The Leviathan of Parsonstown

Published in 18th–19th - Century History, Features, Issue 4 (Winter 2001), Volume 9

During the middle of the nineteenth century an eccentric and wealthy young man who happened to like machines and astronomy embellished Ireland’s place in scientific history by building one of the largest and most dangerous telescopes in the world; his name was William Parsons, third Earl of Rosse to Birr Castle, County Offaly. Born on 17 June 1800, William Parsons (or Lord Oxmantown) was the eldest of three sons. His education began at home, where private tutoring (the norm for a boy of his background), together with his own instinctive curiosity, instilled in him a practical and engineering interest in all aspects of the castle demesne that surrounded him daily. This dual education served as an excellent foundation for his future, and at the age of twenty-two, he graduated with a first class honours in mathematics from Magdelen College, Oxford.
Naturally enough, politics had its calling, and for the next eleven years (1823-1834) William Parsons served as a MP for King’s County, where his honesty and perseverance became renowned—most particularly in his support for Catholic Emancipation (1829) and the Reform Bill (1832). Much as he enjoyed his political career, William had another calling. The scientific and engineering interests in which he dabbled increasingly occupied his mind. Eventually, it beckoned him so much that he retired from politics to pursue his astronomical ambition to build a large telescope.

Mirror making

To make a telescope mirror even today is a very complex procedure. One has to have a wealth of practical experience in casting and polishing techniques, detailed knowledge of optics, and a lot of patience when it comes to the final finishing. For William, the procedure and requirements weren’t any different. He had had previous experience of making small telescope mirrors and had in fact written and worked extensively on the subject, most notably on the finer aspects of grinding and polishing. His ambition to build a larger mirror, however, met many obstacles along the way.
Mirrors in those days were not like the low thermal expansion, glass-ceramic ones we know of today—far from it. Instead, many telescope makers used a type of highly reflective metal alloy of copper and tin called speculum (originally devised by Isaac Newton). It was relatively easy to work with, but it tarnished rather easily and was very brittle, making it impossible to cast large mirrors.
Previously, William Herschel, the Hanoverian telescope-maker and astronomer (discoverer of the planet Uranus), had tried to add more copper in the speculum mix so as to make it less likely to shatter. While a larger mirror could be cast, the mixture lowered the reflectance characteristic and thus made imaging impossible. Likewise, the opposite happened when he reversed the mixture—an increase in the reflectance, but a very breakable mirror. William knew all about this, and more, carrying out several experiments of his own.
His main investigations centred on the type of moulding into which the metal would be poured. He tried various mould configurations but each caused problems: some mirrors would fracture at the last moment after pouring; others trapped air bubbles. After exhaustive changes in mould shapes, he persisted until finally he had an inspiration. By constructing the base of the mould with strips of wound wrought iron, set on edge and packed tightly together, it allowed the gases to escape more easily, while at the same time heat loss was controlled evenly over a prolonged period of time. Using this new technique, he succeeded, in 1839, in casting a perfect thirty-six-inch diameter mirror (three and three-quarters of an inch thick), weighing one and a quarter tons, and of excellent quality.
With a tube length of twenty-six feet attached to the mirror, William supported the telescope using a Ramage styled altazimuthal mount, which allowed him to view any part of the sky. Under a magnification of 100 to 600 he could see much new detail in star clusters and nebulae never before seen. In a famous planetary nebula called the ‘Ring Nebula’, he could see a very faint blue star, and for once the Moon, though relatively a well observed object at the time, showed greater detail never before seen in any telescope. Many more new discoveries followed, and word got around of his extraordinary achievement. His skill as a mirror maker was immediately recognised, leading to publication of several papers on his remarkable and unique technique.
Though the images were wonderful, William still wasn’t satisfied with the resolving power he knew could be achieved with a much larger and better telescope. By the middle of the 1840s, and after extensive planning, he decided it was time to build his next major telescope—the ‘Leviathan of Parsonstown’—a whopping-sized mirror of seventy-two inches in diameter (twice the size of his previous mirror). Little did he realise the enormous task and trouble he was about to take on.

Monster for the heavens

In order that a single casting of the speculum could be made, he had to set up three cast iron crucibles within the castle grounds. Each weighed half a ton and measured two-and-half feet deep by two feet in diameter internally—each seated in its own furnace. These were preheated for ten hours or so and it was estimated that it would take approximately another sixteen hours of turf-burning (some 2,000 cubic feet in all taken from the local bog) to melt the metal ingots.
With everything in place, the pouring took only seconds. Three iron baskets, which held the crucibles of heated metal, surrounded the mould, and were tilted slowly in synchronisation. Within the space of twenty minutes the whole lot had solidified. The casting was then placed into an annealing oven for six weeks, and by the time the essential secondary heating had cooled down the metal satisfactorily, a good seventy-two-inch mirror blank was created.

Fifth time lucky

A steam driven grinding machine was then used for figuring the mirror into the required parabola shape, taking up to two months to complete the task. It was important to get the shape right to ensure that all light falling on the mirror was focused at a single point—an essential requirement for any telescope mirror. Next was the polishing procedure. Unfortunately William’s casting contained more tin than he intended: the weight of the polishing apparatus broke the mirror into bits. No doubt devastated and depressed, nevertheless William’s determination and enthusiasm soon had his workers casting another mirror. This too failed due to fissures appearing in the metal and it wasn’t until the fifth attempt that William finally had two usable specula on his hands (the second for replacement of the first when it needed repolishing).
While all this was going on, work had already started on the design of a proper mounting and support structure for the bulky telescope. He thought of using the same rotational track that he had used for the thirty-two-inch telescope, but previous experience with winds (the thirty-two-inch had blown over in 1839) had taught him a hard lesson. In the end, he decided to suspend the new telescope between two fifteen-metre high walls (twenty-three metres long) aligned in a north-south direction. A metal structure made up of 150 tons of iron castings, allowed the seventeen metres of wooden tubing (fixed and pivoted at one end) and its four-ton mirror to access the full range of altitudes, as it moved up or down vertically. Because of its design, however, the walls restricted azimuthal (left or right) movement.
As the Earth rotated, a celestial object would pass between the viewing gap of the walls, and by means of pulleys, cables and cranes attached to metal structures, the observer—standing precariously on an observing platform that moved at the top of the telescope—could then view the object for no more than thirty minutes at a time on either side of the meridian, as it passed between the intervening gap.

Mountain climbing skills recommended

For larger adjustments in pointing the telescope to different parts of the sky, instructions had to be shouted to helpers down below, who accordingly adjusted the pulleys to suit. It was said that anyone using the telescope, especially when it came to observing objects at high zenith positions, would have been better off with mountain climbing skills rather than astronomical! Under high winds the three moveable viewing galleries at the top sometimes rocked and swayed, and viewer could easily have been blown off. However, over the period of the telescope’s use, no serious accidents were reported except for the occasional fumbled eyepiece dropped on one of the helpers down below.
When it was time for replacing a tarnished mirror with the spare polished one (always a nervous and delicate procedure), a score of men slowly rolled the spare to the northern end of the telescope, which at this stage was hoisted vertically into an upright position, and the two were switched—the tarnished one been brought back for repolishing.
On 15 February 1845, Dr Romney Robinson, director of Armagh observatory, and Sir James South, the noted English astronomer, visited the telescope and conducted the first official viewing. They observed the double star Castor in the constellation of Gemini followed by several observations of nebulae, for example, M67 (M here stands for Messier from Charles Messier’s catalogue of brightest nebulae), which it was commented, displayed a mass of faint stars. Several hours later, and many observations taken, the night’s viewing was deemed a success and William’s seventy-two-inch reflecting telescope—the ‘Leviathan of Parsonstown’—was now, officially, the best and biggest telescope in the world.


Though it could be said that it was a clumsy telescope and that image quality suffered under higher magnification (at 6,500 times or greater), many astronomers who came from around the world to use it, passed favourable reviews and comments on its power and optical quality. Unlike other observers at the time, William and his associates always disclosed their observations and discoveries immediately to the public and professionals alike. The scientific community buzzed whenever a publication appeared with Rosse or Leviathan in its heading—knowing full well that another important discovery had been made.
Of note was the one he made within a month of its use, while observing a spiral structure in Nebula M51 in the constellation of Canes Venatici. This was the first galaxy (though he didn’t know it was a galaxy at the time) in which individual stars could be seen in the spiral structure. He was convinced that the dynamic nature of the nebula held important information, in that it suggested some form of motion (It was later dubbed the ‘whirlpool’ galaxy). Many thought his proposal ridiculous, blaming it on an instrumental error of his telescope, but his training as a mathematician, together with his instinct to recognise a problem, persuaded him that it was true.
Another puzzle that bothered astronomers at the time was whether these objects in fact were ‘island universes’—vast collections of stars far beyond the confines of the Milky Way. The excellent views that the Leviathan was providing queried this puzzle to such an extent, that the true nature of these fantastic new phenomena divided the scientific community. The answer would not be found until 1924 when Edwin Hubble, while studying a photographic plate of the so-called Andromeda nebula (one of the nebulae around which the controversy raged) discovered a Cepheid variable star. These stars are intrinsically bright and well known by astronomers for determining distances. By referring to other previous plates, Hubble was able to discover additional Cepheid and soon realised that for these luminous stars to appear dim, as they were on his photographs, they must be extremely far away. Using straightforward calculations he was able to put their distances far beyond the confines of our Milky Way, and the argument was settled; the universe was far larger and populated with far bigger objects than had previously been imagined.
Throughout the controversy, however, William continued with his programme of investigating further the shapes of nebulae of the northern hemisphere, drawing many of them onto sketchpads and notebooks. Some of these sketches that one can still view today, have to be admired for their accurate detail when compared to modern photographs, especially when one considers the conditions under which they were executed. Astronomical photography was in its infancy at the time and crude attempts at photographing astronomical objects onto silver nitrate plates didn’t quite work as well as expected on the Leviathan, as they required a lot of exposure time which the telescope couldn’t give.
Laurence, his son, the fourth earl, followed in his father’s footsteps and pursued further William’s astronomical research. He tried to upgrade the Leviathan by incorporating a clock-driven device with the intention of doing photographic and spectroscopic work, but in the end it didn’t work due to the force required to move it through an east-west orientation. In 1868, he carried out extensive measurements of the moon’s thermal radiation using a series of thermocouples set at the telescope’s focus; the result of which, after taking into account several other important factors, was a very close estimate of 197 degrees Fahrenheit (modern observations give it at around 158 degrees Fahrenheit).
One of the most notable Birr astronomers to use the Leviathan was the Danish-born Johann Dreyer who served as assistant to Laurence between 1874 and 1878. After resigning from Birr to take up a post in the National Observatory at Dunsink, he and many others had discovered so many nebulae and star clusters using the Leviathan that a new catalogue was needed. He was assigned the task by the Royal Astronomical Society, and in 1888, the New General Catalogue (NGC) was published with 7,840 objects. Later, two more Index Catalogues (IC) added 5,386 objects, a total of over 13,000, the core of today’s comprehensive databases of deep sky wonders.
Though William, it was said at the time, was ‘no Herschel’, he continued to make important contributions from time to time. Towards the end, however, his renewed political life and family demanded more time. After returning home with his family from a holiday in Germany and Switzerland, he died on the 31 October 1867.
The telescope finally saw its end in 1908, when the fourth earl died, and over the next few years it fell into disrepair. One reason why it fell into disuse was possibly due to advancements in astronomical photography. Smaller and better-mounted telescopes were able to take longer exposures and the images they were producing were much better than anything the Leviathan could produce. When the First World War broke out in 1914, nearly all of the iron castings and any other metal were melted down to contribute to the war effort. Anything that was left was either burned as firewood or converted for some other purpose. While the fate of one of the two original mirrors remains a mystery, the second is on display in London’s Science Museum today.


Attempts were made in 1968 to restore the telescope but they never got off the ground. It wasn’t until 1996 that an official restoration, initiated by the seventh Earl of Rosse with European and governmental aid, tackled the problem seriously. By referring to old documents, photographs, newspaper clippings and model displays, work began to rebuild parts that had long since rotted and crumbled. Years of vegetative growth were cleaned off the supporting walls, loose masonry and blocks were replaced, and wooden and metalwork remnants, which were either in a decrepit or dilapidated state, were completely renewed. Some original equipment was found on the site and installed too.
A new tube had to be munufactured and a French company was commissioned to cast a new seventy-two-inch aluminium mirror. It was then passed on to the Optical Science Laboratory, London, to have its surface nickel-plated for figuring and polishing. The new mirror weighs in at about a ton, is non-tarnishing, has a high temperature range and is ten times the strength of glass. The restored mounting and telescope look exactly like they did in the 1840s, but with the added advantage of a computerised motorised system that allows for daily demonstrations of how the telescope actually moved.
William’s giant remained the largest until 1917, when the 100-inch telescope on Mount Wilson, California, began observing. The new Leviathan stands proudly today. Its historical past bear’s testament to a man whose engineering and observational skill conceived a universe that only great insight and determination could bring. His recognition and contribution to astronomy is well and truly deserved.

John Moore is a freelance writer in science and astronomy.

Further reading:

P. Moore, The Astronomy of Birr Castle (1981).

A. Pannebrook, A History of Astronomy (1961).

W.G. Scaife, From Galaxies to Turbines (2000).

The Cambridge Illustrated History of Astronomy (Cambridge 1997).


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