The Leviathan of Parsonstown

William Parsons, third earl of Rosse, had the idea to build at Birr a telescope far surpassing any of its predecessors – an instrument with a 6ft-diameter speculum that would acquire notoriety as the Leviathan of Parstonstown.

Construction of the Leviathan, which took almost two years and entailed many setbacks, was an astonishing feat. Casting the speculum required a huge foundry – created in the dry moat surrounding the castle – in which peat-fired furnaces were used to heat three iron crucibles, each 24in in diameter and containing 1.5 tons of a tin and copper alloy. The molten metal was transferred by means of cranes into an enormous mould, where it was carefully cooled over a period of 16 weeks.

The next stage was the laborious and time-consuming process of grinding and polishing. ‘The speculum was successfully cast, but the surface was covered with minute fissures, about the breadth of a horse-hair,’ explained William Rosse later. ‘These we resolved to grind out.’ The grinding continued for nearly two months, the machinery working for part of the time at night. The speculum was then polished, and its performance equalled expectations. Since the process of polishing was slow and the metal tarnished quickly, two mirrors were built – making it possible for one to be re-polished while the other was being used for observation.

The telescope’s 56ft-long wooden tube and hoist were fixed between two castellated brick walls 50ft high and 70ft long. Movement of the tube was controlled by chains, pulleys and counterweights. A platform for observing objects at low altitude was built at the southern end of the walls. For high altitudes, a long gallery mounted on the west wall moved across the central space to follow the tube’s lateral motion. At least three assistants were needed to help the observer by moving the winch and shifting the galley.

It had long been recognized by astronomers that the larger a telescope’s mirror the more light could be ‘grasped’, or collected, allowing fainter and more distant objects to be studied – and what made the Leviathan so powerful was its tremendous light-grasp. At the time of its creation in the 1840s, it was by far the biggest telescope ever built, and it would remain the largest telescope in the world for almost three-quarters of century, until it was overtaken in 1917 by the 100-inch Hooker telescope on Mount Wilson in California.

One issue that obsessed early 19th-century astronomers was the true nature of nebulae – objects resembling luminous clouds of gas among the stars, many of which are now known to be galaxies outside the Milky Way. William Rosse used his Leviathan to determine that some of these nebulae had a spiral structure, and he and his assistants recorded their observations in meticulous notes and drawings – at a time when photography was not a practical option. News of Rosse’s discoveries spread like wildfire, and visitors flocked to Birr, eager to see and use the wondrous instrument for themselves, while guidebooks and newspaper articles helped to establish the earl’s astronomical and technological reputation. Some reports distinguished between the world BR and AR – Before Rosse and After Rosse.