BULLIALDUS, BOUILLAUD, or rather BOULL1AUD, IsmAEL, a celebrated French astronomer, was born at Loudun, in the department of Vienne, on the 28th Sep tember 1605. After having, gone through a course of philosophy at Paris, and a course of civil law at Poitiers, he travelled into Holland, Italy, Germany, Poland, and the Levant, and established a correspondence with the most distinguished philophers of the times. The atten tion which he had paid to mathematics, civil law, theolo gy, and sacred and prophane history, obtained for him the reputation of an universal genius—a title by no means ill applied, when we consider the great variety of subjects upon which he wrote. Although Bullialdus was born and educated in the Protestant faith, he became a Catho lic priest in the year 1639, at the age of 27. After an active life, during which he made ntienernns cal observations, he retired to the abbey of St Victor at Paris, in 1689 ; and he died there on the 25th November 1694, in the 89th year of his age.
In the year 1638, Bullialdus published at Amsterdam an astronomical dissertation, entitled, Philolaus, sive de vero Ristemate mundi. This work was republished in 1645, under the title of Astronomia Philolaica—a work founded on the hyotheses of the earth's motion round the sun. In this valuable work, which obtained a high repu tation for its author, Bullialdus skews, that the inequali ties of the planets may be represented, by supposing them to move uniformly in an ellipse round one of its foci, and that the calculation of these is very simple, by imagining a circle and an epicycle, in place of the elliptical orbit. Our countryman, Seth Ward, Bishop of Salisbury, pub lished, in 1656, his ?strononzia Gcometrica, in which he has given a very simple method of calculating the equa tion of a planet in an elliptical orbit. Hence, the elliptic hypothesis which we owe to Bullialdus, has been called Ward's hypothesis.* Dr Ward supposed, that the mo tion of each planet is so tempered about the focus in which the sun is not placed, as to form at it angles pro portional to the times ; while Bullialdus makes every planet move in an ellipse, formed by the section of a cone, whose axis passes through one of the foci of the ellipse, and makes the motion of the planet about the cone, an4 in circles parallel to the base of the cone, to be equal. Hence he supposes, what is just the hypothesis of Ward. that the angles at the focus in which the sun is not, are proportional to the times. Dr Ward has pointed thi.:
out in his work, entitled, In Ismaeli Bullialdi Astronomie Inquisitio, 1653 ; and has also mentioned several mis takes of our author. These mistakes, Bullialdus has very ingenuously confessed, in his reply to the learned Bishop, entitled, Astronomic Philolaica fundamenta cla rius explicata et asserta adversus Zethi Wardi impugna tionem ; but he has, at the same time, pointed out a very ingenious correction of the elliptic hypothesis. " This correction," says Dr David Gregory, " is well enough, if it be taken only for a correction of the approximation to the true system, as it ought to be ; because, by it, the cocquated anomaly at length might be found from the mean, a priori, and, at the same time, the calculation sa tisfy observation, which no one had done before him, (according to Mercator's judgment) in the elliptic hypo thesis. But when Bullialdus puts it upon us as the true and genuine system, and derives the physical causes of it, after his manner, from the cone, he not only leaves the foundations of his Philolaic astronomy unexplained, but disputes as if there were no physical foundations of as tronomy at all." In 1638, Bullialdus published a work, entitled, De Natura Lucis. In 1644, he published his translation of Theo, the Platonist of Smyrna. His treatise De Lincis Spiralibus Exercitatio Gcometrica et ?stronomica, ap peared at Paris in 1657. In 1663, he pulished a treatise of Ptolemy's De judicandifacultate. In 1666 appeared his Illonita duo ad a.s.tronomos ; prinzuni de stella Ceti, alterum de Nebulosa in Andromeda cinguli parte borea, ante biennium iterum orta. He supposes that the change able star in the Whale has a part of its surface obscure, and a rotatory motion about its axis : the best hypothesis which has yet been employed to explain these singular phenomena. In 1682, he published his Opus Novunz ad Arithmetic= infinitorum, fol. Paris. Bullialdus wrote also some pieces concerning ecclesiastical rights ; and he published the History of Ducas in the original Greek, with a Latin version, and notes. It was printed at the Louvre in 1649. The manuscripts of Bullialdus were preserved in the king's library, a1id there was also a copy of them in the depot of the Marine. See Perrault's Ilommes Illustres, Paris, 1697 ; Niceron Menzoires des Hommes Illustres ; and the Journal des Siavan•, Feh. 7, 1675. (o)