acidulated water, so as to keep up a constant flow, which would travel any distance so long as the circuit was not broken. Here, you will see, was the first step towards the electric telegraph. It was but a commencement, and for nearly forty years no further advance was made; but the seed was sown, and when we reap the benefits we must always remember the names of Franklin, Galvani, and Volta, as the great pioneers in the science of electricity. - Chief Works consulted. Lardner's Cyclopedia, Electricity, Magnetism, and Meteorology;' 'Encyclopædias Britannica,' and 'Metropolitana,' art. 'Electricity;' Franklin's 'Experiments and Observations on Electricity,' 1749; Priestley, 'On Electricity,' 1785 ; Thomson's Hist. of Royal Society,' 1812; 'Life of Franklin,' by himself, 1833; Bennett's 'Text-Book of Physiology;' Fownes's 'Chemistry;' Wilkinson's 'Galvanism.' CH. XXX. BRADLEY AND DELISLE. 265 CHAPTER XXX. SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). Bradley and Delisle, Astronomers-Aberration of the Fixed StarsNutation of the Axis of the Earth-Delisle's Method of Measuring the Transit of Venus-Lagrange and Laplace-Libration of the Moon accounted for by Lagrange-Laplace works out the Long Inequality of Jupiter and Saturn-Lagrange proves the Stability of the Orbits of the Planets-Sir William Herschel constructs his own Telescopes-Discovery of a New Planet-Discovery of Binary Stars - Herschel studies Star-clusters and Nebula-Theory of Nebula being matter out of which Stars are made-The Motion of our Solar System through Space-Weight of the Earth determined by the Schehallien Experiment-Summary of the Science of the Eighteenth Century. Astronomical Labours of Bradley and Delisle.-And now, as we approach the end of the eighteenth century, we must take up once more the history of Astronomy, which we have neglected since Newton died in 1727. Since that time many good astronomers had been occupied in making different observations, but the only two who need be mentioned were Bradley, the Astronomer-Royal (born 1692, died 1762), and Delisle (born 1688, died 1768). Bradley explained two difficult astronomical problems; the first of these is called the 'aberration of the fixed stars,' which is an apparent movement of each fixed star in a small circle in the heavens, but which is really the combined effect of the yearly motion of our own earth, and of the time which light occupies in coming down from the stars to us. This question is very difficult, as is also his second discovery of the nutation, or slight oscillation, of the earth's axis; but it is necessary to bear in mind that he made these observations, for they are very important in astronomy. Delisle will interest you because he proposed a second method of measuring the transit of Venus, which is now used at stations where Halley's rule (see p. 160) cannot be applied. Delisle's method consists in marking the time at which the transit is seen to begin in one part of the world, and to end in another; instead of measuring, as Halley did, the duration or length of time occupied by the whole transit as seen at each place. It requires that the clocks of all the different stations from which the transit is observed should be set exactly to the same time, and then it answers as well as Halley's. These discoveries are all that need be mentioned during the first half of the eighteenth century, but during that time there had been born within a few years of each other three men, Lagrange, Laplace, and Herschel, who were to light up the close of the century with the most brilliant discoveries. The two first of these were Frenchmen, the last we may fairly claim as an Englishman; for though he was born at Hanover in 1738, of German parents, still Sir William Herschel came over to England at the age of twenty-one, and all his discoveries were made here. It was our King George III. who gave him the pension which enabled him to devote himself to science; and his son Sir John Herschel was, like his father, one of our greatest astronomers, and made England his home and country. Lagrange and Laplace.-Louis de Lagrange was born at Turin in 1736. His father, who had been Treasurer of War, lost all his fortune when his son was quite a child, and CH. XXX. LAGRANGE AND LAPLACE. 267 Lagrange often said that it was partly owing to this mischance that he became a mathematician. His talent showed itself so early that before he was twenty he was appointed Professor of Mathematics in the Military College of Turin, where nearly all his pupils were older than himself. From there he went to the Academy of Sciences at Berlin, and remained twenty years, during which time he worked out most of his celebrated problems. In 1787 he settled in Paris, where he died in 1813, at the age of seventy-seven. Pierre Simon Laplace was the son of a farmer, and was born at Beaumont-en-Auge, near Honfleur, in 1749. He, too, began work very early in life, for in 1769 the famous geometer D'Alembert was so struck with his talents that he procured for him the chair of Mathematics in the Military School of Paris, and from that time for more than fifty years Laplace devoted himself to the pursuit of science, never letting his active life as a politician interfere with his scientific studies. He died in 1827. The work which was done both by Lagrange and Laplace in astronomy was purely mathematical, and dealing as it did with some of the most complicated movements of the heavenly bodies, it cannot be rightly understood by any but mathematicians. But some general idea may be formed of the problems they solved, and we will take these in the order of time, for they treated so much of the same questions, one taking up the subject where the other left it, that it is difficult to separate their work. Libration of the Moon accounted for by Lagrange, 1764-1780.-Long before the time of Lagrange it had been known from observation that the moon always turns the same side of her globe towards our earth as she goes round it, so that we never see, and never can see, more than one side of her surface, so long as she has the same movement as at present. In 1764 the Académie des Sciences offered a prize for a complete explanation of this curious fact, and Lagrange was thus led to study the question, which he solved quite satisfactorily in 1780. Many people find it very difficult to understand how the moon can be always turning round upon her own axis, as a top spins, and yet always keep the same side towards us; therefore, it will be as well to make a simple experiment which explains it quite clearly. Take a round ball and stick a pin in one side of it, then turn the ball slowly round like a teetotum, and notice as it goes round that the pin points successively to each of the sides of the room one after the other; then sew a piece of cotton to the side of the ball opposite the pin, and fasten the other end down to the table (as at E, Fig. 46). If you now roll the ball round the E FIG. 46. M Diagram showing why one side of the M, Ball representing the moon. E, Point , Pin to mark the side of the moon which is never turned towards the earth. table, you will observe that the pin points to each side of the room in succession, as it did before, showing that it has been turning slowly once upon its own axis while going once round the point E, and that, for this reason, the same side has been facing E all the time. This is the case with the moon as she travels round our earth, and Lagrange proved mathematically that it must be so, as Newton had already suggested, on account of the attraction of the earth upon the bulge at the moon's equator. But Lagrange also showed that as the moon moves in an |