May 26, 1 9 10] 



NATURE 



387 



Ephemeris for i2h. Berlin M.T. 



1910 

 May 30 



June I 

 „ 2 

 » 3 



R.A. 

 h. m. 

 9 45 o 



9 50'9 



9 56 o 



10 0"4 



10 4"3 



Decl. 



+ '2 53-0 

 ^2 23-5 



+ 1 57-2 

 + 1 34*2 

 + 1 134 



The estimated brightness for May 30 is about 0-5 mag., 

 and for June 2, i-o mag. 



HALLEY AND HIS COMET.' 



WHAT do we know of the nature of a comet's tail? 

 Near the head we notice that there is a dome-shaped 

 envelope which suggests that something has been spouted 

 from the head and turned back by some repelling force 

 of the sun, much as water spouted from a jet into the 

 air is turned back b}- the attraction of the earth. The 

 shape of a fountain is, in fact, closely like that of a comet 

 near the head. The same curve, a parabola, characterises 

 both, and accordingly the adopted view of the nature of 

 the tail is on these lines. But Mr. Eddington has recently 

 directed attention to a grave difficulty ; he has calculated 

 from comet Morehouse, which appeared eighteen months 

 ago. what must be the magnitude of the sun's repelling 

 force to accord with this view, and he finds it almost 

 inconceivably great. The suggestion was made, accord- 

 ingly, that possibly a different view might fit the facts 

 better, viz. that the matter was not spouted from the 

 comet's head and turned back by the sun, but came from 

 the sun and was turned back by the comet. Mr. 

 Eddington has done something already to examine this 

 view, and finds several striking facts in its favour. In 

 the first place, the shape of the curve, the parabola, which 

 has been taken as good evidence for the former view, 

 equally fits the latter. Secondly, since the matter is 

 streaming out from the sun, and the comet is moving 

 across it, we must take into account the motion of the 

 matter relative to the moving comet. It should be possible 

 to test the correctness of the view by observing the direc- 

 tion of the comet's tail, which should not lie accurately 

 in the line away from the sun, but should be slightly 

 inclined to it. Measurements of several photographs seem 

 to support this view. 



Besides the particles of solid matter which exist in the 

 tail (whether spouted from the comet or from the sun), 

 there are also certain gases. Sir William Huggins found 

 with his spectroscope, as early as 1881, that there were 

 certain carbon compounds in the head of a comet which 

 are doubtless also present to some extent in the tail. Some 

 very interesting spectroscopic observations made on the 

 daylight comet showed that there was sodium vapour, not 

 only in the nucleus, but extending out into the tail on 

 either side. The strong yellow lines in the spectrum have 

 been thought to have possibly some other origin, perhaps 

 helium or a carbon compound. It may appear curious 

 that there should be any doubt, but it must be remem- 

 bered that when an object is in motion the lines in its 

 spectrum never correspond exactly with the lines of a 

 terrestrial substance. They are displaced to one side or 

 another according as the object is moving towards or from 

 us. However, in the present instance measures made at 

 the Lick Observatory seem to identify the lines as sodium 

 lines when due allowance is made for the comet's motion 

 away from the earth. Prof. Fowler has recently made 

 an interesting identification of the spectrum of a comet's 

 tail with that of some substance which at present he has 

 not completely recognised, but which he believes to be 

 a compound of carbon. h\-drogen, and oxygen in a state 

 of great tenuity. He can obtain a spectrum in his labora- 

 tory when the pressure is reduced to less than ioo,oooth 

 of atmospheric pressure. 



The particular interest of Halley's comet lies, not in 

 the fact that we shall pass through its tail, not in the 

 magnificence of the spectacle which we hope to see (for 

 this will be much less sensational than, for instance, the 

 comet of 1858, or even of 1882), but in the circumstances 



1 From the Aldred Lecture delivered before the Royal Society of Art; on 

 May 4 by Prof. H. H. Turner, F.R.S. 



which first brought its existence to the knowledge of the 

 world. It was the first comet of which the return was 

 predicted witli success. H alley reaped this reward, the 

 great reward of having an enduring monument in the 

 skies, as the well-deserved outcome of his devotion to 

 science. He was one of those men who seem to have an 

 instinct for discerning at any particular moment the most 

 important thing to be done, and the energy to do it, such 

 a man as would have made a great general or a great 

 statesman, or succeeded in many other walks of life, and 

 such a man as is only too rare in the history of science. 

 When an undergraduate at Oxford (Queen's College) he 

 was struck by the fact that our knowledge of the stars 

 was practically limited to those of the northern hemi- 

 sphere, and he determined personally to rectify this at the 

 earliest possible moment. Accordingly, he did not wait 

 even to take his degree, but started for St. Helena to 

 make a catalogue of southern stars. Bad weather 

 rendered his expedition far less successful than he hoped, 

 but, nevertheless, a beginning was made, and the founda- 

 tions of our knowledge of the southern hemisphere were laid. 

 He gracefully acknowledged the help and patronage of the 

 King by naming a small constellation " Robur Caroli II->" 

 but the name was afterwards omitted by a German editor. 

 His name for a striking double star, Cor Caroli, has 

 survived. In later years his studies of the positions of 

 the stars convinced him that they were not really fixed 

 in their places, but that some of them must have moved 

 since the observations of Ptolemy. From this beginning, 

 due to Halley, springs our knowledge of the proper 

 motions of the stars and all that has come from it. 



Another of Halley's great enterprises was the attempt to 

 determine longitude at sea. It is quite easy for a sailor to 

 determine his latitude (how far, that is, he is north or 

 south of the equator), but his longitude is a different 

 matter, and the uncertainty- often led to grave difficulties 

 and often disasters. Here, again, Halley saw that a great 

 effort must be made to remove this disabilit>-. He 

 obtained the loan of a ship from the King (not Charles II. 

 by this time, but William of Orange), and was instructed 

 to make a long voyage, especially in the southern hemi- 

 sphere, to observe the magnetic variation, that is to say, 

 the amount by which the compass points east or west of 

 due north in various localities. In spite of the mutiny 

 of his lieutenant (which caused his return home after first 

 setting out), he completed his voyage. He made a chart 

 of the variations, and thus solved, for the first time, this 

 problem of determining the longitude. His solution was 

 verv soon superseded by a better, in the invention of the 

 chronometer by John Harrison : but this does not alter 

 the fact that Halley was the first to give a solution at 

 all. Even his own defects seem to have inspired him to 

 a method of compensating them. He was himself not a 

 verv accurate observer, nor skilled in the use of the 

 instruments of a fixed observatory. Possibly his experi- 

 ence with rough and ready methods at sea may have con- 

 tributed to this end. Be that as it may, he was so far 

 out of personal sympathy with accuracy in observation 

 that he used to say, " Take care of the minutes of arc 

 and the seconds will take care of themselves." _ It is, 

 therefore, not to be wondered at that his observations as 

 Astronomer Royal were not of any value ; and it might 

 have happened that he would look for a successor equally 

 indifferent to accuracy, but he was too great a man to 

 make such a mistake. He set his heart upon being 

 succeeded by Bradley, the most painstaking and accurate 

 obser%-er of the time (possibly of all past time), and 

 Bradley's skill soon made up for any deficiencies of the 

 previous twent}' years. 



But Halley ' showed his greatest insight in connection 

 with the discovery of the great law of gravitation. He 

 shared with others the suspicion (for in i6S^ it amounted 

 to little more) that a law of attraction, varying as the 

 inverse square of the distance, would explain the move- 

 ments of the planets round the sun ; but while others 

 were content to let the matter rest there, or to claim, 

 falsely, that they had satisfactorily proved the proposition, 

 Halley saw the vital necessity of sifting the matter to the 

 bottom. When other inquiries had failed, he determined 

 to visit Newton, at Cambridge, with the idea of enlisting 

 his help in solving this great problem. To his great delight 

 he found that it was already solved. Newton had already 



NO. 21 17, VOL. 83] 



