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Originally appearing in Volume V13, Page 712 of the 1911 Encyclopedia Britannica.
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JEREMIAH HORROCKS (1619-1641), English astronomer, was born in 1619 at Toxteth Park, near Liverpool His family was poor, and the register of Emmanuel College, Cambridge, testifies to his entry as sizar on the 18th of May 1632. Isolated in his scientific tastes, and painfully straitened in means, he those which have a schistose character. The andalusite may be pink and is then often pleochroic in thin sections, or it may be white with the cross-shaped dark enclosures of the matrix which are characteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded in quartz. In the rocks of this group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms which are divided up into six sectors when seen in polarized light. In biotite hornfelses a faint striping may indicate the original bedding of the unaltered rock and corresponds to small changes in the nature of the sediment deposited. More commonly there is a distinct spotting, visible on the surfaces of the hand specimens.. The spots are round or elliptical, and may be paler or darker t han the rest of the rock. In some cases they are rich in graphite or carbonaceous matters; in others they are full of brown mica; some spots consist of rather coarser grains of quartz than occur in the matrix. The frequency with which this feature reappears in the less altered slates and hornfelses is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. " Tourmaline hornfelses " are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl which under the microscope are dark brown and richly pleochroic. As the tourmaline contains boron there must have been some permeation of vapours from the granite into the''sediments. Rocks of this group are often seen in the Cornish tin-mining districts, especially near the lodes. A second great group of hornfelses are the talc-silicate-hornirises which arise from the thermal alteration of impure lime-stones. The purer beds recrystallize as marbles, but where there has been originally an admixture of sand or clay lime-bearing silicates are formed, such as diopside, epidote, garnet, sphene, vesuvianite, scapolite; with these phlogopite, various felspars, pyrites, quartz and actinolite often occur. These rocks are fine-grained, and though often banded are tough and much harder than the original limestones. They are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites. When perfused with boric and fluoric vapours from the granite they may contain much axinite, fluorite and datolite, but the aluminous silicates (andalusite, &c.) are absent from these rocks. From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of felspar with hornblende (generally of brown colour) and pale pyroxene. Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure. Where the original mass was decomposed and contained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities, there are usually rounded a reas or irregular streaks containing a suite of new minerals, which may resemble those of the talc silicate hornfelses above described. The original porphyritic, fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the less advanced stages of hornfelsing, but become less, evident as the alteration progresses. In some districts hornfelsed rocks occur which have acquired a schistose structure through shearing, and these form transitions to schists and gneisses which contain the same minerals as the hornfelses, but have a schistose instead of a hornfels structure. Among these may be mentioned cordierite and sillimanite gneisses, andalusite and kyanite mica schists, and those schistose talc silicate rocks which are known as cipolins. That these are sediments which have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed is not always clear. The essential features of hornfelsing are ascribed to the action of heat, pressure and permeating vapours, regenerating a rock mass without the production of fusion (at least on a large scale). It has been argued, however, that often there is extensive chemical change owing to the introduction of matter from the granite into the rocks surrounding it. The formation of new felspar in the hornfelses is pointed out as pursued amid innumerable difficulties his purpose of self-education. His university career lasted three years, and on its termination he became a tutor at Toxteth, devoting to astronomical observations his brief intervals of leisure. In 1636 he met with a congenial spirit in William Crabtree, a draper of Broughton, near Manchester; and encouraged by his advice he exchanged the guidance of Philipp von Lansberg, a pretentious but in-accurate Belgian astronomer, for that of Kepler. He now set himself to the revision of the Rudolphine Tables (published by Kepler in 1627), and in the progress of his task became convinced that a transit of Venus overlooked by Kepler would nevertheless occur on the 24th of November (O.S.) 1639. He was at this time curate of Hoole, near Preston, having recently taken orders in the Church of England, although, according to the received accounts, he had not attained the canonical age. The 24th of November falling on a Sunday, his clerical duties threatened fatally to clash with his astronomical observations; he was, however, released just in time to witness the punctual verification of his forecast, and carefully noted the progress of the phenomenon during half an hour before sunset (3.15 to 3'45). This transit of Venus is remarkable as the first ever observed, that of 1631 predicted by Kepler having been invisible in western Europe. Notwithstanding the rude character of the apparatus at his disposal, Horrocks was enabled by his observation of it to introduce some important corrections into the elements of the planet's orbit, and to reduce to its exact value the received estimate of its apparent diameter. After a year spent at Hoole, he returned to Toxteth, and there, on the eve of a long-promised visit to his friend Crabtree, he died, on the 3rd of January 1641, when only in his twenty-second year. To the inventive activity of the discoverer he had already united the patient skill of the observer and the practical sagacity of the experimentalist. Before he was twenty he had afforded a specimen of his powers by an important contribution to the lunar theory. He first brought the revolutions of our satellite within the domain of Kepler's laws, pointing out that her apparent irregularities could be completely accounted for by supposing her to move in an ellipse with a variable eccentricity and directly rotatory major axis, of which the earth occupied one focus. These precise conditions were afterwards demonstrated by Newton to follow necessarily from the law of gravitation. In his speculations as to the physical cause of the celestial motions, his mind, though not wholly emancipated from the tyranny of gratuitous assumptions, was working steadily towards the light. He clearly perceived the significant analogy between terrestrial gravity and the force exerted in the solar system, and by the ingenious device of a circular pendulum illustrated the composite character of the planetary movements. He also reduced the solar parallax to 14" (less than a quarter of Kepler's estimate), corrected the sun's semi-diameter to 15' 45", recommended decimal notation, and was the first to make tidal observations. Only a remnant of the papers left by Horrocks was preserved by the care of William Crabtree. After his death (which occurred soon after that of his friend) these were purchased by Dr Worthington, of Cambridge; and from his hands the treatise Venus in sole visa passed into those of Hevelius, and was published by him in 1662 with his own observations on a transit of Mercury. The remaining fragments were, under the directions of the Royal Society, reduced by Dr Wallis to a compact form, with the heading Astronomia Kepleriana defensa et promota, and published with numerous extracts from the letters of Horrocks to Crabtree, and a sketch of the author's life, in a volume entitled Jeremiae Horroccii opera posthuma (London, 1672). A memoir of his life by the Rev. Arundel] Blount Whatton, prefixed to a translation of the Venus in sole visa, appeared at London in 1859. For additional particulars, see J. E. Bailey's Palatine Note-Book, ii. 253. iii. 17; Bailey's " Writings of Horrocks and Crabtree " (from Notes and Queries, Dec. 2, 1882); Notes and Queries, 3rd series, vol. v., 5th series, vols. ii., iv.; Martin's Biographia philosophica, p. 271 (1764) ; R. Brickel, Transits of Venus, 1639–1874 (Preston, 1874); Astronomical Register, xii. 293; Hevelii, Mercurius in sole visus, pp. 116-140; S. Rigaud's Correspondence of Scientific Men; Th. Birch, History of the Royal Society, i. 386, 395, 470; Sir E. Sherburne's Sphere of M. Manilius, p. 92 (1675); Sir J. A. Picton's Memorials of Liverpool, ii. 561; M. Gregson's Fragments relative to the Duchy of Lancaster, p. 166 (1817); Liverpool Repository, i. 570 (1826); Phil. Trans. Abridged, ii. 12 (1809); C. Hutton's Phil. and Math. Dictionary (1815); Penny Cyclopaedia (De Morgan); Nature, viii. 117, 137; J. B. J. Delambre, Hist. de l'astronomie moderne, ii. 495; Hist. de l'astronomie au X VIII siecle, pp. 28, 61, 74; W. Whewell, Hist. of the Inductive Sciences, i. 331; R. Grant, Hist. of Physical Astronomy, pp. 420, 545; J. Madler, Geschichte der Himmelskunde, i. 275; M. Marie, Hist. des Sciences, iv. 168, vi. 9o; J. C. Houzeau, Bibl. Astr. ii. 167. (A. M. C.)
End of Article: JEREMIAH HORROCKS (1619-1641)
JOHN HORROCKS (1768–1804)

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