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(27.) Huygens ami Fontana, before (he middle of the 17(1)Century, greatly improved the construction of telescopes. Cas-sini with telescopes ot 200 and 30!) feet focus, saw five satellites ofSaturn, with his belts, and the shadows of Jupiter ’s satellites passingover his body. In 1666, Azont applied a micrometer to teles-copes, to measure the diameters of the planets, and oilier smalldistances in the heavens: but an instrument of this kind had been■ invented before, by Gascoigne. Marseiunis first started the ideaof making telescopes of reflectors, instead of lenses, in a letter toDescartes ; and in 1663, James Gregory , of Aberdeen, shewed how-such a tele.'Cope might be constructed.
(28.) Sir Isaac Newton , after spending some time on the con-struction of ,bpth sorts of telescopes, found out the great inconve-nience ihich arises to refractors from the dilferent reirangibilily ofthe rays of light; which inconvenience Dollond has since fullyobviated. In 1672, Newton presented to the Royal Society , tworeflectors, constructed with spherical speculunis. Towards theend of the 17th, and beginning of the 18th century, practical astro-nomy rather languished ; but the speculative part was carried to thehighest perfection by Newton, in his Principia , by David Gregor)-,Keil, and others. Soon after this, great improvements of astrono-mical instruments began to take place, particular:)’ in Britain .Graham greatly improved astronomical instruments and time-pieces,lie constructed the old eight-feet mural arch at the Itoyal Obser-vatory, Greenwich : but lie is chiefly remarkable for contrivingthe zenith-sector of 24 feet radius, and afterwards one of 12f feet,with which Bradley discovered the aberration of the fixed stars.The reflecting telescope of Gregory and Newton was greatly im-proved by Iladlev, who presented a very powerful instrument ofthat kind'to the Itoyal Society, in 1719. He invented also thereflecting quadrant ’or sector, now called by his name, presentedto the society in 1731, and now so universally useful at sea. Itappears, however, that an instrument similar to this, in its prin-ciples, had been invented by Newton ; and that Godfrey, of Phi-ladelphia, in America , made the same discovery, and the first in-strument of this kind.
(29.) About the middle of this century, the constructing anddividing of large astronomical instruments were carried to greatperfection by Bird ; and reflecting telescopes were not less im-proved by Short, who first executed the divided object - glassmicrometer, which had been proposed and described by Louvilleand others. Dollond also improved refracting telescopes, bymeans of his acromatic glasses; and the discoveries of Herschylare owing to the amazing powers of reflectors of bis own construc-tion. Roemer, a celebrated Danish astronomer, first made useof a meridional telescope; and, by observing the eclipses ofJupiter ’s satellites, first discovered the progressive motionof light. Flamsteed, appointed the first Astronomer Royal, atGreenwich, in 1675, observed for 44 years, and gave a catalogueof 3000 stars with their places, to the year 1689. Cassini, the firstFrench Astronomer Royal, made many observations on the sun,moon, planets, and comets, from which he greatly impiovcd theelements of their motions.
(30.) Flamsteed was succeeded, in 1719, by Dr. Halley, asAstronomer Royal, at Greenwich. In 1705, he published his“ Synopsis Astronomi® Cometica?,” in which he ventured to predictthe return of a comet, in 1758, or 1759. He first discovered theacceleration of the moon, and gave a very ingenious method forfinding her parallax, by three observed phases of a solar eclipse ;published in the Philosophical Transactions many learned papers,and among them, some concerning the use that might be made ofthe next transit of Venus, in determining the distance of the sunfrom the earth ; composed tables of the sun, moon, and all theplanets, which are still in great repute; and recommended the me-thod of determining the longitude, by the moon’s distances fromthe sun, and certain fixed stars. A dispute concerning the figureof the earth, took place about this time. New ton had determined,from a consideration of the laws of gravity, and the diurnal mo-tion of the earth, that the figure of it was an oblate spheroid, andflattened at the poles ; but Cassini from the measure's of Picart; sup-posed it to be an oblong spheroid, or lengthened at the poles. Tosettle this dispute, it was resolved, under Lewis XV’. to measuretwo degrees of the meridian ; one near the equator, and the otheras near the pole as possible. For this purpose, the Royal Aca-demy of Sciences sent to Lapland, Maupertuis , Clairault, Camus,and Le Monicr; who were accompanied by Outliier, and Celsus, ,von. i.— no. 17.
professor of anatomy at Upsal. On the southern expedition weresent Godin, Condamine, and Bouguer, to w hom the king of Spain joined George Juan, and Antonio de Uiloa. These set out in 1735,and returned at different times, in 1744, 1745, and 1746 ; but theformer party who set out only in 1736, returned the year follow-ing; having both fulfilled their commissions. Picart’s measure wasrevised by Cassini and de la Caillc, w hich, after his errors were cor-rected, was found to agree very well with the other two ; and theresult of the whole served to confirm the determination of the figurebefore laid down by Newton . On lm southern expedition, theattraction of the great mountains of Peru caused the plumb-line ofone of their largest instruments, to dellect seven or eight secondsfrom the true perpendicular.
(31.) In 1742, Dr. Bradley succeeded Dr. Halley, as Astrono-mer Royal, at Greenwich. The accuracy of iiis observations-enabled him to detect the smaller inequalities in the motions of theplanets and fixed stars. The consequence of his accuracy was, thediscovery of the aberration of light, the nutation of the earth’s axis,and a much greater degree of perfection in the lunar tables. He ob-served the places, and computed the elements of the comets whichappeared in the years 1723, 1736, 1743, and 1757 ; made new and.accurate tables of die motions of Jupiter ’s satellites; and from amultitude of observations of the luminaries, constructed the mostaccurate table of refractions yet extant. He also, w ith a very largetransit instrument, and a new mural quadrant of eight feet radius,constructed by Bird in 1750, made an immense number of ob-servations for settling the places of all the stars in the British catalogue, together with near 1500 places of the moon, the greaterpart of which lie compared with Mayer’s tables. Bradley died in 1767.
(32.) Astronomers elsewhere were equally assiduous. Thetheory of the moon was particularly considered by Clairault,D’Alembert, Euler , Mayer, Simpson, and Wahnsly; Mayer’slunar tables for their superior accuracy, were rewarded with a pre-mium ot 3000/. and used in the computation of the NauticalAlmanac, published by the Board of Longitude. Among themany French astronomers who contributed to the advancement ofthe science, it was particularly indebted to De la Caille for an ex-cellent set of solar tables; who, in 1750, went to the Cape of GoodHope to make observations in concert with the most celebratedastronomers in Europe, for determining the parallax of Mars andthe moon, and thence, that of the sun, which it was concludeddid not much exceed ten seconds. Here he re-examined and ad-justed, with great accuracy, the stars about the southern pole ; andalso measured a degree of the meridian. In Italy , the science wasassiduously cultivated by Bianchini, Boscovich , Frisi,,Manfredi,.Zanotti, and many others ; in Sweden , by Wargentin, Blingen-stern, Mallet, and Planman ; and in Germany, by. Euler , Mayer,.Lambert, Grischow, and others.
f33.) In 1760, all the learned Societies in Europe made prepa-rations for observing the transit of Venus over the sun, which-hadbeen predicted by Kepler, 134.years before. The same exertionswere repeated, to observe the transit in 1769, by sending observersto different parts ot'lhe world ; and from the whole, Short.computedthat the sun’s parallax was about 84 seconds, and consequently thedistance of the sun from the earth about 2.4114.of the earth’s semi-diameters, or 96 millions- of miles. Bradley was succeeded in1762, in his office of Astronomer Royal, by Bliss, Savilian pro-fessor of astronomy ; who being in a declining state of health, did:not long enjoy it..
(34.) In 1765, Bliss was succeeded by the learned Nevil Mas-kelyne, the present Astronomer Royal, whose assiduity to pro-mote the interests of astronomy and navigation is perhaps une-qualled ; he, among many other improvements, brought into ge-neral practice the Umar method of determining, the longitude,,proposed the project and superintended the experiment for accu-rately measuring the etfect of a mountain, on the plumb-line : it was .found that the sum of its deflections, from. the perpendicular on .the opposite sides of Schehallien, a mountain in Scotland , wasIlf", and proved, to the satisfaction of the. whole world, the uni-versal attraction of matter. From this Dr. Hutton has computedthe mean density of the whole earth to. be about 4j times that ofeommon.wateri
(35.) The discoveries of Dr. Ilerschel form a new a:ra in astro-nomy. In 1781, he began with observations on the 0 , a periodicalstar in Collo Ceti, and a new method of measuring the Ulnarmountains, none of which lie made more.than half a mile in,3 Z. height j.