388 



NATURE 



[May 26, 1910 



proved that an object attracted by the sun in this way 

 would move round it in an ellipse, and, since Kepler 

 had already shown that the planets did move in ellipses, 

 the existence of gravitation was established. Halley saw, 

 further, how important it was to publish this discovery, 

 and paid for the publication from his own pocket when 

 other means failed. It might also be said that in this one 

 particular of recognising the thing to be done and doing 

 it Halley compared favourably with his great friend 

 Newton, for Newton, after proving the proposition, had 

 tossed it aside, shrinking from publication, which had 

 sometimes engaged him in disagreeable controversy. Had 

 it not been for Halley we might have lost this discovery, 

 at any rate for a number of years ; but Newton, though 

 he was diffident as to publication, and failed perhaps to 

 realise the importance of his discovery to the world, proved 

 his own remarkable insight in a different manner. He, 

 and he alone, saw that, even after the proof about the 

 ellipse had been formulated, there was j'et a grave difficulty 

 in accounting completely for a universal gravitation as 

 the cause, not only of the planetary movements, but of 

 the falling of objects towards the earth. He realised that 

 there was need of a further proposition, which he ulti- 

 mately succeeded in proving, viz. that a sphere, however 

 large, would attract another sphere, however close to it, 

 as though it were concentrated at the centre. This com- 

 pleted the great law which was given to the world in 

 1686 ; but without this great proposition the law of gravi- 

 tation would have remained a mere rough approximation 

 instead of taking its place at once, and thereafter, as the 

 most accurate law known to science. 



Not only were the movements of the planets explained, 

 but it became possible to calculate the orbits of comets, 

 and Halley took the earliest opportunity of calculating 

 as many as he could. This opportunity did not, however, 

 come to him for nearly twenty years, since he was full 

 of other projects. It was in the interval that he made 

 his great voyage for determining the longitude ; and it 

 was not until 1704, when he had been elected Savilian 

 professor of geometry at Oxford, that he found the leisure 

 to make the cometary calculations required. The result 

 was a list of twenty-four orbits, representing all the comets 

 which had been sufficiently observed. He had the insight 

 to see that this was a thoroughly important thing to do, 

 though neither he nor anyone else suspected what was to 

 be the most important outcome. Three of the twertty-four 

 orbits were found to be nearly the same, those of the 

 comets of 153 1, 1607, and 1682. The figures were so 

 closely the same that he felt sure they must relate to 

 the same comet, though there was one difficulty, in that 

 the interval between the two returns was not the same. 

 This discrepancy, however, he rightly ascribed to the 

 influence of the planets, especially Jupiter and Saturn. 

 He pointed out that these planets disturbed one another, 

 and therefore would disturb a comet too. He put very 

 clearly also the point that the disturbance would be much 

 greater in the case of a comet. We have said that a 

 comet travels away to a great distance from the sun, and 

 comes almost to a resting point ; indeed, as Halley re- 

 marked, a very slight change in its velocity would send 

 it away altogether from the sun's influence, that is to 

 say, would make all the difference between a finite and an 

 infinite time for its return. Hence a smaller disturbance 

 might easily affect the time of its return seriously. There 

 was no difficulty that he could see in identifying the three 

 comets 33 the same, and in predicting a subsequent return 

 in 1758. Halley did not live to see the return, but died 

 in 1742, at the age of eighty-five, but he left on record 

 his conviction that the comet would appear again, and 

 his hope that posterity would remember to credit an 

 Englishman with <-he prediction. 



It was first seen on Christmas Day, 1758. It returned 

 again in 1835, and now it is with us once more. The 

 interval between the returns varies a few years, from 

 about seventy-five to nearly eighty, the variation being 

 due, as Halley surmised, to the interference of the planets 

 with the comet's movements. It is one thing to realise, 

 as Halley did, the general nature of this interference, and 

 quite another matter, involving much laborious calcula- 

 tion, to determine it accurately. Halley, with his national 

 pride, would have been delighted at the skill with which 

 this problem has recently been attacked by two of his 



NO. 21 17, VOL. 83] 



countrymen, Mr. Cowell and Mr. Crommelin, of the Royal 

 Observatory at Greenwich. Mr. Cowell, after trying old 

 methods with more or less success, found it advisable to 

 employ a totally new method. It has been the custom, 

 hitherto, in such calculations to consider the path of the 

 comet as an ellipse, according to Newton's great discovery, 

 but an ellipse which is continually changing in detail 

 owing to the disturbances of the planets. Mr. Cowell, 

 however, has found it desirable to abandon the idea of tlie 

 ellipse altogether, and to follow the comet, step by step, 

 along its orbit, first forecasting a little ahead, then calcu- 

 lating the consequent attractions of the planets, verifying 

 his forecast or modifying it, if required, and so, by 

 making each step secure before proceeding to the next, 

 obtaining, ultimately, the complete and accurate history 

 of the comet between two returns. It will readily be 

 understood that a process of this kind involves much 

 labour. During seventy-four years the comet was followed 

 by calculation alone, for it was hopeless to attempt to 

 observe it. As a result of the calculations, it was pre- 

 dicted that it would be seen in a certain place in the sky 

 in August and September, 1909, and on taking a photo- 

 graph of this region the comet was found close to the 

 predicted place. The honour of first identifying it in this 

 way falls to Dr. Max Wolf, of Heidelberg, but it is some 

 compensation to Englishmen that, with the help of Dr. 

 Wolf's information, they were able to identify earlier 

 images of the comet on photographs taken at the Royal 

 Observatory, Greenwich, and earlier still at the new 

 Observatory of Helwan, in Egypt. The generosity of Mr. 

 Reynolds, of Birmingham, has furnished this last observa- 

 tory with a fine reflecting telescope, the mirror of which, 

 30 inches in diameter, was, like that at Greenwich, made 

 by the late Dr. Common, of Ealing ; and with this fine 

 instrument, in the splendid climate of Egypt, the first 

 picture of Halley's comet at its present return was secured. 

 Messrs. Cowell and Crommelin have not only calculated 

 the recent history of the comet, they have carried it back 

 through the centuries as far as 240 b.c. We have see" 

 that Halley collected all the observations of comets 

 sufficiently good and numerous to enable him to calculate 

 orbits, and these did not include any comet earlier than 

 1337, nor any appearance of his own particular comet 

 before 1531. But more observations have come to light 

 since then, especially in Chinese annals, and on further 

 search being made, in the first instance by Mr. Hind some 

 half a century ago, it was found that bright comets had 

 appeared in 1456, in 1378, in 1301, and so on, which could 

 with great probability be identified with Halley's. This 

 probability Messrs. Cowell and Crommelin have now con- 

 verted into a certainty. It was not possible, or even 

 necessary, to carry back the computations into the past 

 with the same accuracy as was adopted for the last re- 

 turn, for the observations available for identification were, 

 in many instances, very rough, specifying, for instance, 

 the position of the comet by the constellation in which^ it 

 was seen ; but they adopted something of the same prin- 

 ciple of proceeding step by step, making the appearance 

 at one return secure before working back from it to the 

 preceding, and thus they were able to correct severa 

 mistakes in Hind's original list. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — The Council recommends that a grant 

 lOoL from the Worts Fund be made to Mr. E. A. Wilson 

 of Gonville and Caius College, who has been entruste( 

 with the organisation of the scientific department _ of ih 

 British Antarctic Expedition, 1910, towards defraying tb 

 expense of the equipment. 



The scientific staff of the expedition includes Messrs 



D. G. Lillie, of St. John's College; E. W. Nelson, 

 Christ's College; T. G. Taylor, of Emmanuel Colleg| 



E. A. Wilson, of Gonville and Caius College ; and C. S 

 Wright, of Gonville and Caius College. 



Grants of 50Z. to Mr. C. E. Moss, curator of the Uni- 

 versity Herbarium, towards defraying the expense of 

 botanical investigations which he proposes to make on the 

 Continent of Europe; and of 25?. to Mr. R. H. Rastall, 

 towards defraying the expense of a visit which he proposes 



