The Incas lived in South America from the 14th century to mid 15th century, when they where conquered by Spanish explorers lead by Francisco Pizarro. The Incas had one of the largest empires in the world. They were located in modern day Peru and around the Andes Mountains, but their empire stretched from Colombia to Chile [13].

The Inca Empire originated around the city of Cuzco as a relatively minor group: the Quechuas. Gradually, as early as the thirteenth century, they began to expand and incorporate their neighbors. Inca expansion was slow until about the middle of the fifteenth century, when the pace of conquest began to accelerate, particularly under the rule of the great emperor Pachacuti Inca Yupanqui (1438-71). Pachacuti has been compared with Alexander and Napoleon,for his will of conquer and ability in government. Under his rule and that of his son, Topa Inca Yupanqui (1471-93), the Incas came to control a third of South America, with a population of 9 to 16 million inhabitants under their rule. Pachacuti also created a comprehensive code of laws to govern his vast empire called Tiwantinsuyu(6). In the words of Garcilaso de la Vega (1539--1616), Pachacuti "...governed his empire with so much industry, prudence and resolution, as well in piece as in war, that not only did he increased the boundaries of all the four quarters {Tiwantinsuyu was also known as the land of the four quarters.}, but also he enacted many laws..."(1).

Pizarro and his men arrived just at the end of a disastrous civil war. The Inca Huayna Capac had died in 1527 without leaving clear the succession to the throne. Two of his sons, Huascar and Atahualpa, raved the empire through five bloody years. The Spanish arrived just as Atahualpa's army had won the final decisive battle, but enmities were still strong and either side was willing to make an alliance with the newcomers against the other(8). The Spanish deceived Atahualpa by pretending to be in a diplomatic mission. The Spanish attacked when they were talking with the Inca. Francisco de Xeres (Pizarro's military secretary and an eyewitness of the events) wrote an account of this action: ``During the whole time no Indian raised his arm against a Spaniard. So great was the terror of the Indians at seeing the Governor force his way through them, at hearing the fire of the artillery, and beholding the charging of the horses, a thing never before heard of, that they thought more of flying to save their lives than of fighting''(2) . The Spanish asked for gold and silver in order to release Atahualpa. After a full room with treasures was given to the Spaniards, they killed Atahualpa.

The Incas were most famous for their masonry. Masonry were blocks of stone that were cut, ground, and polished until the surface was smooth and shaped to perfection. The Incas developed two types of masonry: coursed and polygonal. In coursed masonry, all stones were rectangular, placed in even horizontal rows, and tightly joined with sunken joints. The stones became smaller as they were placed higher on the wall to make the building look balanced. Coursed masonry was apparently valued more than polygonal masonry, because the walls of palaces and temples used coursed masonry. Polygonal masonry was generally used for daily buildings and huts. Polygonal masonry is when the stones interlock at random with the convex of one stone fitting into the concave of another stone. The finishing product is composed of many different shapes that all fit together perfectly. The masonry built by the Incas still stands today which proves that the quality of the masonry is superb(13).

City planning was a consequence of their broad pragmatism and the economic sense that an urban center represented. The Incas, being a farming society had to reserve the best lands for their main activity without wasting them for temples or villages that were built in rustic terrains. Even inside the towns, the streets were always narrow to take a maximum advantage of the land(4). The architecture of the Inca cities still amazes and puzzles most scientists. Stone steps lead up to the top of the cities, which consist of stone houses and religious buildings. The blocks of stones weigh several tons and they are fit together so tightly that not even a razor blade can fit through them. The central city was mainly used for government purposes, while the citizens occupied surrounding areas. Their homes were made from the same stone material and have grass rooftops(14).

Some structures were especially designed for astronomical observations such as the Intihuatana at Machu Pichu, which was similar to a sundial, and the sucanca, pillars set up to the east and west of Cuzco to measure solstice. The Incas believed that the sun had two ``seats'', the main one in the north and a secondary one in the south. The year began with the summer solstice, when the sun settled into its southern seat(3).

There were historically only two road systems: the Roman roads, which covered 56 thousand linear miles through Europe, the Near East, and Africa, and that of the Incas, which goes across the surface of the Andes from Argentina to Colombia and along the entire length of desert coast, amounting to more than 10 thousand miles of all-weather highways. In order to hold the Inca realm together (with desert, mountain, and jungle regions) into an empire, the best of communications was needed. The result was the Inca road, a system only comparable to the Roman roads, an American labor which Alexander von Humboldt (who knew both) characterized as ``the most useful and stupendous works ever executed by man''(5). There were two main sections of the road system. The Andean royal road(Capac�an), which moved through the Andes from Colombia down through Ecuador, Peru, Bolivia, Argentina and then to Chile. It was 3,250 miles in length, making it longer than the longest Roman road which goes from Hadrian's Wall in Scotland to Jerusalem. The coastal road was 2,520 miles in length and crossed the desert coast of Peru and Chile.

Even though the Incas never had access to the wheel, they constructed an extensive system of well-built and constantly maintained roads to connect the villages, which spanned at least 23,000 km. The road network facilitated communications and the movement of people (especially the armies) and goods. They were paved with flat stones and barriers to protect the messengers, or chasqui, from falling down a cliff. In the cities, the streets were narrow allowing only two men to pass on each side. Down the middle of the road, there was running water in a small stone-lined path. All the streets were paved and well constructed. To cross the many steep ravines found in the Andes, they built impressive bridges.

The Incas built several types of bridges: slabs resting on stone pillars called rumichacas, simple tree trunks, and suspension bridges made from liana and plait agave fibres, like that near Penip\'e or the one that still hangs over the Apurimac river(9). Alternatively the traveler could use an oroya, a kind of basket in which he or she could slide along a cable. A chaca suyuyoc, or ``governor of the bridges'', was in charge of all these civil engineering works.

The total surface available for farming on the coast without irrigation was strictly limited in the Inca Empire. It consisted of narrow strips along each side of the rivers, and it probably could not have supported more than a tenth of the three million people who eventually lived in the coastal valleys. Beyond the immediate vicinity of the river, the water level dropped rapidly until, at the outer edges of the delta, it may have been as much as 100 feet below the surface of the ground. Maize and squach cannot live in such an environment(8).

People from Tawantinsuyo had a deep farming conviction, where all the valleys and flatland were cultivated. They took advantage even of dry and rocky mountain slopes where farming terraces were built; to construct them all urged qualified technology and very intensive work. When making the terraces, at first stone retaining walls were built and then the empty spaces were filled up with stones or sand in the lower part and the upper side with fertile soil brought in certain cases from some kilometers away. All these farming terraces were totally irrigated with aqueducts that almost always went over many kilometers from their harnessing in springs, rivers or lakes. Moreover, in order to keep humidity of the fertile soil that was about one meter deep, there was a clay layer between fertile and sterile soils(4).

Irrigation served several purposes in ancient Peru: the expansion of farm land, the conservation of water through dry seasons, and the fair distribution of water. The conformation of the coastal valleys was such that they never could have been irrigated efficiently with small ditches leading to individual plots. In order to downgrade a ditch sufficiently, it must be several miles long, which is not economically viable if the ditch is to water only one's family land. The only feasible solution was to take the water high up in the valley, run it through a few major canals, and distribute it to all of the farms through a network of ditches leading from the main canals. At the same time, they cleared more land for farming in the areas reached by the new ditches and canals. The entire system, both main canals and secondary ditches, had to be cleaned once a year to keep the accumulated dirt from blocking the flow of water(8). These channels are still in use, for example in the valleys of the chilean rivers which go from the Andes to the Pacific Ocean, crossing the Atacama desert.

The program of expanding farm land, the distribution of limited water resources, the clearing of the woods, and the upkeep of the irrigation systems all required a central authority capable of ordering the activity of every farmer in a given valley. It is clear that the system could never have functioned on the basis of simple cooperation between families or settlements. The choice lay between a strong central government for each valley or economic disaster. Present evidence indicates that the irrigation systems were not built until the requisite authority patterns had come into existence. The very existence of nucleated cities or ceremonial centers with monumental architecture implies a stratified and specialized socioeconomic and political system capable of running an irrigation system of these proportions. Therefore, it cannot be said that irrigation led to the centralization of authority but rather that, once authority was centralized, it became possible to build and maintain irrigation systems. Irrigation was thus a product of civilization, not a cause of it(8).

Masonry, roads and irrigation channels are Inca achievements that have survived the Inca Empire for centuries. But from the human point of view, one of the biggest realizations of the Tawantinsuyo was to eradicate starvation, through constant biological research, acclimatization, domestication of wild plants for human consumption, and very intensive work(4), in addition, of course, to the very efficient irrigation system. Inca society was based on sharing. People did not work for money. The good they produced were distributed as people needed them. Young married couples received a house and some land for farming. For each child born, the couple was given more land. Children helped in the fields and at home. Older people were given simple tasks such as collecting firewood and teaching the children. After a lifetime of working, elderly Incan citizens were not required to earn their share. The state provided them with food and clothing(7). In return, the subjects of the Inca Empire were required to give a fixed amount of their labor to the state, which could have been dedicated to anything from building roads to making pottery. This system was called the mita(11). In this way the fabulous masonry and road system of the Incas was built, in addition to maintaining a privileged class of aristocrats and the Inca himself.

The tribute system in the Inca empire was based on retribution. This means an equilibrium between what the Inca received from the people and what he gave back to the people. Francisco Pizarro arrived to Tiwantinsuyu in 1532. This system was broken by the Spanish keeping it only in one direction: they got from the people but they did not return to them. As Nathan Wachtel wrote ``The tribute system imposed on the natives by the Spaniards was inspired by and modeled after the Incas' own system of tribute. It failed, however, to take into consideration the ideology of reciprocity that had legitimized it under the Inca''(12).

One of the most savage excesses of the Spanish was their abuse of the mita. The Spanish used the mita to force the natives to work in the mines. Whole families were compelled to travel with their animals over hundreds of miles, leaving behind them entire regions uninhabited, to work in places like Potosi in south Bolivia, famous for its silver mine, the Cerro Rico. At any one time, 14% of all men were on the mita, which lasted for four months each year. Men spent up to 36 hours continuously underground, and many thousands died in the mines. The natives earned little money, and after paying compulsory taxes, they often found themselves in debt. These debts were in turn inherited by children and grandchildren, who could only repay by a lifetime work in the mines. When they were not working in the mines, the natives were forced into agricultural service for the spaniards who had taken over their lands: by a system known as encomienda, the new land lords could acquire a native work force in return for instructing them in the christian faith(10).

Throughout the Americas, the impact of the Spanish conquest and subsequent colonization was to bring about a cataclysmic demographic collapse of the indigenous population. The Incas would be no exception. War, exploitation, socioeconomic change, and the generalized psychological trauma of conquest all combined to reinforce the main contributor to the demise of the native peoples: epidemic disease. Isolated from the Old World for millennia and therefore lacking immunity, the Andean peoples were defenseless against the deadly diseases introduced by the Europeans. From an estimated population of 16 millions(16,000,000) in 1530, before the arrival of the europeans, the population decreased to 2� millions (2,500,000) in just 40 years (in 1570)! This disaster can be better understood noting that Peru recover its 16 millions inhabitants only 400 years later (in 1980)!(6).

The Tiwantinsuyu empire had very advanced building technologies and with the help of these tools this society was making steady progress towards the improvement of the quality of life of its people. Their road system was well paved and maintained and its main road was longer than the longest roman road. Their buildings were so well constructed that lasted for centuries, and some of them were made with huge stones polished to fit perfectly together. Their irrigation system was very sophisticated and only a very well organized society could have built it, making fertile desertic areas and steep slopes with the help of terraced farming.

The great building technologies of the Incas was made possible by a perfectly well organized society. But when the spanish arrived they destroyed that social balance and, in that way, the knowledge and techniques were forgotten. This historical example show us that technology is nothing without the social organization that supports it.

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