BIRR CASTLE DEMESNE VISIT
9 JULY 2011

A note by Eamon Henry.
Date 12 July 2011

A visit to Birr Castle Demesne in County Offaly was made on Saturday 9 July 2011 by eleven members of the Active IT Society (AITS), organised and led by Mrs Angela Hickey. A two-hour journey by car got us there about noon, and after a light snack in the coffee-shop we started our tour, for a group charge of €6.50 per person. The day was dry and cloudy.
Our first venue, beside the coffee-shop, was the Historic Science Centre, which gives much attention to the famous telescope to be described below, but also treats the pioneer photography of Mary Countess of Rosse in the 1840s, and the Steam Turbine invented by her son Charles and used later in ships. We next walked through the extensive grounds, trying to absorb some of the rich variety of very old trees and exotic plants, labelled to give description, and including “The Carroll Oak” which is at least 500 years old. The rivers Camcor and Little Brosna meet within the demesne and provide a wildlife sanctuary, before flowing west into the Shannon.

The Castle itself is still private, as the family home of the Parsons, living there since 1620 in the castle they built on the site of a Norman castle dating from 1170. The Parsons family formal title is “Earl and Countess of Rosse”.

The inventor of the great telescope was the Third Earl of Rosse, William Parsons, born in Yorkshire on 17th June 1800 and departed this life on 31st October 1867. He was educated at Trinity College Dublin and at Magdalen College Oxford, where he graduated in Mathematics with First Class Honours in 1822. He married an English woman, Mary Field, in 1836 and they had four sons. He served as MP for Kings County (now named County Offaly) in the British House of Commons during 1821-1834 (starting while still at Oxford, and became an Irish Peer in the House of Lords from 1845 onwards. Recognition of his research work won him many honours, including the Order of St. Patrick. Unlike many other scientists of his time, he published full technical details of his telescopes and stellar discoveries in the best scientific journals available.
William the Earl in fact designed and built two telescopes of the “Reflecting” kind, one having a mirror of 36 inches diameter, and the second, named “Leviathan”, having a mirror of 72-inch diameter, and being the world’s largest from its first use in 1847 up to roughly 1917, a span of some seventy years. A “reflecting” telescope lets the light from a distant object come down inside an empty cylindrical tube to reach a curved (parabolic) mirror at the bottom, which reflects and focuses it to form an image back up inside the tube. This image is caught precisely by a small slanting mirror which reflects it out through a hole in the side of the tube, so as to reach the “Eyepiece” lens which magnifies the image as required. Sir Isaac Newton (1642-1727) also designed and used a telescope of this kind.

A restored version of Leviathan stands where it was, in the grounds. It comprised a hollow iron tube (of wood & metal)some 54 feet long weighing 16 tons and having at its lower end a parabolic metal mirror of diameter 72 inches. The central viewing direction was due south. It could be raised from horizontal to vertical, but with only some 10 degrees rotation to left or right possible, because of the two big sidewalls of cut stone which carried the heavy tube in a steel frame attached to the walls by ropes and geared pulley-wheels with attached counter-weights. The mirror was made of “Speculum” metal, a mix of two-thirds copper and one-third tin, giving a highly polished surface, but reflecting only two-thirds of the light received, by comparison with more modern mirrors of glass and mercury which reflect some 90 percent of light received. With the help of only two men, the Earl was able to manipulate the 16-ton telescope so as to counteract (to some extent) the effect of Earth’s rotation, which in a few seconds can move the point of interest out of the telescope’s field of view if no continuous tracking adjustment is made. Nowadays an electric motor attachment can keep track of the point being viewed.

No photography was available at that time to make pictures of images in telescopes, so the Earl had to make sketches by hand of what he saw. Among many, the Owl Nebula and the Whirlpool Galaxy are notable. He discovered 226 new items as now listed in the “New General Catalogue” (NGC) series, and gave much improved views of some 70 objects already known. The power of Leviathan is illustrated by a remark of an Irish MP colleague, Thomas Langlois Lefroy: “Planet Jupiter is seen twice as large as the Moon appears to the naked eye”.

Only some of the data given above emerged from looking at the structure in the castle grounds. However, this writer hopes to be excused for concentrating on a world-famous telescope of its day, designed, built and used successfully by the brilliant Third Earl of Rosse, and published in detail so that others could know and use his discoveries.