Troy-Bilt Bronco 42 in. 19 HP Briggs and Stratton Engine Automatic Drive Gas Riding Lawn Mower

Nineteen or 19 may refer to:

• 19 (number)
• One of the years 19 BC, AD 19, 1919, 2019

An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.

Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power generation), heat energy (e.g. geothermal), chemical energy, electric potential and nuclear energy (from nuclear fission or nuclear fusion). Many of these processes generate heat as an intermediate energy form; thus heat engines have special importance. Some natural processes, such as atmospheric convection cells convert environmental heat into motion (e.g. in the form of rising air currents). Mechanical energy is of particular importance in transportation, but also plays a role in many industrial processes such as cutting, grinding, crushing, and mixing.

Mechanical heat engines convert heat into work via various thermodynamic processes. The internal combustion engine is perhaps the most common example of a mechanical heat engine in which heat from the combustion of a fuel causes rapid pressurisation of the gaseous combustion products in the combustion chamber, causing them to expand and drive a piston, which turns a crankshaft. Unlike internal combustion engines, a reaction engine (such as a jet engine) produces thrust by expelling reaction mass, in accordance with Newton's third law of motion.

Apart from heat engines, electric motors convert electrical energy into mechanical motion, pneumatic motors use compressed air, and clockwork motors in wind-up toys use elastic energy. In biological systems, molecular motors, like myosins in muscles, use chemical energy to create forces and ultimately motion (a chemical engine, but not a heat engine).

Chemical heat engines which employ air (ambient atmospheric gas) as a part of the fuel reaction are regarded as airbreathing engines. Chemical heat engines designed to operate outside of Earth's atmosphere (e.g. rockets, deeply submerged submarines) need to carry an additional fuel component called the oxidizer (although there exist super-oxidizers suitable for use in rockets, such as fluorine, a more powerful oxidant than oxygen itself); or the application needs to obtain heat by non-chemical means, such as by means of nuclear reactions.

Gas is one of the four fundamental states of matter. The others are solid, liquid, and plasma. A pure gas is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom (elements such as oxygen) or from different atoms (compounds such as carbon dioxide). A gas mixture, such as air, contains a variety of pure gases. What distinguishes gases from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer.

The gaseous state of matter occurs between the liquid and plasma states, the latter of which provides the upper-temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention. High-density atomic gases super-cooled to very low temperatures are classified by their statistical behavior as either Bose gases or Fermi gases. For a comprehensive listing of these exotic states of matter, see list of states of matter.

A lawn () is an area of soil-covered land planted with grasses and other durable plants such as clover which are maintained at a short height with a lawn mower (or sometimes grazing animals) and used for aesthetic and recreational purposes—it is also commonly referred to as part of a garden. Lawns are usually composed only of grass species, subject to weed and pest control, maintained in a green color (e.g., by watering), and are regularly mowed to ensure an acceptable length. Lawns are used around houses, apartments, commercial buildings and offices. Many city parks also have large lawn areas. In recreational contexts, the specialised names turf, parade, pitch, field or green may be used, depending on the sport and the continent.

The term "lawn", referring to a managed grass space, dates to at least the 16th century. With suburban expansion, the lawn has become culturally ingrained in some areas of the world as part of the desired household aesthetic. However, awareness of the negative environmental impact of this ideal is growing. In some jurisdictions where there are water shortages, local government authorities are encouraging alternatives to lawns to reduce water use. Researchers in the United States have noted that suburban lawns are "biological deserts" that are contributing to a "continental-scale ecological homogenization." Lawn maintenance practices also cause biodiversity loss in surrounding areas. Some forms of lawn, such as tapestry lawns, are designed partly for biodiversity and pollinator support.

A mower is a person or machine that cuts (mows) grass or other plants that grow on the ground. Usually mowing is distinguished from reaping, which uses similar implements, but is the traditional term for harvesting grain crops, e.g. with reapers and combines.

A smaller mower used for lawns and sports grounds (playing fields) is called a lawn mower or grounds mower, which is often self-powered, or may also be small enough to be pushed by the operator. Grounds mowers have reel or rotary cutters. Larger mowers or mower-conditioners are mainly used to cut grass (or other crops) for hay or silage and often place the cut material into rows, which are referred to as windrows. Swathers (or windrowers) are also used to cut grass (and grain crops). Prior to the invention and adoption of mechanized mowers, (and today in places where use a mower is impractical or uneconomical), grass and grain crops were cut by hand using scythes or sickles.

Riding may refer to:

• In equestrianism, riding a horse
• Riding (division), administrative division of a county, or similar district
• Electoral district (Canada), Canadian term for an electoral district
• Riding, Northumberland, a former parish, now in Broomhaugh and Riding, England
• Riding (surname)
• "Riding", a 2022 song by Bently and No Money Enterprise

Troy (Hittite: 𒆳𒌷𒋫𒊒𒄿𒊭, romanized: Truwiša/Taruiša; Ancient Greek: Τροία, romanized: Troíā; Latin: Troia) or Ilion (Hittite: 𒌷𒃾𒇻𒊭, romanized: Wiluša; Ancient Greek: Ἴλιον, romanized: Ī́lion) was an ancient city located in present-day Hisarlık (near Tevfikiye), Turkey. The place was first settled around 3600 BC and grew into a small fortified city around 3000 BC. During its four thousand years of existence, Troy was repeatedly destroyed and rebuilt. As a result, the archeological site that has been left is divided into nine layers, each corresponding to a city built on the ruins of the previous. Archaeologists refer to these layers using Roman numerals. Among the early layers, Troy II is notable for its wealth and imposing architecture. During the Late Bronze Age, Troy was called Wilusa and was a vassal of the Hittite Empire. The final layers (Troy VIII-IX) were Greek and Roman cities which in their days served as tourist attractions and religious centers because of their link to mythic tradition.

The archaeological site is open to the public as a tourist destination, and was added to the UNESCO World Heritage list in 1998. The site was excavated by Heinrich Schliemann and Frank Calvert starting in 1871. Under the ruins of the classical city, they found the remains of numerous earlier settlements. Several of these layers resemble literary depictions of Troy, leading some scholars to conclude that there is a kernel of truth underlying the legends. Subsequent excavations by others have added to the modern understanding of the site, though the exact relationship between myth and reality remains unclear and there is no definitive evidence for a Greek attack on the city.^{(ppxiv, 180–182)}