interested, and of which he afterwards published a chart; the second to ascertain the latitudes and longitudes of the principal points in the British Channel, and the course of the tides. In 1703 he was elected Savilian Professor of Geometry, on the death of the celebrated Wallis. He received, about the same time, the degree of Doctor of Laws, which is conferred without requiring subscription to the Articles of the Church. In his connexion with the University he superintended several parts of the edition of the Greek Geometers, which was printed at the University press.
Halley succeeded Sir Hans Sloane, in 1713, as Secretary to the Royal Society; and, in 1719, on the death of Flamsteed, he was appointed Astronomer Royal at Greenwich. In this employment he continued till his death, under the patronage of Queen Caroline, wife of George II., who procured for him the half-pay of the rank he formerly held in the navy. In 1737 he was seized with a paralytic disorder; but nevertheless continued his labours till within a short time of his death, which took place in January, 1742, at the age of eighty-five. He was interred at Lee, near Blackheath, where a monument was erected to him and his wife by their two daughters.
In person Dr. Halley was rather tall, thin, and fair, and remarkable as well for energy as vivacity of character. He cultivated the friendship and acquired the esteem of his most distinguished contemporaries, and particularly of Newton, spite of their very different opinions. Indeed it may be said that to him we owe, in some degree, the publication of the ‘Principia;’ for Halley being engaged upon the consideration of Kepler’s law, as it had been discovered by observation, viz., that the squares of the periodic times of planets are as the cubes of their distances, and suspecting that this might be accounted for on the supposition of a centripetal force, varying inversely as the square of the distance, applied himself to prove the connexion geometrically, in which he was unable to succeed. In this difficulty he applied to Hook and Wren, neither of whom could help him, and was recommended to consult Newton, then Lucasian Professor at Cambridge. Following this advice, he found in Newton all he wanted; and did not rest until he had persuaded his new acquaintance to give the results of his discoveries to the world. In about two years after this, the first edition of the ‘Principia’ was published, and the proofs were corrected by Halley, who supplied the well-known Latin verses which stand at the beginning of the work.
In conversation, Halley appears to have been of a jocose and somewhat satirical disposition. The following anecdote of him, which is told by Whiston, displays the usual modesty of the latter, when speaking of himself: “On my refusal from him of a glass of wine on a Wednesday or Friday, he said he was afraid I had a pope in my belly, which I denied, and added somewhat bluntly, that had it not been for the rise now and then of a Luther or a Whiston, he would himself have gone down on his knees to St. Winifred or St. Bridget, which he knew not how to contradict.” It is related that when Queen Caroline offered to obtain an increase of Halley’s salary as Astronomer Royal, he replied, “Pray, your Majesty, do no such thing, for should the salary be increased, it might become an object of emolument to place there some unqualified needy dependant, to the ruin of the institution.” And yet the sum which he would not suffer to be increased was only £100 a-year.
To give even a catalogue of the various labours of Halley, would require more space than we can here devote to the subject. For a more detailed account both of his life and discoveries, we must refer the reader to the Biographia Britannica, to Delambre, Histoire de l’Astronomie au dix-huitième Siecle, livre II., and the Philosophical Transactions of the time in which he lived; or better perhaps to the Miscellanea Curiosa, London, 1726, a selection of papers from the Transactions, containing the most remarkable of those written by Halley. We shall, nevertheless, proceed briefly to notice a few of the discoveries on which the fame of our astronomer is built.
The most remarkable of them, to a common reader, is the conjecture of the return of a comet. Some earlier astronomers, as Kepler, had imagined the motion of these bodies to be rectilinear. Newton, in explaining the principle of universal gravitation, showed how a comet might describe a parabola, and also how to calculate its motion, and compare it with observation. Hevelius had already indicated the curvature of a comet’s path, and Dörfel, a Saxon clergyman, had calculated the path of the comet of 1680 upon this supposition. Halley, in computing the parabolic elements of all the comets which had been well observed up to his time, suspected, from the general likeness of the three, that the comets of 1531, 1607, and 1682, were the same. He was the more confirmed in this, by knowing that comets had been seen, though no good observations were recorded, in the years 1305, 1380, and 1456, giving, with the former dates, a chain of differences of 75 and 76 years alternately. Halley supposed, therefore, that the orbit of this comet was, not a parabola, but a very elongated ellipse, and that it would return about the year 1758. The truth of his conjecture was fully confirmed in January, 1759, by Messier. The first person, however, who saw Halley’s comet, as it is now called, was George Palitzch, a farmer in the neighbourhood of Dresden, who had studied astronomy by himself, and fitted up a small observatory.
But a much more useful exertion of Halley’s genius and power of calculation is to be found in his researches on the lunar theory. It is to him that we are indebted for first starting the idea of finding the longitude at sea by means of the moon’s place, which is now universally adopted. The principle of this problem is as follows. An observer at sea can readily find the time of day by means of the sun or a star, and can thereby correct a watch. If he could at the same moment in which he finds his own time, also discover that at Greenwich, the difference between the two, turned into degrees, minutes, and seconds, would be his longitude east or west of Greenwich. If, therefore, he carries with him a Nautical Almanac, in which the times of various astronomical phenomena are registered, as they will take place at Greenwich, or rather as they will be seen by an observer placed at the centre of the earth with a Greenwich clock, he can observe any one of these phenomena, and reduce it also to the centre. He will then know the corresponding moments of time, for his own position and that of Greenwich. The moon traverses the whole of its orbit in little more than 27 days, and therefore moves rapidly with respect to the fixed stars, its motion being nearly a whole sign of the zodiac in 48 hours. If we observe the distance between the moon and a star, and find it to be ten degrees, the longitude of the place in which the observation is made can be known as aforesaid, if the almanac will tell what time it was at Greenwich when the moon was at that same distance from the star. In the time of Halley, though it was known that the moon moved nearly in an ellipse, yet the elements of that ellipse, and the various irregularities to which it is subject, were very imperfectly ascertained. It had, however, been known even from the time of the Chaldeans, that some of these irregularities have a period, as it is called, of little more than eighteen years, that is, begin again in the same order after every eighteen years; the periods and quantities of several other errors had also been discovered with something like accuracy. To make good lunar tables, that is, tables from which the place of the moon might be correctly calculated beforehand, became the object of Halley’s ambition. He therefore observed the moon diligently during the whole of one of the periods of eighteen years, that is, from the end of 1721 to that of 1739, and produced tables which were published in 1749, after his death, and were of great service to astronomers. He also made another observation on the motion of the moon, which has since given rise to one of the finest discoveries of Laplace. In calculating from our tables the time of an ancient eclipse, observed at Babylon, BC 720, he found that, had the tables been correct, it would have happened three hours sooner than, according to Ptolemy, it did happen. This might have arisen from an error in the Babylonian observation; but on looking at other eclipses, he found that the ancient ones always happened later than the time indicated by his table, and that the difference became less and less as he approached his own time. From hence he concluded that the moon’s average daily motion is subject to a very small acceleration, so that a lunar month at present is in a very slight degree shorter than a month in the time of the Chaldeans. This was afterwards shown by Laplace to arise from a very slow diminution in the eccentricity of the earth’s orbit, caused by the attraction of the planets. For a further account of Halley’s astronomical labours, we may refer to the History of Astronomy in the Library of Useful Knowledge, page 79.
We must also ascribe to Halley the first correct application of the barometer to the measurement of the
