chemist1. And if during his life he made only too many concessions to the dominant spirit of the time, this is only another example of that pliancy of character which even Laplace's greatest admirers constantly deplored. When he came to die and had nothing further to hope for from the world, he sent for a priest and devoted himself to settling his account with heaven 2.

III. ASTRONOMY.

A religious of the Catholic Church stands by a twofold right at the head of the Astronomy of the 19th Century, in virtue both of a brilliant discovery and of the enterprise which led up to it.

1 Discours et Éloges Académiques II 255: Laplace "fournit aux matérialistes leurs plus spécieux arguments, sans partager leurs convictions".

2 Nr. 66, March 7th 1827, of La Quotidienne contains the followng: "Paris, 6 mars. M. le marquis de Laplace, pair de France, membre de l'Institut, auteur de la Mécanique Céleste et de plusieurs autres ouvrages qui l'ont fait placer parmi les plus grands géomètres de ces derniers temps, est mort hier dans son hôtel Rue du Bac, entre les bras de ses deux pasteurs, M. le curé des Missions Étrangères et M. le curé d'Arceuil, qu'il avait fait appeler pour en recevoir les derniers secours de la religion. Nous aurons à publier une notice sur la vie de ce savant célèbre; mais nous devons dès ce moment faire remarquer ce que sa mort a présenté d'édifiant à sa famille, à ses amis et à ses admirateurs. C'est un contraste que nous aimons à opposer au récit de morts scandaleuses qui font la joie des ennemis de la religion. Ses obsèques auront lieu demain mercredi, 7, en l'église des Missions Étrangères.... The same information is found in the paper, L'ami de la Religion et du Roi LI, Paris 1827, 107 126 (cf. J. de Joannis in Études LXXI 655). M. Marie (Hist. des sciences math. et phys. X, Paris 1887, 70) calls the wavering Laplace "réactionnaire et ultra-royaliste", assumes him to "afficher des sentiments religieux outrés qu'il ne partageait pas" and ascribes to him "Palinodes", which however would not have been able to retard the progress for which his Cosmogony had cleared the way.

"The first day of the century", says the celebrated astronomer Frederick William Bessel', "was marked by a brilliant discovery: Piazzi of Palermo on the first day of January 1801 found a new planet, 'Ceres'. His discovery was a by-product of a great and admirable undertaking, the determination, namely, by a long series of observations, of the positions of some 7000 fixed stars." Bessel proceeds to explain the significance for astronomical science of this determination of the latitude and longitude of so many stars with the greatest possible accuracy, and recounts the laborious efforts of Tycho Brahe and his successors to arrive at the same result. "Piazzi", he continues, "had striven strenuously to secure the erection of an observatory at Palermo, and to equip it with splendid instruments, the work of the never-to-be-forgotten Ramsden; and when he had succeeded in this he stepped at once to the head of astronomical science. He published in 1803 after incalculable labour a catalogue of the positions of some 7 000 stars, and so resolute was his determination to secure the most accurate results attainable with his instruments that he repeated all his observations, and in 1814 was able to publish a second and much improved edition of his catalogue. Here was in truth a worthy beginning of the century. The appetite for thorough observation was roused from the slumber in which it had lain since the death of Bradley." At first sight the figures mentioned by Bessel may not seem very formidable. Some may not consider it a colossal undertaking to direct a telescope on one after another of 7000 stars, and read off their positions on the graduated circle. But the life of Piazzi supplies an effective reply to such a criticism. During the course of the observations he believed that he had found the long-sought parallax of certain fixed stars that is, the dis

1 Populäre Vorlesungen über wissenschaftliche Gegenstände von F. W. Bessel. Nach dem Tode des Verfassers herausgegeben von H. C. Schumacher, Hamburg 1848, 21-23; cf. 239 538.

placement which the position of the stars must undergo from the fact that in the course of the earth's annual movement they are observed from widely separated points of observation. On closer examination, however, he discovered that it was not the stars but his telescope that had suffered displacement, in all probability from the fact that the tower on which the telescope rested received more heat from the sun on one side that on the other. This will give a hint of the minute accuracy requisite in astronomical observations. Further, every instrument has its trifling defects, which import an element of error into all observations, and is further liable to disturbance by the most trivial occurrences, so that to arrive at practically useful results there is needed not only inexhaustible patience and persistence, but also acute insight, so as to discover the various sources of error and either devise a telescope that will make them inoperative, or correct the results mathematically. After the practical work of observation the astronomer must be prepared for weary hours at his desk. Every star must be observed several times, for no one observation ever corresponds precisely with another: and to the list of figures thus obtained, the Calculus of Probabilities must be applied so as to arrive at the closest possible approximation to the truth.

No one could appreciate better than Bessel the difficulty of compiling such a catalogue, for he himself stood in the first rank of observers, and had reduced to workable form Bradley's essay in the same direction, which contained only single observations.

"Nearly all the Flamsteed stars", he writes, "were observed five times, so that I had to reduce to shape a total of more than 25000 observations. ... On Piazzi's catalogue

which contains still more observations than Bradley's two astronomers were engaged and the task occupied many years: I am now convinced that that gigantic work has been estimated rather below than above its value." 1

Bessel drew a rich yield from the works of Piazzi and Bradley. When he had resolved on his plan for the revision of Bradley's catalogue he wrote to Olbers:

"Thanks to the known dexterity of Bradley, and the excellent instruments of the Greenwich Observatory, Bradley's catalogue yields little in point of accuracy to Piazzi's. An interesting feat, surely, to attain the same accuracy in 1750 as in 1800!" 2

The collation and revision of the two catalogues helped, in the event, to bring to light a very interesting fact: the self-movement of the fixed stars. It showed "that nearly one half of the stars contained in both catalogues (numbering 2959) possess a self-movement amounting to the tenth of a second annually, or, in certain cases to more" 3. The greatest movement was that exhibited by a star of the fifth magnitude Nr. 61 of the Swan: it was as considerable as five seconds a year. Bessel selected this double star on which to renew the search for the long-sought parallax. His labours were successful. He determined the parallax, about the third of a second, and with it the distance of one of the nearest fixed stars. "Nearest" is, however, hardly a word to accentuate in this connection. Its distance from the sun is according to Bessel about 657,700 times the radius of the earth's orbit. Light takes ten years

1 Bessel an Olbers, 26th February 1809: Briefwechsel, herausgegeben von Erman, I 205.

2 10th May 1807: ib. I 97-98.

3 Bessel, Populäre Vorlesungen 248.

to travel that distance: a train covering 200 miles a day would take two hundred million years 1.

These discoveries count, it is true, to the credit of Bessel, not of Piazzi, but it is clear that but for Piazzi's work they could never have been made. Bessel himself speaks elsewhere of the "invaluable services" 2 of the Italian astronomer, and always mentions his name with the most marked respect. And a greater than Bessel, Gauss himself, honoured Piazzi so far as to call him his first-born son Joseph 3.

It was his practice of observing each star more than once that led up to the significant discovery of the first asteroid, Ceres. On January 1st he noted down the position of a small star, on January 2nd he made a second observation of it but found figures different from the first. Either then, one of the observations was incorrect, or the star possessed a movement of its own. Further investigation decided in favour of the latter alternative.

Piazzi was a member of the Order of Theatines founded by St. Cajetan of Thiene. Born in 1746 at Veltlin, he made his first studies at Milan and there entered the Order. He received a decisive impulse towards the cultivation of science from the two editors and expounders of Newton, Thomas Leseur († 1770),

4

1 Bessel, Vorlesungen 261.

2 Ib. 239.

3 Cf. his Briefwechsel mit Bolyai 184.

Philosophiae naturalis principia mathematica; auctore Isaaco Newtono, Eq. Aurato; Perpetuis Commentariis illustrata, communi studio PP. Thomae Le Seur et Francisci Jacquier, ex Gallicana Minimorum Familia, Matheseos Professorum. 4 vols. Genevae 1739-1742. It was due in a great measure to this essay that Newton's work, the style of which was very involved, became generally