(153.) The Nau tical Almanac. of moun systematic reduction of all the lunar observations of Maskelyne and Pond, and a comparison with Damoiseau's tables, under the direction of the present Astronomer Royal, Mr Airy. But, for the application of the method of lunar distances to navigation, farther aid than the construction of good tables was required. This Maskelyne provided by obtaining the regular publication of the Nautical Almanac, superintended by himself, and containing the distances of the moon from the principal fixed stars at predetermined hours for the meridian of Greenwich, a comparison of which with the distance observed by means of the sextant in any part of the world enabled the seaman (after proper reductions) to infer the exact Greenwich time of the observation, and thence, by comparison with the local time obtained by the usual methods, to obtain his longitude. To this, long admitted to be the best practical solution of the celebrated problem of "the longitude at sea," Maskelyne contributed probably more than any other person. His "Lunar Distances" were reprinted in the French Almanac (Connaissance des Tems) for a considerable number of years. (154.) II. The determination of the Attraction of ScheAttraction hallien, and of the Earth's Density. The deviation tains. of the plumb-line from the vertical by the neighbourhood of a mountain had been pointed out by Newton1 as a direct consequence, and also as a test, of the principle that gravity resides in every part of the earth as well as in the earth as a whole. Bouguer had the merit of pointing out the form in which the experiment might be made, and of making the trial, though in a rude and insufficient manner, in the Peruvian Andes in 1738. He observed the effect of the mountain on the south side only, but at two stations unequally distant from its centre of attraction. The numerical result being (as the author himself admitted) without value, Maskelyne proposed to the Royal Society in 1772 to repeat the observation on some British mountain. A "Committee of Attraction" was named, which, besides Maskelyne, included Cavendish, Franklin, and Horsley. Cavendish, as might have been expected, took an earnest part in it. The search for a suitable hill was confided to Mr Charles Mason in 1773. Skiddaw and the Yorkshire Hills were first thought of, but finally Schehallien in Perthshire was preferred. Thither Maskelyne himself proceeded in 1774, with his assistant Burrows, and by these two, with the aid of a local land-surveyor, the labour of the astronomical and servations Ramsden's 9-inch theodolite, was 4364-4 feet, which Maskelyne's obin the latitude of Schehallien corresponds to 42"-94 of latitude. The observed difference of latitude by at Schehal337 observations with Sisson's 10-feet zenith sector lien. was 54" 6. The excess, or 11"-6, is the double attraction of the hill drawing the plumb-line towards itself at the two stations. The sine of this angle, or 17, represents the actual ratio between the double attraction of the hill and the attraction of the earth. But by the computation of the attraction which the hill ought to exert, from its figure, as determined by Maskelyne's gauges, were its density the same as that of the globe generally, this ratio should amount to 33, which can only be accounted for by assuming the earth to be denser on the average than the hill of Schehallien in the proportion of 17804 to 9933. This deduction was made by Dr Hutton by means of a troublesome calculation of the summation of the attractive effects of a number of vertical prisms into which the hill was imagined to be divided. The artifices of calculation were, however, due to Cavendish (who it will be recollected was on the "Committee of Attractions,") as Mr Airy ascertained from his manuscripts. A careful lithological survey of the hill Earth's enabled Professor Playfair to deduce the probable density demean specific gravity of the globe to be between 4.56 and 4.87, which was somewhat greater than Dr Hutton assumed it. duced. and Caven This is the proper place to mention an experiment (156.) on the density of the Earth perhaps still more re- Michell markable, devised by the Rev. Mr Michell, who con- dish's expestructed the apparatus, but first put in practice by riment for Mr Cavendish in 1797-8. It consisted in measuring the same the force of gravitation between two spheres of such end. small size that they could be moved by the hand nearer to or farther from one another. The essential part of the invention was to contrive a balance so delicate as to measure the almost inappreciable tendency of such small bodies to unite. Newton had shown that the attraction at the surface of any sphere is directly as its radius, which he observed must always be incomparably smaller than their tendency towards the earth, that is, their weight. In the largest and heaviest masses with which it has hitherto been found practicable to operate, this tendency 1 De Mundi Systemate, § 22. Newton, in a very remarkable passage of the Third Book of the Principia (Prop. X.), conjectures that "the quantity of matter in the earth may be five or six times greater than if the whole were composed of water." 2 A laughable mistake of Zach in his account of the Schehallien experiment (in his Attraction des Montagnes) is commented on by Playfair in the Edinburgh Review. In a note to the word Schehallien, Zach says, "Montagne appelée dans le pays en langue Erse Maiden pap, qui veut dire orage perpetuel." It is needless to add that these two alleged synonyms are different interpretations given by Gaelic scholars of the word. "From this inaccuracy," adds his reviewer, "his residence in London ought to have delivered him, for though he could not learn there what was Erse, he might have learned what was English." 3 Zach obtained the same result exactly by including all the observations, as Maskelyne had provisionally obtained by using only those stars on which he most depended. (157.) The Torsion Result. sult. amounts to only a very minute fraction of a grain. Mitchell imagined for this purpose the balance of tor- periment with his usual patience, judgment, and suc- (158.) It would be difficult to mention in the whole range Baily's re- of physics a more beautiful and more important experiment. It has been repeated since by Reich of Freiberg and Baily of London. The former obtained 5.44, the latter 5-66 for the Earth's specific gravity; this last result being worthy of much confidence from provements Maskelyne continued his zeal for the promotion of (159.) astronomy to the last. He superintended the publi- Maskecation of 45 annual volumes of the Nautical Alma- lyne's imnac. He left the whole of the observatory work in at Greenperfect order, and the greater part printed. He had wich. the well-earned satisfaction of finding his observations in request in every civilized country, the bases of the most useful tables, and the tests of the most advanced theories. He cultivated the friendly correspondence of astronomers in every country. Not given to change, he preserved the instruments and chief methods of the immortal Bradley; but sufficiently alive to the necessity of progress in the sciences, he introduced many simple but practical improvements in the art of observation. Even in his last years, satisfied that the celebrated quadrant of Bird was no longer the best instrument for its purpose, and was besides sensibly deteriorated by use, he adopted the circle instead (then recently come into notice, though first used more than a century before by Römer), and directed the construction of that by Troughton, though it was not placed at Greenwich until after his death, which occurred in 1811, in the 79th year of his age. His biographer Delambre mentions, that a considerable number of his posthumous memoirs were put into the hands of Professor Vince for publication. They have not, however, appeared. (160.) Its superi ority to Practical astronomy was on the Continent far behind its state in England at the period of which we speak. The various national observatories contri- Continentbuted comparatively little to the progress of science; al observabut there were of course exceptions, a few of which tories at we will here briefly notice. that time. (161.) on the Con The discovery of the four small planets2 Ceres, 1 The experiments of Cavendish are related in the " Philosophical Transactions" for 1798; those of Reich in a separate small 2 In the Fifth Dissertation, page 789, Harding instead of Olbers is named as the discoverer of Vesta. pose of ascertaining whether no planetary body filled the void between Mars and Jupiter. To Piazzi, of Palermo, we owe a most excellent catalogue of fixed stars from observations with a moveable circle of four feet radius by Ramsden. Oriani of Milan was likewise one of the best informed practical astronomers of his time. In France, after the death of the celebrated Lacaille, perhaps Lalande (who was exactly Maskelyne's contemporary) was the most active astronomer. To him and his nephew we owe a very valuable catalogue of 50,000 stars, lately edited by the British Association. But practical astronomy was seriously neglected in France generally. The national observatory was feebly superintended by the later members of the Cassini family; of the French expedition under Maupertuis to measure the length of a degree in Lapland, the Abbé Outhier alone, it is said, knew how to use a quadrant, and the celebrated Lagrange was as ill informed until instructed by Lalande.1 (163.) But the most important labours of the French French arc astronomers at the close of the last, and at the comof the me- mencement of the present century, were in carrying out the measurement of the arc of the meridian from Dunkirk to the Balearic Isles, with the more immediate object of fixing the length of the mètre, but contributing to the solution of far more considerable problems connected with the FIGURE Of the Earth. We connect this labour with the respectable name of Delambre, who was more intimately associated with it than perhaps any other person, though united with such eminent men as Méchain, Biot, and Arago. ridian. ter and talents. (164.) DELAMBRE was the pupil of Lalande, who used to Delambre; say that his disciple was his best work. He first obhis charac- tained distinction as a computer of tables. Those of the motions of the Sun, Jupiter, Saturn, and Uranus, and of the satellites of Jupiter were deservedly prized, and some of them are still the best of their class. He was a man in whom the love of truth and accuracy was conspicuous. Learned and patient, he spared no pains in acquiring knowledge, and in using it to the best purpose. As a calculator he was eminent. Physical astronomy he did not cultivate, except with a view to compare its deductions with facts. He was intimately conversant with all properly astronomical methods and formulæ. He knew the his tory of every problem, and the details and modifications of every astronomical instrument. He has embodied the results of this vast industry in a series of works (forming six quarto volumes) on the history of his favourite science, which are without a parallel for fulness and impartiality. He laboured as conscientiously to ascribe the due credit to Hipparchus and Ptolemy as to hold an equal scale between the merits of French and British astronomers. His critical knowledge of the ancient languages (for he could speak Greek with fluency) was not more remarkable than his complete freedom from national prejudices. Both attributes qualified him pre-eminently for the office of an historian. He published also a large treatise on astronomy, and numerous memoirs on practical subjects in the Connaissance des Tems between 1788 and 1817. Of his original labours the measurement of the (165.) History of French Arc of the meridian, of which he has given a full the French account in his Base du Systême Mètrique Decimal, is Arc. the most important. As some account of this undertaking has been given in Sir John Leslie's Dissertation, I shall state concisely a few particulars not there mentioned. Not the least singular feature of this gigantic work was the political crisis under which it was conducted. So early as August 1790, the French Constituent Assembly, on the motion of Talleyrand, desired the king to write to the English government, to represent the advantage of the two nations uniting to adopt a common unit or standard of weight and measure, which it was proposed should be done by a joint committee of the Royal Society and the French Academy.2. This application was probably never made, at least nothing came of it; but the Academy named their own committee, who, after discussing three sorts of natural standards,— the length of the pendulum in lat. 45° (first proposed by Huygens in his Horologium Oscillatorium), the length of a quadrant of the equator, and that of a quadrant of the meridian from the equator to the pole assumed to be elliptic,-adopted the latter, and this labour was committed to Méchain for the southern part, from Rodez to Barcelona (170,000 toises), and to Delambre for the northern, from Rodez to Dunkirk (380,000 toises). The southern arc was afterwards extended to Formentera in the Balearic Isles, and the whole length of the arc was found astronomically to be 12° 22′ 12′′ 6. Two bases were measured (both by Delambre), one at Perpignan of 6006 toises, the other at Melun of 6076 toises (each about 7.3 miles). When the length of the former was computed by triangulation from the latter, the difference of the observed and inferred amount is said to have been only ten or eleven inches. 1 Both anecdotes are told by Moll, who had them from Delambre. 2 The two governments and their respective learned societies had already co-operated in 1784 for a survey to connect Paris and Greenwich Observatories. were the principal means of extricating him from his difficulties but his danger was often imminent, and he appears to have sometimes heard the dreadful words which, as an eloquent author has expressed it, were the last sounds that vibrated in the ear of many an unhappy victim." The operations were actually suspended for a time by a decree of Robespierre and his colleagues, who deposed Delambre, along with Laplace, Lavoisier, Borda, and others, from the Commission of Weights and Measures, as being deficient in "republican virtues and their hatred of kings." They were, however, resumed, and Delambre had finished his share of the work long before his colleague Méchain, whose shorter task was conducted amidst a people rude and uneducated, indeed, yet far more to be trusted than were then those of the north. Méchain was apparently wayward and impracticable, somewhat too aged for so great a work, yet a really good astronomer. The want of agreement to within 3" of two sets of observations for latitude at Barcelona, the southern end of the arc at that time, led him to the suppression of one of them, and he was tormented ever after by the consciousness of the evasion, which deprived him of the tranquillity necessary to resume and complete his work, which was done chiefly by Delambre after vexatious delays.1 The error, which may be said to have cost Méchain his life, was probably owing to the instrument employed on this survey, the repeating circle of Borda, only fourteen inches diameter, with a rather weak telescope. The opinion generally entertained in Britain is, that the repeating circle was quite inadequate to the prodigious accuracy required of it, especially in the determination of latitudes. The errors of mere division are often trivial compared to those inherent in other parts of an instrument. Of these a deficiency of optical power, and the want of absolute security of the clamps, upon which the entire success of the principle of repetition depends, are amongst the most obvious. The arc was finally prolonged from Barcelona to Formentera by Biot and Arago in 1806. The conclusion of the survey was not destitute of the adventurous character of its commencement. The French astronomers ran many risks, underwent much suffering, and Arago narrowly escaped finishing his days in the dungeons of Spain. The English survey carried on by Roy and Mudge has been also noticed in the previous Dissertation, The arc from Dunnose to Burleigh Moor amounts to 3° 57′ 13′′-1, the measured length to 1442953 feet. An arc of parallel was also measured from Dover to Falmouth. We shall say something of its later progress in the concluding part of this essay, but we have still to regret the postponed publication of the British Arc of the Meridian, which we have no reason to doubt will bear a favourable comparison with the work of Delambre. The practical The appliances, the three-feet theodolite of Ramsden for To Delambre was confided the drawing up of the (168.) trigonometric formulæ used in the calculations of the French trigonosurvey, which were published in a separate work; metric forDe Prony conducting the laborious calculation of an mulæ and altogether new set of logarithmic tables, with the tables. aid of an immense staff of computers, the results of whose labour (still in MS.) are preserved at Paris in 17 folio volumes. Delambre carried his personal exertions so far as to compute his own triangles— which were also independently calculated by Legendre, Van Swinden, and Tralles. awarded to As an acknowledgment of his merit, the highest (169.) indeed in their power to bestow, the Institute of Prize France decreed to him in 1810 one of the Decennial Delambre. Prizes instituted by Napoleon. But the Emperor, though professing to be the warm encourager of science, suffered some meaner motive to interfere, and refused to ratify the decision. "Ce fut," writes Dupin, "un pas dans la route qui le menait à sa chûte." After the siege of Paris in 1814 Delambre wrote a characteristic letter to his friend Moll. The tranquil spirit which had braved the horrors of the Revolution was not to be moved by the sounds of the artillery of the allied armies. In spite of the cannonade which he heard from his library, he laboured from eight in the morning until midnight; and, conscious of rectitude, he feared little the revolution of circumstances, which changing dynasties might call forth. 'Labour," he says, "occupies all my time and all my faculties." As Secretary to the Academy of Sciences for the (170.) 1 The history is minutely given by Delambre himself in his Biography of Méchain.—Astron. du XVIII. Siecle. His impar-Mathematical Sciences, he had to make frequent re- Herschel ; cal credibility of the Scriptures, especially with regard We reserve some farther remarks on the geodetic § 2. SIR WILLIAM HERSCHEL.-History of Sidereal and Telescopic Astronomy to 1820. Her- (172.) The eighteenth century was not, generally speakSir Wm.. ing, distinguished by original observations. The exceptions stand out with all the brighter lustre. Amongst these, the discoveries of Sir William Herschel occupy perhaps the foremost place. Arago has affirmed that Slough is the spot on the earth's surface signalized by the most numerous discoveries, and in a certain sense this is strictly true. No one, before or since (with possibly the exception of Mr Hind) has added so many new bodies to the known planetary system, and this at a time when such discoveries were rarer, more unexpected, and more difficult than now. His researches on the fixed stars are not of a nature to have their importance numerically estimated. (173.) sive works; Sir William Herschel's career was one of the his exten- longest and of the most sustained labour in the history of science. To have contributed papers-often several in one year to the Philosophical Transactions for thirty-nine consecutive years, from 1780 to 1818, with but two exceptions, is a feat of astonishing perseverance; but if we recollect that many of these papers contain announcements of capital discoveries, that every one of them is stored with original matter, and that the author had already passed his fortieth year when he commenced the production of this astronomical library, we cannot withhold a tribute of the warmest admiration. (174.) early history. England cannot claim Herschel as her own, except by adoption. He was born at Hanover in 1738, and was one of a numerous family who supported themselves chiefly by their musical talents. William Herschel, the third son, came to England in 1759 with his elder brother, and after struggling with many difficulties, found himself in comparatively comfortable circumstances as an organist at Bath. In 1774 he had executed a reflecting telescope with his own hands, and soon acquired so much dexterity as to construct instruments of ten and twenty feet in focal length. In 1780 he contributed his first paper on (171.) From this brief sketch it will appear how great (175.) were the obstacles which Herschel had to vanquish before he became a man of science, and that, besides the claims to distinction already enumerated, his knowledge and his skill were acquired in spite of every disadvantage. Practical Instru Practical Astronomy naturally divides itself into (176.) 1 It may be doubted whether any other similar annuity was given at that period on scientific grounds alone, and it is difficult |