mere change of place of the earth. It is related * that he was led to the true explanation by observing that the vane at the top of a boat's mast changed its direction a little whenever the boat was put about, and made to go in a contrary direction; and that on his remarking that it was curious the wind should shift every time the boat was put about, he was assured by the boatman that the same thing always happened. Be this as it may, he proposed to the Royal Society, in 1728, his beautiful explanation of the annual motion which he had observed in the stars; namely, that it is caused by the alteration in the apparent direction of the rays of light, arising from the earth being in motion. Suppose a stream of bullets fired into a carriage in motion, in a line perpendicular to its side, and so directed as to hit the middle of the first window, but not with sufficient velocity to reach any part of the second window. It is plain that they will strike the hinder pannel, which the motion of the carriage brings forward, and that to passengers in the inside the direction of the stream will appear to be from the middle of the window at which it enters to the opposite hinder pannel: whereas, had the carriage. been at rest, it would have appeared to pass through the centre of both windows. And to make the stream really pass through both windows it must, if the carriage be in motion, be directed through the nearer window towards the foremost pannel on the other side. A ray of light is in the same situation with regard to the spectator, both as to the diurnal and the annual motion of the earth. The former gives an insensible aberration only; the latter, one

Professor Rigaud gives this story on the authority of Dr. Thomson's History of the Royal Society,' in which work we find no authority cited We cannot find it in any other place, but are credibly informed that it rests on good traditional evidence.

for it.

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 nutation, 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 con

ceiving 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.

"I was born," says Hogarth in his Memoirs of himself, "in the city of London, November 10, 1697. My father's pen, like that of many authors, did not enable him to do more than put me in a way of shifting for myself. As I had naturally a good eye, and a fondness for drawing, shows of all sorts gave me uncommon pleasure when an infant; and mimicry, common to all children, was remarkable in me. An early access to a neighbouring painter drew my attention from play; and I was, at every possible opportunity, employed in making drawings. I picked up an acquaintance of the same turn, and soon learnt to draw the alphabet with great correctness. My exercises when at school were more remarkable for the ornaments which adorned them, than for the exercise itself. In the former I soon