style had actually shortened their lives, they had more serious cause for alarm than the zealots of orthodoxy in former times, who made themselves unhappy about the notion of merely celebrating Easter on the wrong day.

In the earlier part of the eighteenth century, we ought not to omit to mention, was invented the ingenious and valuable instrument called Hadley's Quadrant (since improved into a sextant, and still more recently into an entire circle), either by John Hadley, who was a fellow of the Royal Society, and who gave an account of it in the Philosophical Transactions for 1731, or by Thomas Godfrey, a glazier in Philadelphia, who is generally believed to have been in possession of it a year before the date of Hadley's communication. But it appears that a similar instrument had been described to Dr. Halley by Newton, some time before his death in 1727. And this age is also marked in the history of optics and astronomical observation by the important correction of the Newtonian views as to the dispersion of refracted light, of which the honour belongs to John Dollond, and by the invention of the Achromatic Telescope, with which that sagacious and philosophical experimentalist followed up his discovery. Dollond's account of his "Experiments concerning the different refrangibility of light” appeared in the Philosophical Transactions in 1758; and his achromatic object-glass was contrived the same year.

Of a few other distinguished British mathematicians belonging to the middle portion of the last century the most eminent was Colin Maclaurin, the successor of James Gregory in the mathematical chair at Edinburgh, who was born in 1698, and died in 1746. Maclaurin's

principal works are his Geometria Organica (a treatise on curves), published in 1720; his admirable Treatise on Fluxions, 1742; and his Treatise on Algebra, 1748. Another very able performance printed after his death is his Account of Sir Isaac Newton's Philosophical Discoveries, which also appeared at London in 1748. All Maclaurin's works are distinguished by profoundness and solidity united with elegance, and often by originality in the method of exposition, or novelty in the application of principles. His countryman and contemporary, Dr. Robert Simpson, professor of mathematics at Glasgow (b. 1687, d. 1768), was also a most learned and able geometrician he is the author of a restoration of the "Loci" of Apollonius, and of an English translation of Euclid, which continued down to our own day in common use as an elementary book both in Scotland and England. Along with these may be mentioned James Stirling, the author of a Latin treatise published in 1717, on what are called lines of the third order, and a treatise on fluxions, entitled Methodus Differentialis, 1730. William Emerson, a mathematician and mechanist of great talent, whose death did not take place till 1782, when he had reached his eighty-first year, is the author of a series of works, on fluxions, trigonometry, mechanics, navigation, algebra, optics, astronomy, geography, dialing, &c. His manner of writing is singularly uncouth; but his works often exhibit much scientific elegance, as well as considerable invention. Another author of a remarkable series of mathematical works, of this date, is the self-taught genius, Thomas Simpson, who was born at Market Bosworth, in the humblest rank of life, in 1710, worked at his trade of

He

a weaver till he was seven-and-twenty, and then suddenly came forth as one of the most acute and well-furnished mathematical writers of the day. A Treatise on Fluxions, another on the Nature and Laws of Chance, a quarto volume of Essays on subjects in speculative and mixed mathematics, a work on the doctrine of Annuities and Reversion, a second volume of Mathematical Dissertations, a treatise on Algebra, another on Elementary Geometry, another on Trigonometry, plane and spherical, a new work on the doctrine and application of Fluxions, a volume of Exercises for young proficients in Mathematics, and a volume of Miscellaneous Tracts, were all produced by Simpson in the twenty years between 1737 and 1757. And he also furnished several papers to the Philosophical Transactions, and edited for some years the mathematical annual called The Ladies' Diary. died in 1761. In the same year with Simpson was born in Banffshire, in Scotland, James Ferguson, who was the son of a day-labourer, and who taught himself the elements of mechanics and astronomy while employed as a farmer's boy in tending sheep. Ferguson published his first performance, his Dissertation on the Phenomena of the Harvest Moon, in 1747; his Astronomy in 1756; his Lectures on Mechanics, Hydrostatics, Pneumatics, and Optics, in 1760; and two or three other works between that date and his death in 1776. "Ferguson," it has been observed, "has contributed more than perhaps any other man in this country to the extension of physical science among all classes of society, but especially among that largest class whose circumstances preclude them from a regular course of scientific instruction. Perspicuity in the selection and arrangement of his facts,

and in the display of the truths deduced from them, was his characteristic both as a lecturer and a writer."*

Another department of natural philosophy in which some splendid results were obtained by English experimenters of this era was that of electricity. Francis Hawksbee, who was admitted a fellow of the Royal Society in 1705, published several papers in the Transactions between that year and 1711, giving an асcount of a series of experiments, partly performed with a glass globe, in the course of which he noticed a number of facts connected with electrical attraction and repulsion, and in particular detected for the first time the remarkable phenomenon of the production of light by friction. A few years later the subject was taken up by Stephen Gray, a pensioner of the Charter House, who, with the aid of a very poor apparatus, made out a catalogue, which he published in 1720, of bodies which show electricity on being rubbed, and in 1732 discovered the conducting property inherent in bodies that are not electrical. The two opposite kinds, or exhibitions, of electricity (which he called the vitreous and the resinous) were discovered by Dufay, keeper of the King's Garden at Paris, before 1739; and he also showed that bodies similarly electrified repel, and those dissimilarly electrified attract, each other. The mode of accumulating the electric power by what is called the Leyden phial, or jar, was discovered by Cuneus and Lallemand in 1745. This experiment immediately attracted universal attention: Nollet in France, and Watson in England, in particular, applied themselves to

* Penny Cyclopædia, x. 234.

find out the explanation of it; and the latter is asserted to have first conceived the hypothesis of the redundancy of the electricity on the one side of the jar and its deficiency on the other. The same view occurred to the celebrated Benjamin Franklin, in America, who expounded it in a series of letters written to his friend Collinson, in London, in the course of the year 1747, in which he described the overcharged side of the jar as in a state of positive, and the undercharged of negative, electricity, and showed how all the known phenomena of electric action were to be accounted for on this hypothesis of only one kind of electric matter, or power, in opposite states. Franklin seems to have known little or nothing of what had been done by his predecessors either in France or England; of the theories, at least, either of Dufay or Watson, he appears never to have heard. Although not the first in the field, his penetrating and inventive genius immediately raised him to the first place among the cultivators of the new science. He soon improved the Leyden jar into the much more powerful apparatus of the electrical battery. Some of his earliest experiments had taught him the superior efficiency of sharp points both in attracting and giving out the electric matter; from the year 1749 he had inferred, from a great number of facts which he had observed and collected, the probable identity of electricity and lightning; and at last, in June, 1752, he established that truth by the decisive experiment of actually drawing down the electric matter from the clouds. This was followed by his invention of lightning-conductors, of which, however, none were erected in England till the year 1762.