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The Pennsylvania Railroad (reporting mark PRR), officially the Pennsylvania Railroad Company, also known as the "Pennsy," was an American Class I railroad established in 1846 and headquartered in Philadelphia, Pennsylvania. In 1882 it was the largest railroad (by traffic and revenue), transportation…

The Roman Kingdom, also known as the Roman monarchy and the regal period of ancient Rome, was the earliest period of Roman history, when the city and its territory were ruled by kings. According to tradition, the Roman Kingdom began with the city's founding c. 753 BC, with settlements around the Pal…

The Neolithic or New Stone Age (from Greek νέος néos 'new' and λίθος líthos 'stone') is an archaeological period, the final division of the Stone Age in Mesopotamia, Asia, Europe and Africa (c. 10,000 BC to c. 2,000 BC). It saw the Neolithic Revolution, a wide-ranging set of developments that appear…

In geometry, a polygonal chain is a connected series of line segments. More formally, a polygonal chain ⁠ P {\displaystyle P} ⁠ is a curve specified by a sequence of points ( A 1 , A 2 , … , A n ) {\displaystyle (A_{1},A_{2},\dots ,A_{n})} called its vertices. The curve itself consists of the line s…

The Byzantine Empire, also known as the Eastern Roman Empire, was the continuation of the Roman Empire centred on Constantinople during late antiquity and the Middle Ages. Having survived the events that caused the fall of the Western Roman Empire in the 5th century AD, it endured until the fall of …

Silicon Valley is a region in Northern California that is a global center for high technology and innovation. Located in the southern part of the San Francisco Bay Area, it corresponds roughly to the geographical area of the Santa Clara Valley.…

Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay…

Ecoregions (ecological regions) are ecological and geographical areas that exist on multiple different levels, defined by type, quality, and quantity of environmental resources. Ecoregions cover relatively large areas of land or water, and contain characteristic, geographically distinct assemblages …

A merchant navy or merchant marine is the fleet of merchant vessels that are registered in a specific country. On merchant vessels, seafarers of various ranks and sometimes members of maritime trade unions are required by the International Convention on Standards of Training, Certification and Watch…

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