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Most simply put, a manhole cover is the outer covering used to seal the opening of a manhole. A manhole is a hole in the ground that gives access to utilities (like gas and water lines), sewage lines, or storm drains. It is most commonly associated with the opening to a vertical tube or shaft leading down into a sewage drain. As such, the main purpose of a manhole cover is to protect the manhole from unauthorized entry and also to prevent accidents occurring from people or objects falling in. Another purpose of the manhole cover is to allow the air pressure in the sewer system to equalize without forcing foul smelling air to emerge from drains inside buildings. Notice that most manhole covers have small openings present to allow air to pass through for just this purpose.
The history of the manhole cover dates back to the history of the sewage system. Evidence for the earliest sewage systems dates all the way back to Mesopotamia (modern day Iraq) up to 4000 BC. The first civilization to have a widespread sewage system were the Romans. During all this time, manhole covers would have been made of carved stone or possibly wood. In fact, Roman manhole covers have been excavated. Starting around the 18th century in North America and Europe, manhole covers have been made from cast iron or sometimes concrete. Also around this time, the shape of the manhole cover became standardized to round in shape, although square or rectangular variants are sometimes found. Manholes covers by design have remained largely unchanged since the late 19th century. Something incorporated more recently has been to personalize and decorate manhole covers to give particular cities or areas a more specialized aesthetic.
Manhole covers are usually circular in shape, made of iron and found on top of manholes. Manholes are entry points, located on the ground, which usually provide access into the underground sewerage or utility areas of a city.
Curiously enough, they tend to be located in the middle of roads, and usually have fast-moving traffic passing over them. It is not known whether or not this was or is a basic form of manhole security. If it is, then it isn't a very good one, as at night most roads are deserted.
From as early as 3500BC, slabs of stone or pieces of wood were used to cover the ditches that carried waste. Several civilisations developed this further - for example, the Romans had a complex system of waterworks - but sewerage system designs that are implemented today were not developed until the 18th Century. As time progressed, tunnels or tubes were used to dispose of waste in order to stop the spread of infectious disease. As the population grew in locations with sewers, there became a need to tie these systems together so that the waste could be removed more efficiently. These sewage lines had a hole dug between the last points in each line in order to connect the two systems together. A cover was then placed over the junction point. This junction served as a plumbing point in case the sewer became clogged with debris and backed up. It was at some point during the 19th Century that manhole covers started being made of iron.
The word manhole was used in the past to describe particular areas on decks of sailing ships. These manholes provided access to goods and stores in the hold and measured approximately a metre long by a metre wide. An access hatch was constructed and placed over the top of the manhole to ensure that nobody would gain a practical experience of the laws of gravity while standing directly over the manhole.
Today these openings are known as companionways or hatches. Another odd titbit is that this space is scarcely large enough for a single man to fit through.
Importance of Man Hole Covers:
Manhole covers are essential road furniture: they keep people from falling into holes used to access subsurface utilities. In Victoria alone, there are over 5,553 access holes; some a foot or two deep, others so deep workers need ladders to descend. Weights and sizes vary, but all manhole covers must be heavy enough to stay in place and to discourage people from moving them.
Why is Man Hole Covers Round?
The main reason manhole covers are round is so they won't accidentally fall into the manhole itself. With a round cover, no matter how you hold it, you can't shove it in. It just won't go. If it were square, a prankster could hold the cover diagonally over the hole and drop it in, to be followed by who knows how many scooters and pedestrians.
The fact that most manholes in the USA are round has given rise to much speculation, and several popular explanations have arisen:
- You can roll them, and thus transport them by hand, more easily than triangles or squares.
- A circular tube will displace ground pressure more effectively than any other shape.
- The shape makes them much easier to line up than triangular and square covers.
- Most other shapes could relatively easily be turned on their side and fall through the hole.
Of course, not all manhole covers are round. Rectangular and triangular covers are also known. Whatever the shape of the manhole, the manhole cover bottom is recessed about 2.5cm so it doesn't fall through, since the inner diameter of the manhole is smaller than that of the manhole cover.
Interesting Facts about Manhole Covers
- Nashua, New Hampshire, in the US has triangular manhole covers that point in the direction of flow but that design is currently being phased out in favour of round covers.
- Triangular manhole covers do not rattle like round manhole covers do, but they tend to stick due to expansion from heat.
- Manhole covers are made of iron or concrete for larger systems where weight is a concern.
- Most iron manhole covers have an artistic or aesthetic print forged on them. Some are smooth, while others have a pattern to assist traction of vehicles passing over them.
- Despite generally weighing more than 50kg, manhole cover pictures are sometimes collected as works of art.
- The forged print of a manhole cover almost always contains an identifying mark for the company that made it.
- In Rome, manhole covers carry the legend 'SPQR', an abbreviation for 'Senatus Populusque Romanus', 'The Senate and People of Rome.'
- The size of a manhole cover varies but measures approximately 60.96cm radius in the USA.
- Unlike in movies, where a person would fall through a manhole as a joke and survive with minimal injuries, if you were to fall into a manhole unsuspectingly you would be seriously injured or could possibly even be killed from the fall.
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