to the diurnal and the annual motion of the earth. The former gives an insensible aberration only; the latter, one which though small is sensible. The smallness of the latter aberration arises from the velocity of light being more than ten thousand times that of the earth in its orbit. And it must be remembered that the motion of light was not an hypothesis, invented to form the basis of Bradley's explanation, but was ascertained before his time, by Römer, from a phenomenon of an entirely different nature; namely, the retardation observed in the eclipses of Jupiter's satellites, as the planet moved from the earth. The absolute deduction of the laws of aberration was completed by Bradley.

The other great discovery of Bradley, namely, the mutation, or oscillatory motion of the earth's axis, was completed in 1747. In his Wanstead observations he had observed some minute discrepancies, which at that time might be attributed to errors of observation; but after he was able to clear the apparent place of a star from the effects of aberration, the field became open to consider and assign the laws of smaller variations. By continual observation, he found a small irregularity in the places of the stars, depending upon the position of the moon's node. Newton had already shown it to be a consequence of gravitation, that the sun must produce a small oscillation in the earth's axis: Bradley showed that a larger oscillation must arise from the moon, and be completed in the course of a revolution, not of the moon, but of the point where her orbit cuts the ecliptic. This discovery is therefore not of so original a character as the last, since astronomers had for some time been in the habit of trying to reconcile every discrepancy which they observed by supposing a nutation; but to Bradley belongs the merit of discovering that small irregularity which really can be reconciled to such a supposition, and its physical causes. The easiest way of conceiving the effect of nutation is as follows:-The precession of the equinoxes, discovered by Hipparchus, has this effect, that the fixed stars, so called, appear to move round the pole of the ecliptic, at the rate of a revolution in about 26,000 years. Instead of a star, let a small oval describe the same course, and let the star in the mean while move round that oval in the course of nineteen years. The motion thus obtained will represent the combined effect of precession and nutation.

To these discoveries of Bradley we owe, as Delambre observes, the accuracy of modern astronomy. It must be remarked, that no individual, whose previous labours have caused public opinion to point

him out as most fit for the part of Astronomer Royal, has ever been passed over when occasion occurred, from the time of Flamsteed to that at which we write. It is the fair reward of such a course, that the reputation which each successive occupant brought to that position should be considered as appertaining to him in the public capacity which it gained for him; and this being granted, it may be truly said that there is no institution in the world which has, upon the whole, done so much towards the advancement of correct astronomy as the Observatory of Greenwich.