Blog

Engineering marvel of the Canal Age

Before I knew much about aqueducts (whether ancient or modern), I imagined they would be constructed to bring water to areas that had none. While they can serve that purpose, I’ve since learned that aqueducts have been built in places with plenty of water—such as a river valley.

One example of this is a first-century aqueduct system constructed by the Romans in Southern France to carry water from the Eure spring in Uzès to a water tower in Nîmes, about thirty miles (fifty kilometers) away. In this case, it was the spring water in particular that the Romans coveted, using it to supply the fountains, sewage systems, and spas of the flourishing town. As part of the aqueduct system, the Romans built the now-famous Pont du Gard, a bridge that enabled the channel of water to cross over the Gard river and to continue on its way to Nîmes. In a way, it was like they built a river (or stream, if you will) above another river; rather remarkable to a non-engineer like me.

The Pont du Gard is remarkable for a number of other reasons, including: its longevity, having remained largely intact over the last two thousand years; its size, being the highest aqueduct ever built by the Romans (at 49 meters in height); and its incredible engineering, being composed of large stones that fit together without the use of mortar. Now it’s a UNESCO World Heritage Site, and every year large numbers of visitors from around the world come to see this amazing construction (including me in 2013—it was an awesome sight). Originally built to serve the social and business interests of Nîmes, the Pont du Gard now serves as a potent reminder of human ingenuity.

Channeling Ideas

A slightly less famous and considerably younger version of the Pont du Gard can be found in Northeast Wales. The Pontcysyllte Aqueduct, built between 1795 and 1805, was created to carry the Llangollen canal over the valley of the River Dee; another river-over-a-river construction. In this case, the Llangollen canal, which connects three major rivers—the Dee, the Severn, and the Mersey—was originally part of the Ellesmere canal, a route built to connect the coalfields and ironworks of the town of Wrexham with the sea.

The creation of the Pontcysyllte Aqueduct (about which more in a moment) was one among a series of large-scale construction projects that began in the late 1700s and continued into the 1830s. These projects were sparked by the opening of the Bridgewater canal in 1761, created by the third Duke of Bridgewater (a great example of an aptonym if ever there was one) to provide an efficient means of transporting coal from his coal mines in Lancashire to the booming industrial city of Manchester. This canal, which included a large aqueduct over the River Irwell, proved so profitable to the Duke that it encouraged many others to build canals of their own.

Canal Knowledge

Before the development of railroads, canals were the first means of mass transport of goods in Britain. The canal-building craze started by Bridgewater helped to fuel the so-called Industrial Revolution, which saw a change from a primarily agrarian society to one in which trade and manufacturing could be undertaken on a massive scale. The reason for the incredible success of the canal system lay in its great improvement over the traditional method of transporting goods.

While Britain had always relied upon waterways as a means of transport, being surrounded by water and possessing many large navigable rivers, those areas of the country not in proximity to a body of water could be accessed only by road. However, roads at that time were mostly built of mud, and could become impassible in bad weather. In addition, there was a limit to how much cargo could be transported by horse and cart, usually around one to two tons. In comparison, the new canals could accommodate boats carrying 30 tons, with only one horse needed to pull the load as it walked beside the boat on specially created towpaths. This dramatically increased the rate at which goods could be shipped, and brought incredible profits to the companies that operated the canals.

There were limitations to this mode of transport, though; in order to save costs, canals were often built quite narrow. This meant only specially designed “narrow boats” could navigate the canals. In addition, because canals are constructed bodies of water with no current, the speed of travel was limited to the speed of the “horsepower” involved, although this problem was less prominent in later years as steam and electric powered boats were developed.

Gradually, with the rise of the railroad, the canal system came to be less and less economically viable. Although the canals were still in commercial use well into the 20th century, their dominance was greatly overshadowed by other modes of transport. Unlike other European countries, such as France, Germany, and the Netherlands, which modernized their canal systems to accommodate larger vessels, the British system did not undergo the same kind of overhaul and fell more and more into disuse.

Fortunately, the canal system in Britain was reborn in the 1960s and ’70s when it came to be associated with holiday travel. Now these historic canals are frequently used by boaters and tour operators seeking a new form of vacation activity.

Canal Retentive

The first commercial canal built by Bridgewater was designed by James Brindley, a man with very little formal education, but who nonetheless went on to become one of the best-known engineers of the 18th Century. Following in his footsteps, Thomas Telford was only four years old when the Bridgewater canal was opened, but came to prominence after he oversaw the construction of the Pontcysyllte Aqueduct from 1795 to 1805.

The aqueduct has many notable features, including its great length (1007 feet; about 300m), its structural ingenuity (in its use of tapered support columns), and the construction of the metal trough in which the water is carried. In order to reduce the weight of the masonry pillars, they were built wider at the bottom than at the top, allowing the aqueduct to reach a great height. The masonry was held in place by a mortar made of lime, water, and ox blood. The metal trough carrying the water (and the boats) was built from cast iron sections joined together and caulked using flannel dipped in boiling sugar, and then sealed with lead.

Canal Plus

In 2005, the Pontcysyllte Aqueduct celebrated its 200th Anniversary, and in recognition of its cultural importance, was submitted to UNESCO as a potential World Heritage Site (which it was awarded in 2009). Like the Pont du Gard, it receives many visitors and has become a significant tourist attraction. Boat rides along the aqueduct are popular, often traveling to and from the town of Llangollen, the site of the International Eisteddfod, a music festival that takes place every year in July.

As with the Pont du Gard, a purely commercial endeavor has now become something to celebrate. Although the canal system, including the Pontcysyllte Aqueduct, no longer drives the economic engine of Britain, the ingenuity of those who designed these marvels continues to amaze and inspire all who see them.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on June 21, 2006.

Fairy castles in the air

When people accuse you of building castles in the air, they are not usually congratulating you on an incredible engineering feat, but more likely trying to bring you back down to earth with a thud. Synonymous with daydreams, pipe dreams, and all other dreams unlikely to come to fruition, castles in the air are at best a hopeful vision, and at worst, a hopeless illusion.

Although the phrase “castles in the air” (the original phrase was “castles in Spain”) is most often used to describe imaginary constructions, it can also be used to describe a very real optical phenomenon—the fata morgana effect—in which different levels of hot and cold air distort the appearance of objects on the horizon to make them look like, well, castles in the air.

Tempting Fata

Fata Morgana is the Italian name for Morgan le Fay, the half-sister of King Arthur in Arthurian legend. Reputedly a sorceress and able to change shape at will, Morgan le Fay was sometimes said to live below the sea in a crystal palace that could also rise above the surface. The fata morgana effect was so named for the superstitious belief among sailors that she created illusory visions to lure men into a false port and to their death. The term first entered English usage in 1818, when it was used to describe an occurrence of the phenomenon in the Strait of Messina, a narrow body of water between Sicily and the region of Calabria in southern Italy.

Technically, fata morganas are a type of mirage, related to those visions of water in the desert, or less exotically, to those seeming pools of water on the highway on a hot day. However, the latter two are examples of inferior mirages, while fata morganas are classified as superior mirages. It’s not that fata morganas are inherently better than the others; the difference lies in the way each mirage is produced.

Refract Up

Although the word mirage is derived from the French verb se mirer, meaning “to be reflected,” a more apt description of a mirage is that it is refracted. As light passes through layers of air with varying densities (density being determined by factors such as pressure and temperature), it bends, or more specifically, refracts, according to each layer’s characteristics.

In the case of inferior mirages, light bends upwards when it moves from a denser layer of cold air into a less dense layer of hot air, like that created above a highway on a hot day. As light hits the surface of the road and bends upwards, it looks to our eyes as if we are seeing a reflection in the road of what is just above it—in this case, the blue sky. This is because we perceive that light travels in a straight line to our eyes, even when that is factually not so.

Lake Superior

A superior mirage is the reverse of this; what we perceive to be higher in the sky is actually lower to the ground. Light is bent downwards when it hits a layer of cold air, making it appear as if what is below our sight line is actually straight ahead or above us because we are seeing the inverted image of what is on the horizon projected above it. This can be further complicated when there are multiple layers of hot and cold air, creating a highly distorted image as the light refracts through them.

Superior mirages occur wherever the surface temperature is colder than the air above it, usually over bodies of water and areas with ice or snow on the ground. The term fata morgana is most often used to describe superior mirages occurring over water. In these instances, objects on the horizon, such as ships, islands, cliffs, or icebergs, appear taller than they are because their inverted image is reflected above or superimposed on them. This elongation of objects on the horizon may make it appear as if there are turrets or towers rising up from the water, leading to the description of fata morganas as castles in the air.

As this effect can occur with ships, making them look higher above the horizon than they are, some have speculated that this is the origin of the Flying Dutchman legend, in which a ghostly ship is doomed to sail the seas for eternity.

There are many other types of superior mirages; one of them, the fata bromosa, or “fairy fog,” is created under the same conditions as the fata morgana, but has a different appearance. It appears as a bank of fog, with varying degrees of brightness, but without the fine detail of the fata morgana.

Fata Complete

Since its introduction into regular usage, the term fata morgana has come to mean more than just an optical phenomenon; although it has kept its original meaning of referring to something that is illusory, its use has been expanded throughout popular culture. It provided the title for a Henry Wadsworth Longfellow poem, an 1868 polka by Johann Strauss, and an Agatha Christie crime novel. It’s the name of a French publishing house, a character in Sergei Prokofiev’s opera, The Love for Three Oranges, and a film by Werner Herzog composed solely of desert landscape images.

The enduring popularity of the term shows how compelling it is as an idea—that there are mysterious phenomena, benign or malevolent, that are beyond our understanding. Or it may be that we continue to be enamored of our castles in the air, despite the knowledge of their illusory nature, as the last stanzas of Longfellow’s poem conclude:

So I wander and wander along,
And forever before me gleams

The shining city of song,
In the beautiful land of dreams.

But when I would enter the gate
Of that golden atmosphere,
It is gone, and I wonder and wait
For the vision to reappear.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on July 24, 2006.

This week, another book just for Mac users! As the amount of data we store continues to grow, figuring out where to put it and how to access it becomes more complicated. Every Mac includes internal storage in the form of a hard drive, SSD, or Fusion drive. But you may also have one or more external devices (such as hard drives, flash drives, SD cards, or RAID devices), not to mention network-attached storage (NAS) devices or cloud storage (like Dropbox or iCloud Drive). Making sense of all your options, managing your stored data, choosing new devices or services when you’re running out of space, or even just figuring out what’s where can drive anyone to distraction.

Jeff Carlson covers all this and much, much more in his book Take Control of Your Digital Storage. For example, the book helps you choose a new (internal or external) hard drive, SSD, or hybrid drive; determine how much storage space you need; understand APFS, Apple’s new filesystem; format, partition, and repair disks using Disk Utility; choose and use a NAS, RAID, flash drive, or SD card for use with your Mac; work with disk images; and decide among local, network, and cloud storage for various types of files.

This book, like all Take Control titles, comes as an ebook, and you can download any combination of formats—PDF, EPUB, and/or Kindle’s Mobipocket format—so you can read it on pretty much any computer, smartphone, tablet, or ebook reader. The cover price is $14.99, but as an Interesting Thing of the Day reader, you can buy it this week for 30% off, or just $10.49.