198 



SCIENCE, 



[Vol. IV., No. 83. 



periods, on the one hand, and, on the other, the 

 planets' distances from the sun, their diameters and 

 masses. More than thirty years ago, Professor Kirk- 

 wood supposed that he had discovered the relation in 

 the analogy which hears his name. The materials 

 for testing and establishing it were then, however, 

 insufficient, and still remain so, leaving far too many 

 of the data uncertain and arbitrary. Could such a 

 relation be discovered, it could hardly fail to have 

 a most important significance with respect to theo- 

 ries of the origin and development of the planetary 

 system. 



The great problem of the absolute dimensions of 

 our system is, of course, commanded by the special 

 problem of the solar parallax; and this remains a prob- 

 lem still. Constant errors of one kind or another, 

 the origin of which is still obscure, seem to affect the 

 different methods of solution. Thus, while experi- 

 ments upon the velocity of light and heliometric 

 measurements of the displacements of Mars among 

 the stars agree remarkably in assigning a smaller 

 parallax (and greater distance of the sun) than seems 

 to be indicated by the observations of the late transits 

 of Venus, and by methods founded on the lunar 

 motions, on the other hand, the meridian observa- 

 tions of Mars all point to a larger parallax and smaller 

 distance. While still disposed to put more confidence 

 in the methods first named, I, for one, must admit 

 that the margin of probable error seems to me to 

 have been rather increased than diminished by the 

 latest published results deduced from the transits. 

 I do not feel so confident of the correctness of the 

 value 8".80 for the solar parallax as I did three years 

 ago. In its very nature, this problem is one, how- 

 ever, that astronomers can never have done with. 

 So fundamental is it, that the time will never come 

 when they can properly give up the attempt to in- 

 crease the precision of their determination, and to 

 test the received value by every new method that 

 may be found. 



The problems presented by the sun alone might 

 themselves well occupy more than the time at our 

 disposal this evening. Its mass, dimensions, and 

 motions, as a whole, are, indeed, pretty well deter- 

 mined and understood ; but when we come to ques- 

 tions relating to its constitution, the cause and nature 

 of the appearances presented upon its surface, the 

 periodicity of its spots, its temperature, and the main- 

 tenance of its heat, the extent of its atmosphere, and 

 the nature of the corona, we find the most radical 

 differences of opinion. 



The difficulties of all solar problems are, of course, 

 greatly enhanced by the enormous difference between 

 solar conditions and the conditions attainable in our 

 laboratories. We often reach, indeed, similarity suf- 

 ficient to establish a bond of connection, and to afford 

 a basis for speculation ; but the dissimilarity remains 

 so great as to render quantitative calculations unsafe, 

 and make positive conclusions more or less insecure. 

 We can pretty confidently infer the presence of iron 

 and hydrogen and other elements in the sun by ap- 

 pearances which we can reproduce upon the earth ; 

 but we cannot safely apply empirical formulae (like 



that of Dulong and Petit, for instance), deduced 

 from terrestrial experiments, to determine solar tem- 

 peratures : such a proceeding is an unsound and un- 

 warrantable extrapolation, likely to lead to widely 

 erroneous conclusions. 



For my own part, I feel satisfied as to the substan- 

 tial correctness of the generally received theory of the 

 sun's constitution, which regards this body as a great 

 ball of intensely heated vapors and gases, clothed out- 

 wardly with a coat of dazzling clouds formed by the 

 condensation of the less volatile substances into drops 

 and crystals like rain and snow. Yet it must be ac- 

 knowledged that this hypothesis is called in question 

 by high authorities, who maintain, with Kirchoff and 

 Zollner, that the visible photosphere is no mere layer 

 of clouds, but either a solid crust, or a liquid ocean of 

 molten metals ; and there may be some who continue 

 to hold the view of the elder Herschel (still quoted 

 as authoritative in numerous school-books), that the 

 central core of the sun is a solid and even habitable 

 globe, having the outer surface of its atmosphere cov- 

 ered with a sheet of flame maintained by some action 

 of the matter diffused in the space through which the 

 system is rushing. We must admit that the question 

 of the sun's constitution is not yet beyond debate. 



And not only the constitution of the sun itself, but 

 the nature and condition of the matter composing it, 

 is open to question. Have we to do with iron and 

 sodium and hydrogen as we know them on the earth, 

 or are the solar substances in some different and 

 more elemental state? 



However confident many of us may be as to the 

 general theory of the constitution of the sun, very 

 few, I imagine, would maintain that the full explana- 

 tion of sun-spots and their behavoir has yet been 

 reached. We meet continually with phenomena,, 

 which, if not really contradictory to prevalent ideas, 

 at least do not find in them an easy explanation. 



So far as mere visual appearances are concerned, 

 I think it must be conceded, that the most natural 

 conception is that of a dark chip or scale thrown up 

 from beneath, like scum in a caldron, and floating, 

 partly submerged, in the blazing flames of the photo- 

 sphere which overhang its edges, and bridge across 

 it, and cover it with filmy veils, until at last it settles 

 down again and disappears. It hardly looks like a 

 mere hollow filled with cooler vapor, nor is its appear- 

 ance that of a cyclone seen from above. But then, 

 on the other hand, its spectrum under high disper- 

 sion is very peculiar; not at all that of a solid, heated 

 slag, but it is made up of countless fine dark lines, 

 packed almost in contact, showing, however, here 

 and there, a bright line, or at least an interspace 

 where the rank is broken by an interval wider than 

 that which elsewhere separates the elementary lines, — 

 a spectrum, which, so far as I know, has not yet found 

 an analogue in any laboratory experiment. It seems, 

 however, to belong to the type of absorption spectra, 

 and to indicate, as the accepted theory requires, that 

 the spot is dark in consequence of loss of light, and 

 not from any original defect of luminosity. Here, 

 certainly, are problems that require solution. 



The problem of the sun's peculiar rotation and 



