What reason is given for the stability of the arch?

W: An arch is the spanning of a wall opening by means of separate units, such as bricks or blocks, which are assembled into an upward curve．The invention of the arch made it possible to span wide openings without large timbers．Before the arch, large window and door openings required a large wooden timber, and a supply of large trees．The arch was very important in regions where there were few large trees．The Babylonians, Egyptians, and Etruscans all used the arch, and it was later adopted by the Romans．The arch became a basic principle in the Roman system of construction in brick and concrete．The Romans contributed a lot to the development of architecture, especially its engineering aspects．They perfected a system of arch and vault construction．As a result, a larger variety of buildings appeared, each with a form suited to its function．The Roman approach to the building arts was always practical．The Romans undertook huge public work projects, including a network of bridges and aqueducts, all of which utilized the arch．To supply their vast public baths and fill the water needs of their citizens, they needed water in large amounts for their cities．Using their considerable engineering skills, the Romans built aqueducts so sturdy that many are still standing．The Roman semi-circular arch is the most common and most elementary type of arch．It was made by taking wedge shaped blocks-large bricks with angled sides-and placing them side by side, forming an upward curving, semi-circular opening．The block at the highest point-the center of the arch-is called the keystone．The keystone is the most important block because it holds the other blocks in place．The keystone locks the other stones together．The round arch is extremely strong and stable．It’s primarily the wedge shape of the blocks that gives the arch its stability．The wedge-shaped blocks are locked together tightly, transferring their weight downward on either side．The arch is very sturdy and can support a heavy load．The arch is supported only from the sides．This is because of lateral thrust．The downward pressure is transformed into lateral thrust through the piers, the two vertical structures on either side of the opening．What I mean is, the ..．um ..．the weight of the arch and its load is converted from downward pressure into sideways-lateral-pressure．The massive piers that frame each side of the opening receive the pressure and therefore support the arch．Until the nineteenth century, arches were still widely used in public buildings in North America．However, the invention of steel beams in the nineteenth century-yes, did you have a question?
M: Yes．I was wondering ..．um ..．what you said implies that we don’t see arches in public buildings anymore, I mean, in newer buildings．But what about the arches in our administration building, here on campus? It has lots of arches, and I’m pretty sure the building is fairly new.
W: I’m glad you mentioned our administration building because that’s a perfect illustration of the decorative arch．You see, behind all that beautiful brickwork is a system of steel beams．The beams are doing the real work of holding up the building．The brick arches are just for show．Steel beams for wide spans have reduced the arch to a mostly decorative function．And, as you can see in our administration building, decorative arches remain popular today.