5°6 



NA TURE 



{Sept. 18, 1884 



which appear to be diametrically opposite. Dr. Huggins con- 

 siders that he has succeeded in photographing the corona in full 

 sunshine, and so in establishing its objective reality as an im- 

 mense solar appendage, sub-permanent in form, and rotating with 

 the globe to which it is attached. One may call it " an atmo- 

 sphere," if the word is not to be too rigidly interpreted. I am 

 bound to say that pla'es which he has obtained do really show 

 just such appearances as would be produced by such a solar 

 appendage, though they are very faint and ghost-like. I may 

 add further, that, from a letter from Dr. Huggins, recently re- 

 ceived, I learn that he has been prevented from obtaining any 

 similar plates in England this summer by the atmi spheric haze, 

 but that Dr. Woods, who has been provided with a similar 

 apparatus, and sent to the Riffelberg in Switzerland, writes that 

 he has "an assured success " 



Our American astronomer, on the other hand, at the last 

 eclipse (in the Pacific Ocean), observed certain phenomena which 

 seem to confirm a theory he had formulated some time ago, and 

 to indicate that the lovely apparition is an apparition only, a 

 purely optical effect due to the diffraction (not ; (fraction, nor 

 reflection either) of light at the edge of the moon —no more a 

 solar appendage than a rainbow or a mock sun. There are 

 mathematical considerations connected with the theory which 

 may prove decisive when the paper of its ingenious and able 

 proposer comes to be published in full. In the meantime it 

 must be frankly conceded that the observations made by him are 

 very awkward to explain on any other hypothesis. 



Whatever may be the result, the investigation of the statu- and 

 possible extent of a nebulous envelope around a sun or a star is 

 unquestionably a problem of very great interest and importance. 

 We shall be compelled, I believe, as in the case of ■ 

 recognise other knees than gravity, heat, and ordinary gaseous 

 elasticity, as concerned in the phenomena. As regards the 

 actual existence of an extensive gaseous envelope around the sun, 

 I may add that other appearances than those seen at an eclipse 

 seem to demonstrate it beyond question, — phenomena such as 

 the original formation of clouds of incandescent hydrogen at high 

 elevations, and the forms and motions of the loftiest promin- 

 ences. 



But of all solar problems, the one which excites the di 1 

 and most general interest is that relating to the solar heat, its 

 maintenance and its duration. For my own part, I find no 

 fault with the solution proposed by Helmholtz, who accounts for 

 it mainly by the slow contraction of the solar sphere. The only 

 objection of much force is that it apparently limits the past 

 duration of the solar system to a period not much exceeding some 

 twenty millions of years, and many of our geological friends 

 protest against so scanty an allowance. The same theory would 

 give us, perhaps, half as much time for our remaining lifetime ; 

 but this is no objection, since I perceive no reason to doubt the 

 final cessation of the sun's activity, and the consequent death of 

 the system. But while this hypothesis seems fairly to meet the 

 requirements of the case, and to be a necessary consequence of 

 the best knowledge we can obtain as to the genesis of our system 

 and the constitution of the sun itself, it must, of course, be con- 

 ceded that it does not yet admit of any observational verification. 

 No measurements within our power can te-t it, so far as we can 

 see at present. 



It may be admitted, too, that much can be said in favour of 

 other theories ; such as the one which attributes the solar heat 

 to the impact of meteoric matter, and that other most interesting 

 and ingenious theory of the late Sir William Siemens. 



As regards the former, however, I see no escape from the 

 conclusion, that, if it were exclusively true, the earth ought to 

 be receiving, as was pointed out by the late Prof. Peirce, as 

 much heat from meteors as from the sun. This would require 

 the fall of a quantity of meteoric matter, more than sixty 

 million times as much as the best estimates make our present 

 supply, and such as could not escape the most casual observa- 

 tion, since it would amount to more than a hundred and fifty ' 

 tons a day on every square mile. 



1 In an article on astronomical collisions, published in the North 

 American Review about a year ago, 1 wrongly stated the amount at fifty 

 tuns. There was some fatality connected with my calculations for that 

 article. I gave the amount of heat due to the five hundred tons of meteoric 

 matter which is supposed to fall daily on the earth with an average velocay 

 of fifteen miles per second as fifty-three calories annually per square me;re, 

 — a quantity two thousand times too great. Probably the error would have 

 been noticed if even the number given had not been so small, compared with 

 the solar heat, as fuily to justify my argument, which is only strengthened 

 by the c Fraction. I owe the correction to Prof. LeConte of California, who 

 called my attention to the errors.' 



As regards the theory of Siemens, the matter has been, of 

 late, so thoroughly discussed, that we probably need spend no 

 time upon it here. To say nothing as to the difficulties connected 

 with the establishment of such a far-reaching vortex as it 

 demands, nor of the fact that the temperature of the sun's 

 surface appears to be above that of the dissociation point of 

 carbon compounds, and hence above their highest heat of 

 combustion, it seems certainly demonstrated that matter of the 

 necessary density could not exist in inter-planetary space without 

 seriously affecting the planetary motions by its gravitating action 

 as well as by its direct resistance ; nor could the stellar radiations 

 reach us, as they do, through a medium capable of taking up 

 and utilising the rays of the sun in the way this theory supposes. 



And yet I imagine that there is a very general sympathy with 

 the feeling that led to the proposal of the theory, — an uncom- 

 fortable dissatisfaction with received theories, because they 

 admit that the greater part of the sun's radiant energy is, 

 speaking from a .scientific point of view, simply wasted. Nothing 

 like a millionth part of the sky, as seen from the sun, is occupied, 

 so far as we can make out, by objects upon which its rays can 

 fall : the rest is vacancy. If the sun sends out rays in all 

 directions alike, not one of them in a million find, a target, or 

 accomplishes any useful work, unless there is in space tome 

 medium to utilise the rays, or unknown worlds, of which we 

 have no cognisance, beyond the stars. 



Now, for my own part, I am very little troubled by accusations 

 1 f wastefulness against Nature, or by demands for theories which 

 will show what the human mind can recognise as " use " for all 

 energy expended. Where I can perceive such use, I recognise 

 it with reverence and gratitude, I hope ; but the failure to 

 recognise it in other cases creates in my mind no presumption 

 against the wis 'om of Nature, or against the correctness of an 

 hypothesis otherwise satisfactory. It merely suggests human 

 limitations and ignorance. How can one without sight under- 

 stand what a telescope is good for ? 



At the same time, perhaps we assume with a little too much 

 confidence that, in free space, radiation does take place equally 

 in all directions. Of course, if the received views as to the 

 nature and conduct of the hypothetical "ether'' are correct, 

 there is no possibility of questioning the assumption ; but, as 

 Sir John Ilerschel and others have pointed out, the properties 

 which must be ascribed to this "ether," to fit it for its various 

 functions, are so surprising and almost inconceivable, that one 

 ma) bi pardoned for son. e reserve in accepting it as a finality. 

 At any rate, as a fact, the question is continually started (the 

 idea has been brought out repeatedly, in some cases by men of 

 real and recognised scientific and philosophic attainment), 

 whether the constitution of things may not be such that radiation 

 and transfer of energy can take place only between ponderable 

 masses ; and that, too, without the expenditure of energy upon 

 the transmitting-agent (if such exist) along the line of transmission, 

 even in transitu. If this were the case, then the sun would 

 send out its energy only to planets and meteors and sister-stars, 

 wasting none in empty space ; and so its loss of heat would be 

 enormously diminished, and the time-scale of the life of the 

 planetary system would be correspondingly exti tded. So far as 

 I know, no one has ever yet been able to indicate any kind of 

 medium or mechanism by which vibrations, such as we know to 

 constitute the radiant energy of light and heat, can be transmitted 

 at all from sun to planet under such restrictions. Still one ought 

 not to be too positive in assertions as to the real condition and 

 occupancy of so-called vacant space. The "ether" is a good 

 working hypothesis, but hardly more as yet. 



I need not add that a most interesting and as yet inaccessible 

 problem, connected with the preceding, is that of the mechanism 

 of gravitation, and, indeed, of all forces that seem to act at a 

 as, for that matter, in the last analysis, all forces do. 

 If there really be an " ether," then it would seem that somehow 

 all attractions and repulsions of ponderable matter must be due 

 to its action. Challis's investigations and conclusions as to the 

 effect of hydrodynamic actions in such a medium do not seem to 

 have commanded general acceptance ; and the field still lies open 

 for one who will show how gravitation and other forces can be 

 correlated with each other through the ether. 



Meteors and comets, seeming to belong neither to the solar 

 system nor to the stellar universe, present a crowd of problems 

 as difficult as they are interesting. Much has undoubtedly been 

 gained during the last few decades ; but in some respects that 

 which has been learned has only deepened the mystery. 



The problem of the origin of comets has been supposed to be 

 solved to a certain extent by the researches of Schiaparelli, Heis, 



