Arches were known in ancient Egypt and Greece, but they were deemed inadequate for monumental building and were rarely employed. The semicircular arch, on the other hand, was employed by the Romans in bridges, aqueducts, and large-scale building. It is this type of arch that we see used in many Roman buildings. The semicircular arch is more stable than an equilateral arch because it has two equal sides. This gives it two points of resistance instead of one. Thus, it will not collapse under its own weight as easily as an equilateral arch would. Also, the pressure on the top of the arch can be spread out over a larger area, which reduces the risk of cracking from excessive pressure. There are several types of semicircular arches, but the three most common ones are the hypocaust, oculus, and triglyph/tritrope. The hypocaust is the arch under a floor or platform where hot air was kept in heating systems. This would be used in large public buildings such as theaters or arenas where noise might disturb those sleeping below. The oculus is the opening at the top of an arch where rain or snow may enter. This would be used in small buildings like houses or shrines where security was not a concern.