way or the other. Here was an image of his case: the boat was the earth moving in different directions at different seasons, and the wind was the light of a star. He had now to trace the consequences of this idea; he found that it led to his empirical rules, and, in 1729, he gave his discovery to the Royal Society. His paper is a very happy narrative of his labours and his thoughts. His theory was so sound that no astronomer ever contested it; and his observations were so accurate, that the quantity which he assigned as the greatest amount of the change (one ninetieth of a degree) has hardly been corrected by more recent astronomers. It must be noticed, however, that he considered the effects in declination only; the effects in right ascension required a different mode of observation, and a consummate goodness in the machinery of clocks, which at that time was hardly attained.

Sect. 4.-Discovery of Nutation.

WHEN Bradley went to Greenwich as Astronomer Royal, he continued with perseverance observations of the same kind as those by which he had detected aberration. The result of these was another discovery; namely, that very nutation which he had formerly rejected. This may appear strange, but it is easily explained. The aberration is an annual change, and is detected by observing a star at different seasons of the year; the nutation is a change

of which the cycle is eighteen years; and which, therefore, though it does not much change the place of a star in one year, is discoverable in the alterations of several successive years. A very few years' observations showed Bradley the effect of this change'; and long before the half cycle of nine years had elapsed, he had connected it in his mind with the true cause, the motion of the moon's nodes. Machin was then secretary to the Royal Society, and was 66 employed in considering the theory of gravity, and its consequences with regard to the celestial motions:" to him Bradley communicated his conjectures; from him he soon received a table containing the results of his calculations; and the law was found to be the same in the table and in observation, though the quantities were somewhat different. It appeared by both, that the earth's pole, besides the motion which the precession of the equinoxes gives it, moves, in eighteen years, through a small circle ;—or rather, as was afterwards found by Bradley, an ellipse, of which the axes are nineteen and fourteen seconds".

For the rigorous establishment of the mechanical theory of that effect of the moon's attraction from which the phenomena of nutation flow, Bradley rightly and prudently invited the assistance of the great mathematicians of his time. D'Alembert, Thomas Simpson, Euler, and others, answered this

• Rigaud, lxiv.

11 Ib. lxvi.

10 Ib. 25.

call, and the result was, as we have already said in the last chapter, that this investigation added another to the recondite and profound evidences of the doctrine of universal gravitation.

It has been said" that Bradley's discoveries "assure him the most distinguished place among astronomers after Hipparchus and Kepler." If his discoveries had been made before Newton's, there could have been no hesitation as to placing him on a level with those great men. The existence of such suggestions as the Newtonian theory offered on all astronomical subjects, may perhaps dim, in our eyes, the brilliance of Bradley's achievements; but this circumstance cannot place any other person above the author of such discoveries, and therefore we may consider Delambre's adjudication of precedence as well warranted, and deserving to be permanent.

Sect. 5.-Discovery of the Laws of Double Stars. The two Herschels.

No truth, then, can be more certainly established, than that the law of gravitation prevails to the very boundaries of the solar system. But does it hold good further? Do the fixed stars also obey this universal sway? The idea, the question, is an obvious one,—but where are we to find the means of submitting it to the test of observation?

12 Delambre, Ast. du 18 Siéc. p. 420. Rigaud, xxxvii.

If the stars were each insulated from the rest, as our sun appears to be from them, we should have been quite unable to answer this inquiry. But among the stars, there are some which are called double, and which consist of two stars, so near to each other that the telescope alone can separate them. The elder Herschel diligently observed and measured such stars; and as has so often happened in astronomical history, pursuing one object he fell in with another. Supposing such pairs to be really unconnected, he wished to learn, from their phenomena, something respecting the annual parallax of the earth's orbit. But in the course of twenty years' observations he made the discovery (in 1803) that these couples were turning round each other with various angular velocities. These revolutions were, for the most part, so slow that he was obliged to leave their complete determination as an inheritance to the next generation. His son was not careless of the bequest, and after having added an enormous mass of observations to those of his father, he applied himself to determine the laws of these revolutions. A problem so obvious and so tempting was attacked also by others, as Savary and Encke, in 1830 and 1832, with the resources of analysis. But a problem in which the data are so minute and inevitably imperfect, required the mathematician to employ much judgment, as well as skill in using and combining these data; and Herschel, by employing positions only of the line joining the pair of stars,

to the exclusion of their distances, and by inventing a method which introduced the whole body of observations, and not selected ones only, into the determination of the motion, has made his investigations by far the most satisfactory of those which have appeared. The result is, that it has been rendered very probable, that the double stars describe ellipses. about each other; and therefore that here also, at an immeasureable distance from our system, the law of attraction according to the inverse square, prevails. And, according to the practice of astronomers when a law has been established, tables have been calculated for the future motions; and we have ephemerides of the revolutions of suns round each other, in a region so remote, that the whole circle of our earth's orbit, if placed there, would be imperceptible by our strongest telescopes. The permanent comparison of the observed with the predicted motions, continued for more than one revolution, is the severe and decisive test of the truth of the theory: and the result of this test astronomers are now awaiting.

The verification of Newton's discoveries was sufficient employment for the last century; the first step in the extension of them belongs to this century. We cannot at present foresee the magnitude of this task, but every one must feel that the law of gravitation, before verified in all the particles of our own system, and now extended to the all but infinite distance of the fixed stars, presses upon our minds with