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Gravity, in music, is the modification of any sound by which it becomes deep or low in respect of some other sound. The gravity of sounds depends on- the thickness and distension of the chords, or the length and diameter of the pipes, and in general on the mass, extent, and tension of the sonorous bodies. The larger and more lax the bodies, the slower will be the vibrations and the graver the sounds.

GREASE. See Farriery.

GREAT circle sailing, the manner of conducting a ship in, or rather pretty near the arch of a great circle, that passes through the zenith of the two places, viz. from whence she came, and to which she is bound.

GREEK church. In the eighth century there arose a difference between the eastern and western churches, which, in the course of about two centuries and a half ended in a total separation. The Greek, or Eastern, or as it is sometimes called the Russian church, spread itself over the eastern parts of Europe. It bears a considerable resemblance to the church of Rome, but denies the infallibility and supremacy of the Pope: it rejects also the worship of images, and the doctrine of consnbstantiation, or the union of the body of Christ with the sacramental elements. The administration of baptism is performed by immersing the whole body. The Greek church has the same division of clergy, and the same distinction of ranks and offices with the church of Rome.

GREEN, one of the original colours excited by the rays of light. See ChromaTics, Colours, and Optics. The green coTour of plants has been shown, by the French chemists, to depend upon the absorption of carbonic acid, and it is supposed that the leaves of plants have the power of decomposing the carbonic acid and water also; the oxygen they emit, while the carbon and hydrogen enter into the composition of the inflammable parts of the plant.

Green, Brunswick, a pigment used by German artists, and known in our shops under that name. It is made by saturating ct Id water with muriated ammonia, and. adding three times as much copper clipping as ammonia. The moisture is to be evaporated, taking care that no dust be allowed to get to it. The muriate of ammonia is decomposed by the copper, which is itself corroded and converted into a green oxide. Jl is then to be digested in successive portions of alcohol, as long as any green oxide

is taken up ; the solutions are now to be added together, and the liquor to be driven off by a moderate heat; the residue is the pigment required.

Green cloth, a board orconrt of justice, held in the counting-house of the king's household, composed of the lord-steward, and officers under him, who sit daily. To this court is committed the charge and oversight of the king's household in matters of justice and government, with a power to correct all offenders, and to maintain the peace of the verge, or jurisdiction of the court royal; which is every way about two hundred yards from the last gate of the palace where his Majesty resides. It takes its name, board of green-cloth, from a green cloth spread over the board where they sit. Without a warrant first obtained from this court, none of the king's servants can be arrested for debt.

Green finch. See Fringilla.

GREENHOUSE, or conservatory, a' house in a garden contrived for sheltering and preserving the most tender and curious exotic plants, which, in our climate, will not bear to l>e exposed to the open air during the winter season. These are generally large and beautiful structures, equally ornamental and useful.

GREGORIAN calendar, that which shows the new and full moon, with the time of Easter, and the moveable feasts depending thereon, by means of epacts, disposed through the several months of the Gregorian year.

Gregorian epoch, the epoch, or time whence the Gregorian calendar or computation took place. The year 1808 is the 3±>6th year of that epocha.

Gregorian year, the Julian year corrected, Or modelled, in such a manner as that three secular years, which in the Julian account are bissextile, are here common years, and only every fourth secular year is made a bissextile year.

The Julian computation is more than the solar year by eleven minutes, which in one hundred and t.'.irty-one years amounts to a whole day. By this calculation, the vernal equinox was anticipated ten days from the time of the general council of Nice, held in the year Si'", of the Christian ain't, to the time of Pope Gregory XIII. who therefore caused ten days to be taken out of the month of October, in line, to make the equinox fall on the Twenty -first of March, as it did at the time of that council, and to prevent the like variation for the future, he ordered that three days should be abated in every four hundred years by reducing the leap year at the close of each century for three successive centuries to common years, and retaining the leap year at the dose of each fourth century only.

This was at that time esteemed as exactly conformable to the true solar year, but it is found not to be strictly just, because that in four hundred years it gets one hour and twenty minutes, and consequently in 7200 years, a whole day.

The greatest part of Europe have long used the Gregorian style: but Great Britain retained the Julian till the year 1753, when by act of parliament this style was adjusted to the Gregorian ; since which time Sweden, Denmark, and other European states, who computed time by the Julian account, have followed this example.

GREGORY (james), professor of mathematics, first in the university of St. Andrew's, and afterwards in that of Edinburgh, was one of the most eminent mathematicians of the seventeenth century. He was a son of the Rev. John Gregory, minister of Drumoak, in the county of Aberdeen, and was born at Aberdeen, in November 1638. His mother was a daughter of Mr. David Anderson, of Fin. zangh, or Finshaugh; a gentleman who possessed a singular turn for mathematical and mechanical knowledge. This mathematical geuius was hereditary in the family of the Andersons, and from them it seems to have been transmitted to their descendants of the names of Gregory, Reid, &c. Alexander Anderson, cousin gcrman of the said David, was professor of mathematics at Paris in the beginning of the 17th century, and published there several valuable and ingenious works. The mother of James Gregory inherited the genius of her family ; and observing in her son, while yet a child, a strong propensity to mathematics, she instructed him herself in the elements of that science. His education in the languages be received at the grammar-school of Aberdeen, and went through the usual course of academical studies in the Marischal college ; but he was chiefly delighted with philosophical researches, into which a new door had lately been opcued'by the key of the mathematics.

Galileo, Kepler, Des Cartes, &c, were the great masters of this new method: their works therefore became the principal study of young Gregory, who soon began , to make improvements upon their disco

veries in Optics. The first of these improvements was the invention of the reflecting telescope; the construction of which instrument he published in his " Optica Promota," in ICG3, at twenty-four years of age. This discovery soon attracted the attention of the mathematicians, both of our own, and of foreign countries, who immediately perceived its great importance to the sciences of optics and astronomymy. But the manner of placing the two specula upon the same axis appearing to Newton to be attended with the disadvantage of losing the central rays of the larger speculum, he proposed an improvement on the instrument, by giving an oblique position to the smaller speculum, and placing the eye-glass in the side of the tube. It is observable, however, that the Newtonian construction of that instrument was long abandoned for the original, or Gregorian, which is now always used when the instrument is of a moderate size ; though Herschell has preferred the Newtonian form for the construction of those immense telescopes, which he has of late so successfully employed in observing the heavens.

About the year 1664, or 1665, coming to London, he became acquainted with Mr. John Collins, who recommended him to the best optic glass grinders there to have his telescope executed. But as this could not be done for want of skill in the artists to grind a plate of metal for the object speculum into a true parabolic concave, which the design required, he was much discouraged with the disappointment; and, after a few imperfect trials made with an ill-polished spherical one, which did not succeed to his wish, he dropped the pursuit, and resolved to make the tour of Italy, then the mart of mathematical learning, that he might prosecute his favourite study with greater advantage. And the University of Padua, being at that time in high reputation for mathematical studies, Mr. Gregory fixed his residence there for some years. Here it was that be published, in 1667, "Vera Circuli et Hyperbole Quadrature;" in which he propounded another discovery of his own, the invention of an infinitely converging series for the areas of the circle and hyperbola. He sent home a copy of this work to his friend Mr. Collins, who communicated it to the Royal Society, where it met with the commendations of Lord Brounker and Dr. Wallis. He reprinted it at Venice the following year, to which he added a new work, entitled "Geometrise Pars Universalis, inserviens Ijuaiititatuni Curvarum, Transmutationi et Mensune"; in which he is allowed to have shown, for the first time, a method for the transmutation of curves. These works engaged the notice, and procured the author the correspondence of the greatest mathematicians of the age, Newton, Huygens, Wallis, and others. An account of this piece was also read before the Royal Society, of which Mr. Gregory, being returned from his travels, was chosen a member the same year, and communicated to them an account of a controversy in Italy about the motion of the earth, which was denied by Riccioli, and his followers. Through this channel, in particular, he carried on a dispute with M. Huygens, on the occasion of his treatise on the quadrature of the circle and hyperbola, to which that great man had started some objections; in the course of which our author produced some improvements of his series. But in this dispute it happened, as it generally does on such occasions, that the antagonists, though setting out with temper enough, yet grew too warm in the combat. This was the case here, especially on the side of Gregory whose defence was, at his own request, inserted in the Philosophical Transactions. It is unnecessary to enter into particulars : suffice it therefore to say that, in the opinion of Leibnitz, who allows Mr. Gregory, the highest merit for his genius and discoveries, M. Huygens has pointed out, though not errors, some considerable deficiencies in the treatise above-mentioned, and shown a much simpler method of attaining the same end.

In 1688, our author published at London another work, entitled "Exercitationes Geometricae," which contributed still much further to extend his reputation. About this time he was elected Professor of Mathematics in the University of St. Andrew's, an office which he held for six years. During his residence there he married, in 1669, Mary, the daughter of George Jameson, the celebrated painter, whom Mr. Walpole has termed the Vandyke of Scotland, and who was fellow-disciple with that great artist in the school of Rubens, at Antwerp.

In 167S, he published " The great and new Art of weighing Vanity: or a Discovery of the Ignorance and Arrogance of the great and new Artist, in his pseudo-philosophical Writings. By M. Patrick Mathers, Archbedal to the University of St. Andrews.

To which are annexed some Tcntaminu de Mum Penduli et Projectorum." Under this assumed name, our author wrote this little piece to expose the ignorance of Mr. Sinclare, professor at Glasgow, in his liydrostatical writings, and in return for some ill usage of that author to a colleague of Mr. Gregory's. The same year Newton, on his wonderful discoveries in the nature of light, having contrived a new reflecting telescope, and made several objections to Mr. Gregory's, this gave birth to a dispute between those two philosophers, which was carried on during this and the following year, in the most amicable manner on both sides; Mr. Gregory defending his own construction, so far as to give his antagonist the whole honour of having made the catoptric telescopes preferable to the dioptric, and showing that the imperfections in these instruments were not so much owing to a defect in the object, speculum, as to the different refrangibility of the rays of light.—In the course of this dispute our author described a burning concave mirror, which was approved by Newton, and is still in good esteem. Several letters that passed in this dispute, are printed by Dr. Desagulicrs, in an appendix to the English edition of Dr. David Gregory's " Elements of Catoptrics and Dioptrics."

In 1674, Mr. Gregory was called to Edinburgh, to fill the chair of mathematics in that university. This place he had held but little more than a year, when in October 1675, being employed in shewing the satellites of Jupiter through a telescope to some of his pupils, he was suddenly struck with total blindness, and died a few days after, to the great loss of the mathematical world, at only 37 years of age.

As to his character, Mr. James Gregory was a man of very acute and penetrating genius. His temper seems to have been warm, as appears from his conduct in the dispute with Hnygens : and conscious perhaps of his own merits as a discoverer, he seems to have been jealous of losing any portion of bis reputation by the improvements of others upon his inventions. He possessed one of the most amiable characters of a true philosopher, that of being content with his fortune in his situation. But the most brilliant part of his character is that of his mathematical genius as an inventor, which was of the first order; as will appear by the following list of his inventions and discoveries. Among many others may be reckoned his reflecting telescipc ; burning concave mirror ; quadrature of the circle and hyperbola, by an infinite converging series; his method for the transformation of curve*; a geometrical demonstration of Lord Brounkers'series for squareing the hyperbola; bis demonstration that the meridian line is analogous to a scale of logarithmic tangents of the half-complements of the latitude; he also invented and demonstrated geometrically, by help of the hyperbola, a very simple converging series for making the logarithms; he sent to Mr. Collins the solution of the famous Replerian problem by an infinite series; he discovered a method of drawing tangents to curves geometrically, without any previous calculations; a rule for the direct and inverse method of tangents, which stands upon the

. same principle (of exhaustions) with that of, fluxions, and differs not much from it in the manner of application; a series for the length of the arc of a circle, from the tangent, and vice versa. These, with others for measuring the length of the elliptic and hyperbolic curves, were sent to Mr. Collins, in return for some received from him of Newton's, in which he followed the elegant example of this author, in delivering his series in simple terms, independently of each other. These and other writings of our author are mostly contained in the following works, viz.: 1. Optira

'I'romota; 4to. London, 1663. t. Vera Circuli et Hyperbolae Quadrature, 4to. Padua, 1667 and 1668. S. Geometriac Pars Universalis, 4to Padua, 1668. 4. Exercitationes Geometries;, 4to. London 1668. h. The great and new Art of weighing Vanity, 8vo. Glasgow, 1672. The rest of his inventions make the subject of several letters and papers, printed either in the Philos. Trans, vol. iii., the Commerc. Epistoi. Joh. Collins, et alionmi, 8vo. 1715, in the appendix to the English edition of Dr. David Gregory's Elements of Optics, 8vo. 17.}.i, by Dr. Desaguliers, and some series in the Excrcitatio Geometrica of the same author, (to. 1684, Edinburgh; as well as in his little piece on Practical Geometry.

Gregory (dr. David,) Savilian professor of astronomy at Oxford, was nephew of the above-mentioned Mr. James Gregory, being the eldest son of his brother, Mr. David Gregory, of Kinardie, a gentleman who had the singular fortune to see three of his sous all professors of mathematics, at the same time, in three of the British universities, viz. our author David at Oxford, the second son James, at Edin

burgh, and, the third son Charles at SI. Andrew's. Our author David, the eldest son, was born at Aberdeen, in 1661, where he received the early parts of his education, but completed his studies at Edinburgh: and, being possessed of the mathematical papers of his uncle, soon distinguished himself likewise as the heir of his genius. In the 23d year of his age, he was elected professor of mathematics in the university of Edinburgh , and, in the same year he published '' Ex'ercitatio Geometrica de Dimensinne Figurarum, sive Specimen Methods generalis dimetiendi quasvisFigures., Edinb, 1684, 4 to. He very soon perceived the excellence of the Newtonian philosophy, and had the merit of being the first that introduced it into the schools, by his public lectures at Edinburgh. " He had (says Mr. H'his ton in the Memoirs of his own life, i. 32.) already caused several of his scholars to keep acts, as we call them, upon several branches of the Newtonian philosophy ■ while we, at Cambridge, poor wretches, were ignominiously studying the fictitious hypothesis of the Cartesian."

In 1691, on the report of Dr. Bernard's intention of resigning the Savilian professorship of astronomy, at Oxford, our author went to London ; and being patronised by Newton, and warmly befriended by Mr. I'lamstead, the astronomer royal, he obtained the vacant professorship, though Dr. Halley was a competitor. This rivalship, however, instead of animosity, laid the foundation of friendship between these eminent men; and Halley soon after became the colleague of Gregory, by obtaining the Professorship of Geometry in the same university. Soon after his arrival in London, Mr. Gregory had been elected a Fellow of the Royal Society; and previously to his election into the Savilian Professorship, had the degree of Doctor of Physic conferred on him by the university of Oxford.

In 1693, he published in the Philos. Trans, a solution of the Florentine problem, " DeTestndiue vcliformi quadrabili ;* and he continued to communicate to the public, from time to time, many ingenious mathematical papers by the same channel.

1695, he printed at Oxford, "Catoptrics; et DioptricIb Sphxricx Elemcnta," a work which we are informed, in the preface, contains the substance of some of his public lectures read at Edinburgh eleven years before. This valuable treatise was published in English, first with additions by Dr. William Brown,with the recommendstu>n of Mr. Jones and Dr. Desaguliers, and afterwards by the latter of these gentlemen; with an appendix, containing an account of the Gregorian and Newtonian telescopes, together with Mr. Hartley's tables for the construction of both those instruments. It is not unworthy of remark, that, in the conclusion of this treatise, there is an observation which shows that the construction of achromatic telescopes, which Mr. Holland has carried to such great perfection, had occurred to the mind of David Gregory, from reflecting on the admirable contrivance of nature in combining the different humours of the eye. The passage is as follows: "Perhaps it would be of service to make the object lens of a different medium, as we see done in the fabric of the eye; where the crystalline hnmotir (whose power of refracting the rays of light differs very little from that of class) is by nature, who never does any thing in vain, joined with the aqueous and viti eons humours (not differing from water as to their power of refraction) in order that the image may be painted as distinct as possible upon the bottom of the eye."

In 1702, onr author published at Oxford, in folio, " Astronomic Physica* et Geometries Elementa," a work which is accounted his master piece. It is founded on the Newtonian doctrines, and was esteemed hy Newton himself as a most excellent explanation and defence of his philosophy. In the following year he gave to the edition, in folio, of the works of Euclid in Greek and Latin; being done in prosecution of a design of his predecessor Dr. Bernard, of printing the works of all the anrient mathematicians. In this work, which contains all the treatises that have been attributed to Euclid, Dr. Gregory has been careful to point out such as he found reason, from internal evidence, to believe to be the productions of some inferior geometrician. In prosecution of the same plan, Dr. Gregory engaged soon after, with his colleague Dr. Halley, in the publication of the copies of Apollonius; but he had proceeded only a little way in the undertaking, when he died at Maidenhead, in Berkshire, in 17 lit, being the 49th year of his age.

Besides those works published in our author's-life-time, as mentioned above, he had several papers inserted in the Philos. Trans, vol. xviii, xix, xxi, xxiv, and xxv, particularly a paper on the Catenarian curve, first considered by our author.

He left also, in manuscript, a short Treatise of the Nature and Arithmetic of Loga

rithms, which is printed at the end of Kcill'a translations ot Commandine's Euclid, and a, treatise ot Practical Geometry, which was afterwards translated, and published in 1745, by Mr. Marlaurin.

Dr. David Gregory married, in 1695, Elizabeth the daughter of Mr. Oliphant, of Langtown in Scotland. By this lady he had four sons of whom, the eldest, David, was appointed Kegius Professor of modern history, at Oxford, by King George the First, and died at an advanced age in 1767, after enjoying, for many years, the dignity of Dean of Christ Church in that University.

When David Gregory quitted Edinburgh, he was succeeded in the Professorship of that University by his brother James, likewise an eminent mathematician, who held that office for thirty-three years, and retiring in 1725, was surrounded by the celebrated Madam in. A daughter of this Professor James Gregory, a young lady of great beauty and accomplishments, was tin victim of an unfortunate attachment, that furnished the subject of Mallet's weU-kupwn ballad of William and Margaret.

Another brother, Charles, was created Proftssor of Mathematics at St. Andrew's, by Queen Anne, in 1707. This office he held with reputation and ability for thirty. two years; and resigning in 1759, w»s succeeded by his son, who eminently inherited the talents of his family, and died in 1763. v

GRENADE, or grenade, in military affairs, a kind of small bomb or shell, being furnished with a touch-hole and fuse, and is thrown by hand from the tops, lienor they are frequently styled hand-grenades. The best way to secure one's-self from the effects of a grenade, is to lie flat down on the ground before it bursts.

The grenades are of much later invention aud use than the bomb. They are usually about three inches in diameter, and weigh near three pounds. The meal may be one quarter or three-eighths of an inch thick, and the hole about one-sixth.

GREW I. \, in botany, so named in honour of Nehemiah Grew, M. D. F. R. S. the famous author of the " Anatomy tf Vegetables," a genus of the Gynandria Polyandria class and order. Natural order of Columnifene. Tiliaccas, Jussieu. Essential character: calyx rive-leaved; petals five, with a nectareous scale at the base of each ; berry four-celled. There are thirteen species.

GK1 AS, in botany, a genus of the Po

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