Between June 1843 and the following spring Penrose visited the chief cities of Italy, and after a brief return to England started somewhat reluctantly for Greece. He describes Athens as 'by far the most
miserable town of its size I have ever seen' (9 Jan. 1845). But he soon fell under the spell of the 'Pericleian Monuments,' to which his first enthusiasm for Gothic architecture quickly gave way. In August
he made his way home through Switzerland, Augsburg, Munich, and Cologne.
Already Penrose realised the importance of exact mensuration to a critical study of Greek architecture. The pamphlet on the subject by John Pennethome [q. v.] attracted his attention on its publication in 1844. On his arrival in England the Society of Dilettanti had determined to test thoroughly Pennethome' s theories as to the measurements of Greek classical bxiildings, and they commissioned Penrose to undertake the task in their behalf. In 1846 Penrose was again at Athens. His principal collaborator in the work of measurement there was Thomas Willson of Lincoln. They completed their labours in May 1847. Despite corrections in detail Penrose confirmed in essentials Pennethome's theories. When in 1878 Pennethome brought out his 'Geometry and Optics of Ancient Architecture' he adopted with due acknowledgment Penrose's mass of indisputable material.
'Anomalies in the Construction of the Parthenon,' which the Society of Dilettanti published in 1847, was the first result of Penrose's labours, but it was in 1851 that there appeared his monumental work, 'Principles of Athenian Architecture,' of which a more complete edition was issued in 1888. Penrose's exhaustive and minutely accurate measurements finally established that what is apparently parallel or straight in Greek architecture of the best period is generally neither straight nor parallel but curved or inclined. He solved the puzzle which all Vitruvius's commentators had found insoluble by identifying the 'scamilli impares' with those top and bottom blocks of the columns which, by virtue of the inclination of the column or the curvature of stylobate and architrave, are 'unequal' (i.e. they have their upper and lower faces out of parallel). Some important conclusions relating to the Roman temple of Jupiter Olympius at Athens Penrose laid before the Institute of British Architects in 1888.
In 1852 Dean Milman and the chapter appointed Penrose surveyor of St. Paul's Cathedral. The appointment was made with a view to the completion of the interior decoration in accordance with the intentions of Wren. Penrose deemed it necessary to allot, apart from the decorative scheme, 2000l. per annum to the maintenance of the fabric, and a public appeal in 1870 provided substantial financial support. Penrose took up the decorative scheme with enthusiasm, and he insisted on respecting his conception of Wren's generous intentions. In the result he soon found himself at variance with the chapter, who favoured a more restricted plan. Nor was he at one with them on the methods of completing the Wellington monument (see Stevens, Alfred). Counsels prevailed in which the surveyor was neither consulted nor concerned.
Like Wren himself Penrose found relief from the disappointment in astronomical study, which had already attracted him at Cambridge' and in Paris. He was an adept at mechanical inventions, and an instrument for drawing spirals won him a prize at the Great Exhibition of 1851. A theodolite which he had bought in 1852 primarily for use in measurement of buildings, he applied at the suggestion of Dr. G. Boole to such astronomical purposes as accurate determination of orientation and time in connection, for example, with the fixing of sundials. In 1862 came the purchase of a small astronomical telescope which was soon superseded by a larger one with a 5½-inch object-glass (Steinheil), equatorially mounted by Troughton & Simms. In 1866 Penrose, finding the prediction of the time of an occultation of Saturn in the 'Nautical Almanac' inadequate for his purpose, endeavoured with success 'to obtain by graphical construction a more exact correspondence suited to the site of the observer. He published his results in 1869 in 'The Prediction and Reduction of Occupations and Eclipses' (4to), and the work reached a second edition in 1902.
In 1870 he visited Jerez in the south of Spain to view the total eclipse of the sun with his smaller (2¼-inch) instrument. The observation was spoilt by a cloud, but Penrose made the acquaintance of Professor Charles A. Young of America, whom he met again at Denver in 1878. Penrose's observations on the eclipse of 29 July 1878 were published in the Washington observations (Appendix III). He afterwards extended to comets the graphical method of prediction which he had applied to the moon (cf. his paper before the Royal
