formerly called Culebra Cut), an excavated channel 13 km (8.1 mi) long. At the end of the Gaillard Cut is the Pedro Miguel Lock, which has a drop of 9.4 m (31 ft). The lock borders Miraflores Lake, which is 16.8 m (55 ft) above the level of the Pacific. The canal passes 2.1 km (1.3 mi) through Miraflores Lake and reaches the two Miraflores Locks. These locks lower ships to Pacific tidewater level. From the Miraflores Locks the canal runs 4 km (2.5 mi) to Balboa on the Gulf of Panama, from which a dredged channel extends approximately 8 km (5 mi) out into the bay. In addition to the canal itself, auxiliary facilities include the Madden Dam on the Chagres River, which provides a reservoir to maintain the level of Gatun Lake during the dry season; breakwaters to protect the channels at either end of the canal; hydroelectric plants at the Gatun and Madden dams; and the Panama Railroad that extends 76.6 km (47.6 mi) from Colón at the Atlantic end of the canal to Panama City on the Pacific.
In 1993 about 12,080 commercial vessels, carrying more than 160 million metric tons of cargo, passed through the canal. Transit time through the canal is seven to eight hours.

History 
Interest in a short route from the Atlantic to the Pacific began with the explorers of Central America early in the 16th century. Hernán Cortés, the Spanish conqueror of Mexico, suggested a canal across the Isthmus of Tehuantepec; other explorers favored routes through Nicaragua and Darién. The first project for a canal through the Isthmus of Panama was initiated by Holy Roman Emperor Charles V, who in 1523 ordered a survey of the isthmus. A working plan for a canal was drawn up as early as 1529, but was not submitted to the king. In 1534 a local Spanish official suggested a canal route close to that of the present canal. Later, several other canal plans were suggested, but no action was taken.

Renewed Interest 
The Spanish government subsequently abandoned its interest in the canal, but in the early 19th century the books of the German scientist Alexander von Humboldt revived interest in the project, and in 1819 the Spanish government formally authorized the construction of a canal and the creation of a company to build it. Nothing came of this effort, however, and the revolt of the Spanish colonies soon took the control of possible canal sites out of Spanish hands. The republics of Central America subsequently tried to interest groups in the United States and Europe in building a canal, and it became a subject of perennial debate in the Congress of the United States. The discovery of gold in California in 1848 and the rush of would-be miners stimulated U.S. interest in digging the canal (see Clayton-Bulwer Treaty). Various surveys made between 1850 and 1875 indicated that only two routes were practical, the one across Panama and that across Nicaragua. In 1876 an international company was organized; two years later it obtained a concession from the Colombian government-Panama was then part of Colombia-to dig a canal across the isthmus.

U.S. Involvement 
The international company failed, and in 1880 a French company was organized by Ferdinand Marie de Lesseps, the builder of the Suez Canal. His company went bankrupt in 1889. U.S. interest in an Atlantic-Pacific canal, however, continued. In 1899 the U.S. Congress created an Isthmian Canal Commission to examine the possibilities of a Central American canal and to recommend a route. The commission first decided on the Nicaraguan route, but reversed its decision in 1902 when the reorganized Lesseps company offered its assets to the United States at a price of $40 million (see Hay-Pauncefote Treaty). The U.S. government negotiated with the Colombian government to obtain a strip of land 9.5 km (6 mi) wide across the isthmus, but the Colombian Senate refused to ratify this concession. In 1903, however, Panama revolted from Colombia. That same year the United States and the new state of Panama signed the Hay-Bunau-Varilla Treaty by which the United States guaranteed the independence of Panama and secured a perpetual lease on a 16-km (10-mi) strip for the canal. Panama was to be compensated by an initial payment of $10 million and an annuity of $250,000, beginning in 1913. The figure was later revised upward in 1936 to $430,000, and in 1955 to $2 million per year.

Construction 
In 1905 the Isthmian Canal Commission decided to build a canal with locks rather than a sea-level channel, and this plan was approved by the U.S. Congress the following year. President Theodore Roosevelt put the construction work under the direction of the U.S. Army Corps of Engineers; Colonel George W. Goethals was named to head the project.
It was estimated that the canal would be completed in ten years; however, it was in operation by the summer of 1914. The construction involved not only excavating an estimated 143 million cu m (175 million cu yd) of earth, but also sanitizing the entire canal area, which was infested with the mosquitoes that spread yellow fever and malaria. The sanitation work was undertaken by Colonel William C. Gorgas of the U.S. Army Medical Corps, who virtually eliminated the diseases. An unexpected difficulty in the actual construction was the prevalence of slides of earth from the banks of the canal, particularly in the Gaillard Cut. Reexcavation after such slides added about 25 percent to the estimated amount of earth moved. The final cost of the canal was $336 million.
One of the most important later pieces of construction work is the $20 million Thatcher Ferry Bridge, which spans the Pacific entrance to the canal and provides a vital link in the Pan-American Highway. This high-level span, 1647 m (5425 ft) long, was dedicated in 1962. The widening of the Gaillard Cut from 91.5 m (300 ft) to a width of 150 m (600 ft) was completed in 1970. It permitted, for the first time, two-way passage through the entire cut.

New Treaties 
In 1977 the United States and Panama agreed on two new treaties to replace their 1903 agreement. These treaties provided for Panama's sovereignty over the Canal Zone shortly after their ratification and its control of the canal itself at the beginning of 2000, but left the United States the right to defend the canal's neutrality even thereafter. The treaties took effect in 1979.

Hay-Pauncefote Treaty, agreement negotiated in 1901 between the United States and Great Britain, providing for the construction and regulation of a canal across the Isthmus of Panama. It was signed by John Hay, U.S. secretary of state, and Lord Julian Pauncefote, British ambassador to the U.S. The treaty superseded the Clayton-Bulwer Treaty of 1850 as the definitive statement of Anglo-American policy concerning an Atlantic-Pacific canal.
The Spanish-American War of 1898 created an American interest in South and Central America. American public opinion began to demand abrogation of the 1850 treaty, which permitted neither the United States nor Great Britain to act alone in regard to the canal. The French engineer Ferdinand de Lesseps had tried unsuccessfully to dig an isthmian canal in the 1880s, but the U.S., in 1900, decided that such an enterprise should be entirely American. Great Britain was amenable, provided that the canal zone would remain neutral.
Conversations between Hay and Pauncefote resulted in a draft treaty giving the U.S. complete direction of the construction project, establishing permanent neutrality of the zone and a ban on fortifications, and inviting other nations to join in guarantees of neutrality. The U.S. Senate, refusing to ratify the original draft, amended it to permit the U.S. to take any measures for its own defense in the canal zone and deleted the clause concerning other nations. Great Britain opposed these amendments, and negotiations were resumed. A revised draft was presented to the Senate in 1901 and was ratified shortly after its presentation. By the terms of the ratified treaty, the U.S. was given full control of the construction and management of the canal; the U.S. was named sole guarantor of the neutrality of the canal and was permitted to build fortifications; and the canal was opened to ships of any nation under equal terms, although the U.S. could forbid passage in times of war. In 1911 Great Britain claimed that the U.S. had contravened the last clause by passing the Panama Canal Act, exempting American coastal shipping from paying canal tolls; President Woodrow Wilson, agreeing with the British view, persuaded Congress to repeal the act in 1914.

Yellow Fever, noncontagious, infectious disease, caused by a virus, and characterized in severe cases by high fever and jaundice. Originally yellow fever was believed to be exclusively a disease of humans, but research has revealed that it also affects monkeys and other animals. It is believed that diseased monkeys of Africa and tropical America are the primary source of infection and that carrier mosquitoes transmit the infection to humans. This type of the disease, which occurs only sporadically in human beings, is known as jungle yellow fever. If infected individuals move into a populated area, they may be bitten by a semidomestic species of mosquito, such as Aëdes aegypti, which lives close to human habitations. These feed on the blood of humans and are the chief transmitting agents in epidemics of urban yellow fever.

History 
It is probable that the disease initially appeared in Africa and that it was brought to America at the time of the slave trade. The disease was first described in medical terms during the 17th century, when an outbreak was observed in the Yucatán Peninsula, Mexico. Ultimately yellow fever spread to the U.S., Spain, and other countries.
In 1881 the Cuban physician Carlos Finlay advanced the hypothesis that yellow fever is transmitted by mosquito bites. Finlay's theory was verified in 1901 by the work of several investigators, notably the American bacteriologist Walter Reed, who also proved the agent to be a virus. The disease was brought under control by advanced methods of sanitation, including drainage of mosquito breeding grounds and quarantine of ships arriving from infected areas.

Symptoms 
The incubation period of yellow fever is three to six days. In severe cases the onset is sudden, with typical symptoms of headache, backache, and fever. The first stage is also characterized by nausea, vomiting, and presence of albumin in the urine. After the initial fever, the temperature usually returns to normal, but on the fourth or fifth day the temperature again rises. This second stage is marked by jaundice, hemorrhage from mucous membranes, vomiting of blood (the characteristic, so-called black vomit of yellow fever), and fatty degeneration of the liver, kidneys, and heart. The destruction of liver cells results in the accumulation of yellow bile pigments in the skin, giving the disease its name. Death usually occurs between the fourth and eighth day after the onset. In cases of spontaneous recovery, convalescence is rapid, although jaundice may persist for some time. The disease never recurs, one attack providing immunity for life.

Prevention 
No treatment is known for yellow fever. In 1939 the South African physician Max Theiler developed a vaccine that confers immunity to the disease. Vaccination is today required for all persons traveling between endemic regions and other parts of the world.

Malaria, disease of animals, especially birds, monkeys, and humans, caused by infection by protozoans of the genus Plasmodium and characterized by chills and intermittent fever. The causative organisms of human malaria are transmitted by the bite of about 60 species of mosquitoes in the genus Anopheles. The disease may occur in subtropical and tropical regions in almost all parts of the world as well as in other temperature areas. With the advent of control programs based on the use of residual insecticides, the distribution of malaria changed rapidly. Since 1950 malaria has been eliminated from almost all of Europe and from large areas in Central and South America. It remains a major problem in parts of Africa and in southeastern Asia. About 100 million cases of human malaria develop each year; about 1 percent are fatal.

Malaria in Humans 
Human malaria occurs in four forms, each caused by a different species of parasite. In each form, the symptoms usually are chills, fever, and sweating. In untreated cases, these attacks recur periodically. The mildest form of malaria is benign tertian malaria, caused by Plasmodium vivax, in which the fever may occur about every second day after the initial attack (which may occur within two weeks after infection). Jungle fever, malignant tertian malaria, or estivo-autumnal malaria, caused by Plasmodium falciparum, is responsible for most of the deaths from malaria. The organisms in this form of the disease often block the blood vessels of the brain, producing coma, delirium, and finally death. Quartan malaria, caused by Plasmodium malariae, has a longer incubation period than either tertian malaria or jungle fever; the first attack does not appear until 18 to 40 days after infection. The attacks recur about every third day. The fourth and rarest form of the disease, caused by Plasmodium ovale, is similar to benign tertian malaria. In all forms of the disease, periodic fevers may be less regularly spaced in some people.
During the incubation period of malaria, the protozoan grows within cells in the liver; a few days before the first attack, the organisms invade the red blood cells, which they destroy in the course of their development, producing the typical febrile attack.

History 
Since 1638 malaria has been treated with an extract from the bark of the cinchona tree, known as quinine. Quinine, which is somewhat toxic, suppresses the growth of protozoans within the bloodstream. In 1930, German chemists synthesized Atabrine, which is more effective than quinine and less toxic. A new drug, chloroquine, that became available at the end of World War II in 1945, was found to be capable of preventing and curing jungle fever completely and to be much more effective in suppressing the other forms of malaria than Atabrine or quinine. It also had a much lower toxicity than any of the earlier drugs and was effective in less frequent doses.
Recently, strains of Plasmodium falciparum, the organism that causes jungle fever, have shown resistance to chloroquine and other synthetic antimalarial drugs. These strains are encountered most frequently in Vietnam, and also in the Malay Peninsula, Africa, and South America. Quinine is still the agent used against Plasmodium falciparum strains resistant to synthetics. In addition to the occurrence of strains of drug-resistant parasites, the fact that some vector mosquitoes (Anophelines) have become resistant to insecticides such as DDT has led to an upsurge of malaria in certain tropical countries. As a result, malaria has increased among American and Western European travelers to Asia and Central America and among refugees from these areas.
Currently, work is progressing on the development of a malaria vaccine. Several vaccine candidates are now undergoing clinical trials for safety and effectiveness in human volunteers, and scientists look forward to having a vaccine for general distribution.

Gorgas, William Crawford (1854-1920), American sanitarian and army surgeon, born in Mobile, Alabama, and educated at the University of the South and Bellevue Hospital Medical College. He entered the U.S. Army in 1880 as an officer in the Medical Corps, serving mostly in posts in the South. In 1900 the American surgeon Walter Reed discovered that the yellow fever germ is carried by mosquitoes. Four years later Gorgas was sent to Panama as chief sanitary officer of the Panama Canal Commission to combat yellow fever and malaria, which were the greatest obstacles to the building of the Panama Canal. By destroying the mosquitoes' breeding zones and keeping patients separated, within two years he had eliminated yellow fever from the canal region and had brought malaria under control. In 1914 he was appointed surgeon general of the U.S. Army. He was retired from the army in 1918 and, until his death, directed the Rockefeller Foundation's work on yellow fever.

Versailles, Treaty of, peace treaty signed at the end of World War I between Germany and the Allies. It was negotiated during the Paris Peace Conference held in Versailles beginning January 18, 1919. Represented were the United States, Great Britain, France, and Italy; the German Republic, which had replaced the imperial German government at the end of the war, was excluded from the parley. Included in the first section of the treaty was the Covenant of the League of Nations, the world's first peacekeeping body, which was given the responsibility for executing the terms of the various treaties negotiated after World War I. The treaty was signed on June 28, 1919, in the Hall of Mirrors at the Palace of Versailles near Paris. (The U.S. did not ratify the agreement but signed a separate Treaty of Berlin with Germany on July 2, 1921.)

Disarmament and Reparations 
By the Treaty of Versailles, Germany was required to abolish compulsory military service; to reduce its army to 100,000; to demilitarize all the territory on the left bank of the Rhine River and also that on the right bank to a depth of 50 km (31 mi); to stop all importation, exportation, and nearly all production of war material; to limit its navy to 24 ships, with no submarines, the naval personnel not to exceed 15,000; and to abandon all military and naval aviation by October 1, 1919. Germany also agreed to permit the trial of former emperor William II by an international court on the charge of "a supreme offense against international morality." (The trial never took place.)
For damage incurred by the Allied powers during the war, Germany was required to make extensive financial reparation. In addition to money, payment was made in the form of ships, trains, livestock, and valuable natural resources. Difficulty arose in collecting payment, and the situation was not finally settled until the Lausanne Conference in 1932.

Territorial Changes 
Germany recognized the unconditional sovereignty of Belgium, Poland, Czechoslovakia (now the Czech Republic and Slovakia), and Austria and denounced the treaties of Brest-Litovsk and Bucharest. In addition, it lost some 71,000 sq km (about 27,500 sq mi), or slightly more than 13 percent of its European domain. Alsace-Lorraine was returned to France, and the Saar basin placed under a League of Nations Commission for 15 years. Belgium received the small districts of Eupen, Malmédy, and Moresnet. Under plebiscites held in 1920 to determine the status of northern and central Schleswig, the former, comprising 3981 sq km (1537 sq mi), was reunited with Denmark, but the latter remained with Germany. To Poland were ceded large parts of the provinces of Posen and West Prussia. Plebiscites in southeastern Prussia and the Marienwerder district of West Prussia, held in 1920, produced substantial majorities for Germany. The plebiscite in Upper Silesia in 1921 gave a majority for Germany, but the Council of the League of Nations, having been invited to settle the controversy, awarded the richest part of the region to Poland. A portion of Upper Silesia (now in the Czech Republic) was ceded to Czechoslovakia in 1920. The port of Memel with adjacent territory was ceded to the Allies for ultimate transfer to Lithuania. The port of Danzig was ceded to the principal Allied and associated powers, which recognized Danzig (now Gdansk) as a free city administered under the League of Nations but subject to Polish jurisdiction in regard to customs and foreign relations. Germany also lost its entire colonial empire.

League of Nations, international alliance for the preservation of peace. The league existed from 1920 to 1946. The first meeting was held in Geneva, on November 15, 1920, with 42 nations represented. The last meeting was held on April 8, 1946; at that time the league was superseded by the United Nations (UN). During the league's 26 years, a total of 63 nations belonged at one time or another; 28 were members for the entire period (see accompanying table).

The Covenant and the United States 
In 1918, as one of his Fourteen Points summarizing Allied aims in World War I, United States president Woodrow Wilson presented a plan for a general association of nations. The plan formed the basis of the Covenant of the League of Nations, the 26 articles that served as operating rules for the league. The covenant was formulated as part of the Treaty of Versailles, which ended World War I, in 1919.
Although President Wilson was a member of the committee that drafted the covenant, it was never ratified by the U.S. Senate because of Article X, which contained the requirement that all members preserve the territorial independence of all other members, even to joint action against aggression. During the next two decades, American diplomats encouraged the league's activities and attended its meetings unofficially, but the United States never became a member. The efficacy of the league was, therefore, considerably lessened.

League Structure 
The machinery of the league consisted of an assembly, a council, and a secretariat. Before World War II (1939-1945), the assembly convened regularly at Geneva in September; it was composed of three representatives for every member state, each state having one vote. The council met at least three times each year to consider political disputes and reduction of armaments; it was composed of several permanent members-France, Great Britain, Italy, Japan, and later Germany and the Union of Soviet Socialist Republics (USSR)-and several nonpermanent members elected by the assembly. The decisions of the council had to be unanimous. The secretariat was the administrative branch of the league and consisted of a secretary-general and a staff of 500 people. Several other bodies were allied with the league, such as the Permanent Court of International Justice, called the World Court, and the International Labor Organization.

World Involvement 
The league was based on a new concept: collective security against the "criminal" threat of war. Unfortunately, the league rarely implemented its available resources, limited though they were, to achieve this goal.
One important activity of the league was the disposition of certain territories that had been colonies of Germany and Turkey before World War I. Supervision of these territories was awarded to league members in the form of mandates. Mandated territories were given different degrees of independence, in accordance with their stage of development, their geographic situation, and their economic status.
The league may be credited with certain social achievements. These include curbing international traffic in narcotics and prostitution, aiding refugees of World War I, and surveying and improving health and labor conditions around the world.
In the area of preserving peace, the league had some minor successes, including settlement of disputes between Finland and Sweden over the Åland Islands in 1921 and between Greece and Bulgaria over their mutual border in 1925. The Great Powers, however, preferred to handle their own affairs; France occupied the Ruhr, and Italy occupied Corfu, both in 1923, in spite of the league.
Although Germany joined the league in 1926, the National Socialist (Nazi) government withdrew in 1933. Japan also withdrew in 1933, after Japanese attacks on China were condemned by the league. The league failed to end the war between Bolivia and Paraguay over the Gran Chaco between 1932 and 1935 and to stop the Italian conquest of Ethiopia begun in 1935.
Finally, the league was powerless to prevent the events in Europe that led to World War II. The USSR, a member since 1934, was expelled following the Soviet attack on Finland in 1939. In 1940 the secretariat in Geneva was reduced to a skeleton staff, and several small service units were moved to Canada and the United States.
In 1946 the league voted to effect its own dissolution, whereupon much of its property and organization were transferred to the UN.

Legacy 
Never truly effective as a peacekeeping organization, the lasting importance of the League of Nations lies in the fact that it provided the groundwork for the UN. This international alliance, formed after World War II, not only profited by the mistakes of the League of Nations but borrowed much of the organizational machinery of the league.

Membership 
The accompanying table lists the countries that were members of the international organization. Where no date is given, the country was an original member of the league. The year in parentheses is the year of admission to the league unless otherwise indicated.