An Account of Some Books

it somewhat of a Sky-colour, he was, upon owning his surprise thereat, informed, that a dozen of them being put in, they would dye it to almost a full Azure. Which is touch't here, that, the Experiment being so easy to make, it may be tried, when the season furnishes those Insects; meantime, it seems not more incredible, that this Creature should yield a Sky-colour, when put in water, than that Cachineel, which also is but an Insect, should afford a fine red, when steep'd in the same Liquor. An Account Of Some Books. I. Le Tome troisieme et dernier des Lettres de M. DES-CARTES. As the two first Tomes of M. Des-Cartes his Letters, contain Questions, for the most part of a Moral and Physiological Nature, proposed to, and answer'd by him, so this consists of the Contests, he had upon several Subjects with divers Men eminent in his time. To pass by that Sharp Contest, he was engaged in by some Professors of Divinity at Utrecht, who endeavoured to discredit his Philosophy, as leading to Libertinisme and Atheisme, notwithstanding he made it so much his business, as to assert the Existence of a Deity, and the Immortality of a Soul: We shall take notice of what is more to our purpose, vid. the Differences, he had touching his Dioptricks and Geometry. As for his Dioptricks, though a great part of the Learned World have much esteem'd that Treatise, as leaving little to be said after him upon that Subject; yet there have not been wanting Mathematicians, who have declared their disagreement from his Principles in that Doctrine. The first of them was the Jesuit Bourdin, Mathematick Professor in the Colledg of Clermont at Paris; but this difference was soon at an end. A second was Mr. Hobbs, upon whose account he wrote several Letters to Mersennus, containing many remarks conducing to the Knowledge of the Nature of Reflection and Refraction. But the Person, that did most learnedly and resolutely attack the said Dioptricks, was Monsieur Fermat, writing writing first about it to Mersennus, who soon communicated his Objections to M. Des-Cartes, who failed not to return his Answer to them. But Fermat replied, and Des-Cartes likewise; and after many reciprocations, in which each party pretended to have the advantage, the matter rested; until M. Fermat taking occasion to write afresh of it to M. De la Chambre, several years after Des-Cartes's death, upon occasion of a Book, written by M. De la Chambre, Of Light; discoursed with this new Author after the same rate, as he had done before with Des-Cartes himself, and seemed to invite some-body of his friends, to re-assume the former contest. Whereupon M. Clerfeller and M. Rohault took up the Gantlet, to assert the Doctrine of the deceased Philosopher, exchanging several Letters with M. Fermat, all inserted in this Tome, and serving fully to instruct the Reader of this Difference, and withal to elucidate many difficult points of the Subject of Refractions; especially of this particular, Whether the Motion of Light is more easily, and with more expedition, perform'd through dense Mediums, than rare. Besides this, though one would think, Disputes had no place in Geometry, since all proofs there, are as many Demonstrations; yet M. Des-Cartes hath had several scuffles touching that Science. As M. Fermat had assaulted his Dioptricks, so He reciprocally examined his Treatise De Maximis & Minimis, pretending to have met with Paralogismes in it. But the Cause of M. Fermat was learnedly pleaded for, by some of his Friends, who took their turn to examine the Treatise of Des-Cartes's Geometry; whereupon many Letters were exchanged, to be found in this Book, and deserving to be considered; which doubtless the Curious would easily be induced to do, if Copies of this Book were to be obtain'd here in England, besides that one, which the Publisher received from his Parisian Correspondent, and which affords him the opportunity of giving this, though but Cursory, Account of it. As to Physicks, there occur chiefly two Questions, learnedly treated of in this Volume, though not without some heat between M. Des-Cartes and M. Roberval. The one is, touching the Vibrations of Bodies suspended in the Air, and their Center of Agitation: about which, there is also a Letter inserted of M. Des-Cartes M. Des-Cartes to that late Noble and Learned English Knight, Sir Charles Cavendish. The other is, whether Motion can be made without supposing a Vacuum: where 'tis represented, That, if one comprehend well the Nature, ascribed to the Materia subtilis, and how Motions, called Circular, are made, which need not be just Ovals or true Circles, but are only called Circular, in regard that their Motion ends, where it had begun, whatever irregularity there be in the Middle; and also, that all the Inequalities, that may be in the Magnitude or Figure of the parts, may be compensated by other inequalities, met with in their Swiftness, and by the facility, with which the parts of the Subtle Matter, or of the first Cartesian Element, which are found everywhere, happen to be divided, or to accommodate their Figure to the Space, they are to fill up: If these things be well understood and considered, that then no difficulty can remain touching the Motion of the parts of Matter in pleno. Besides all these particulars, treated of in this Tome, there occur many pretty Questions concerning Numbers, the Cycloid, the manner of Working Glasses for Telescopes, the way of Weighing Air, and many other Curiosities, Mathematical and Physical. I.I. ASTRONOMIA REFORMATA, Auctore JOHANNE BAPT. RICCIOLI, Soc. Jesu. For the Notice of this Book, and the Account of the Chief Heads contained therein, we are obliged to the Journal des Savans, which informs us, First, That the Design of this Work is, that, because several Astronomers, having had their several Hypotheses, there is found so great a diversity of opinions, that it is difficult thence to conclude any thing certain; this Author judged it also necessary, to compare together all the best Observations, and upon examination of what they have most certain in them, to reform upon that measure the Principles of Astronomy. Secondly, That this Volume is divided into two Parts; whereof the First is composed of Ten Books; in which the Author con- considers the principal Observations, hitherto made of the Motion of the Planets and the Fixed Stars, of their Magnitude, Figure, and other Accidents; drawing thence several Conclusions, in which he establishes his Hypothesis. The Second contains his Astronomical Tables, made according to the Hypotheses of the First Part, together with Instructions teaching the manner of using them. Thirdly, That Astronomers will find in this Book many very remarkable things, concerning the Apparent Diameter of the Sun and the other Stars, the Motion of the Libration of the Moon, the Eclipses, Parallaxes, and Refractions: And that this Author shews, that there is a great difference between Optical and Astronomical Refraction, which Tycho and many others have confounded, undertaking to prove, that whereas these Astronomers have believed, that the remoter any Star is, the less is its Refraction, on the contrary the Refraction is the greater, the more a Star is distant. And among many other things, he ingeniously expatiates the two contrary Motions of the Sun, from East to West, and vice versa, by one only Motion upon a Spiral, turning about a Cone. Fourthly, That he represents, How uneasy it is to establish sure Principles of this Science, by reason of the difficulties of making exact Observations. So, for example, in the Observation of the Equinox, every one is mistaken by so many Hours, as he is of Minutes, in the Elevation of the Pole, or the Diameter of the Sun, or the Refraction, or in any other circumstance. In the Observation of the Solstice, the error of one only Second causeth a mistake of an Hour and an half: mean time 'tis almost impossible to avoid the error of 2 Seconds; and even the sharpest sight will not be able to perceive it, except it be assisted with an Instrument of a prodigious bigness. For to mark Seconds, though Lines were drawn as subtil as the single threds of a Silk-worms Clew, (which are the smallest spaces to be discerned by the sharpest Eye) by the Calculation made by this Author there would need an Instrument of 48 feet Radius, since Experience shews, that there needs no more at most, than 3600. threds of Silk to cover the space of an inch. But, suppose one could have a Quadrant of this bigness, who can assure himself, that dividing it into 324,000 parts (for so many Seconds there are in 90 Degrees) either in placing it, or in observing, he shall not mistake the thickness of a single third of Silk? He adds, that Great Instruments have their defects, as the small ones: For in those, that are Movable, if the third, on which the Lead hangs, is anything big, it cannot exactly mark Seconds; if it be very fine, it breaks, because of its great length, and the weight of the Lead: And in the Fixed ones, the greater the Diameter is, the less the Shadow or the Light is terminated; so that it is painful enough, exactly to discern the extremities thereof. Yet 'tis certain, that the greater the Instruments are, the surer Astronomers may be: Whence it is, that some Astronomers have made use of Obelisks of a vast bigness, to take the Altitudes; and Signior Cassini, after the example of Egnatio Dante, caused a hole to be made on the highest part of a Wall of 95 feet in a Church at Bononia, through which the beams of the Sun falling on the Floor, mark as exactly as is possible, the height of that Luminary. Fifthly, That the Author reasons for the Immobility of the Earth after this manner. He supposes for certain, that the swiftness of the Motion of heavy bodies doth still increase in their descent; to confirm which principle, he affirms to have experimented, That, if you let fall a Ball into one of the Scales of a Ballance, according to the proportion of the height, it falls from, it raiseth different weights in the other Scale. For example, A Wooden Ball, of 1½ ounce, falling from a height of 35 inches, raiseth a weight of 5 ounces; from the height of 140 inches, a weight of 20 ounces; from that of 315 inches, one of 45 ounces; and from another of 560 inches, one of 80 ounces, &c. From this principle he concludes the Earth to be at Rest; for, saith he, if it should have a Diurnal Motion upon its Center, Heavy Bodies being carried along with it by its motion, would in descending describe a Curve Line, and, as he shews by a Calculus, made by him, run equal spaces in equal times; whence it follows, that the Celerity of their Motion would not increase in descending, and that consequently their stroke would not be stronger, after they had fallen thorow a longer space. III. ANA III. ANATOME MEDULLAE SPINALIS, ET NERVORUM inde provenientium, GERARDI BLASII, M. D. The Author shews in this little Tract a way of taking the entire Medulla Spinalis, or Marrow of the Back, out of its Theca or Bony Receptacle, without Laceration; which else happens frequently, both of the Nerves proceeding from it, and of the Coats investing it; not to name other parts of the same. This he affirms to have been put into practice by himself, by a fine Saw and Wedge; which are to be dexterously used: and he produceth accordingly in excellent Cuts, the Representations of the Structure of the said Medulla thus taken out, and the Nerves, thence proceeding; and that of several Animals, Dogs, Swine, Sheep. He intermixes several Observations, touching the Singleness of this Medulla, against Lindanus and others; its Original, vid. Whether it be the Root of the Brain, or the Brain the Root of it: its difference of Softness and Hardness in several Animals; where he notes, that in Swine it is much softer than in Dogs, &c. He exhibits also the Arteries, Nerves, and Veins, dispersed through this Medulla, and inquires, Whether the Nerves proceed from the Medulla itself, or its Membran; and discourses also of the Principle and Distribution of the Nerves; referring for ampler information in this and the other particulars, to that Excellent Book of the Learned Dr. Willis, De Anatomia Cerebri. Advertisement.