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William Parsons (1800-1867)

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The Right Honourable William Parsons, 3rd Earl of Rosse, (1800-1867)


1868 Obituary [1]

THE RIGHT HONOURABLE WILLIAM PARSONS, third EARL OF ROSSE, was born at York, on the 17th of June, 1800.

The elementary part of his education was acquired under his father’s roof; and in the year 1818 he entered Trinity College, Dublin. After a successful course of study he, in the following year, migrated to Magdalen College, Oxford, where he took his degree of B.A. in 1822, obtaining a first class in mathematical honours. As Lord Oxmantown he represented King’s County in the House of Commons from 1821 till the end of the first reformed Parliament ; and although he seldom took part in the debates, he proved an excellent man of business on Committees. From this time he devoted himself more closely to philosophical pursuits.

Besides his acquirements as an astronomer and a mechanician, he was a political economist of a high order, and devoted much attention to the question of national education. He was a good chemist, and possessed a large amount of military, nautical, and engineering knowledge. As evidence of this it may be mentioned that he had formed the conception of armoured ships some years before the Crimean war, and had shown that vessels of no great tonnage could carry a sheathing of 4-inch plates, which would be proof against the 32-pounder, the normal gun of the time.

But it was as a constructor of reflecting telescope that Lord Rosse was most celebrated. To obtain speculum metal of the highest standard, he combined four equivalents of copper and one of tin. The resulting compound is very brilliant, and resists tarnish far better than if there be a slight excess of either ingredient; but it is nearly the most intractable of all materials harder than steel, more brittle than glass, and 0ying in pieces with any sudden change of temperature. In consequence, all large specula which had been previously made contained a larger proportion of copper than the above, to ensure the necessary toughness.

At first he combined a central disc with annular zones two or three inches broad, and ground and polished them spherical; and by a fit adjustment, each, beginning at the centre, was drawn back till their foci coincided. Yet the complexity of the arrangement, and its liability to change with temperature were weighty objections, and he was led to devise another plan. In polishing these rings he found the outer ones too large for hand-work, and made, in 1828, a machine which gave a rectilinear motion to the polisher while the speculum revolved. This was only expected to produce a spherical figure ; but it caused rings and other irregularities, which he saw would be corrected by adding a second eccentric, giving a lateral motion to the polisher. This changed its course into a curve which might be varied from a right line to an ellipse almost circular. This had the desired effect ; but on watching its action, he saw that it would give what was the great desideratum, a change of curvature from the centre to the circumference, which could be varied at pleasure, and therefore could be made to give a true aplanatic figure. The machine so altered was subsequently employed, with only one change of any importance, which was a means of setting the speculum vertical while on the polishing machine, and testing it by a terrestrial mark. This much facilitates the obtaining a perfect figure. He met the difficulty of obtaining a large speculum, by making a strong frame of a peculiar brass, which has the same expansion as fine speculum-metal, and soldering on its front thin plates of the latter, cast on as large a scale as possible, and closely fitting them on the edges. From what has been mid of the properties of this alloy, it may be judged how much mechanical skill the process required ; but he succeeded in making a 3-feet speculum. It was very light and strong, and defined perfectly, except that, with large stars, the diffraction at the joints produced four minute rays. For those below the fifth magnitude, and for fainter objects, this defect is insensible ; and Lord Rosse believed that in this way only would it be possible to attain the extreme limit of telescopic vision.

The construction is stronger than the solid speculum, with a third of its weight; and the cellular distribution of the greater part of the maw enables it to assume the temperature of the atmosphere far more rapidly. Meanwhile he was led to an important improvement in casting speculum metal. When the melted alloy is poured into a mould, the surfaces in contact with the mould harden, while the interior of the mass is still fluid ; this cools in its turn, and, in contracting, exerts a powerful drag on the outer crust, which gives may, and the whole is shivered. Besides, if the solidification is gradual, the mass assumes a crystalline character, which gives it, when polished, a mottled surface. These difficulties were met by forming the bottom of the mould of iron, the sides of sand, and by leaving the top open. The metal in contact with the iron congealed almost in an instant, the sides of the speculum more slowly, as the sand is a worse conductor than the iron, the upper part remaining fluid longest; so that the contraction occurred chiefly at the back\of the speculum, where it did no harm, while the front presented a layer of uniform and comparatively tough material ; but this was a little more liable to tarnish than that which was cooled slowly. He found, however, that speculum metal, like copper, absorbs in fusion a large quantity of oxygen, much of which it gives out on becoming solid. This always produces microscopic pores, and causes bubbles and cavities, which are found even when no air is entangled in the pouring. To give the gas a free escape, he made the bottom of the mould of hoop-iron, placed on edge and packed so closely that it retained the metal, but was previous to the gas ; and so successful was the experiment, that by the summer of the year 1839 he had succeeded in making a solid 3-feet speculum.

In April, 1842, he had made a perfect cast of a 6-feet speculum, which was accidentally broken after being partly ground ; and finally, in February, 1845, he completed an instrument 52 feet in length and 7 feet in diameter, having a 6-feet speculum. The tube is supported at its lower extremity by a massive universal joint. It is counterpoised by weights, which are constrained to move in a circular arc nearly coinciding with the curve of equilibrium; and a steady strain is kept upon the suspending- chain by means of three weights attached to levers, which successively come into play as the tube approaches the zenith and passes north beyond it. A chain connects the levers at the proper intervals and the tube with them ; and as the tube descends, each lever takes its place successively in a deep recess in the ground, the chain subsiding into a heap. The motion in right ascension is by a rack, the extremity of which bears by rollers on a circular arc of 40-feet radius. This rack is connected with the tube by a pinion, and the pinion is acted upon by an endless screw driven by a pulley, which pulley is driven by a band from a potter's wheel attached to the lower end of the tube. For polar distances there is a circle, 18-inches radius, at the lower end of the tube, furnished with a spirit-level; but for finding objects there is an index of 6-feet radius connected with the transverse axis of the universal joint. By means of three galleries provision is made for enabling the observer to reach the eye-piece in every position of the telescope. Every part of the structure is of great strength, and the telescope is perfectly steady even in a high wind.

By means of this magnificent instrument many of the nebulae, previously seen merely as luminous patches, were resolved into stars, and in others a spiral form and arrangement were detected. Its cost was about £30,000, and its completion waa a work of seventeen years. It deserves notice that all this massive work was executed in Lord Rosse’s laboratory at Birr Castle, and that he had so thoroughly considered the whole, that in no instance was it found necessary to deviate from the drawings which had been prepared.

1836 he married the eldest daughter and co-heir of the late Mr. John Warner Field, of Heaton Hall, Yorkshire, by whom he leaves three sons. He succeeded to the earldom in 1841, and four years later was elected a representative peer of Ireland in the House of Lords. He had been Lord-lieutenant of King’s County since 1831, and Colonel of the King’s County Militia from 1834.

Lord Rosse was a Fellow of the Royal Society, of which he was President from 1848 to 1854. In 1842 the University of Cambridge conferred on him the degree of LL.D., and in the following year he presided over the meeting of the British Association at Cork.

In 1853 he was elected one of the members of the Imperial Academy of Sciences at St. Petersburg ; and he was made a Knight of the Legion of Honour by the Emperor of the French in 1855. He was also a Knight of St. Patrick, and a member of many learned societies on the Continent.

Since 1862 he had filled the post of Chancellor of the University of Dublin. His connection with the Institution of Civil Engineers dated from the 5th of June, 1849, when he was elected an Honorary Member. He died on the 31st of October, 1867, sinking under the effect of t.he removal of a tumour caused by a sprain of the knee.


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