PENDING PROBLEMS OF ASTRONOMY. 57 



planets' distances from the sun, their diameters and masses. More 

 than thirty years ago, Professor Kirkwood supposed that he had dis- 

 covered the relation in the analogy which bears his name. The mate- 

 rials 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 theories of the ori- 

 gin 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 problem still. Constant errors of one kind or another, 

 the origin of which is still obscure, seem to affect the different meth- 

 ods of solution. Thus, while experiments 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 observations of Mars all point to a larger par- 

 allax 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 dimin- 

 ished by the latest published results deduced from the transits. I do 

 not feel so confident of the correctness of the value S'SO" for the solar 

 parallax as I did three years ago. In its very nature, this problem is 

 one, however, that astronomers can never have done with. So funda- 

 mental is it, that the time will never come when they can properly give 

 up the attempt to increase the precision of their determination, and to 

 test the received value by every new method that may be found. 



The problems j)resented by the Sun alone might themselves well 

 occupy more than the time at our disposal this evening. Its mass, di- 

 mensions, and motions, as a whole, are, indeed, pretty well determined 

 and understood ; but when we come to questions relating to its consti- 

 tution, the cause and nature of the appearances presented upon its sur- 

 face, the periodicity of its spots, its temperature, and the maintenance 

 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 en- 

 hanced by the enormous difference between solar conditions and the 

 conditions attainable in our laboratories. "We often reach, indeed, 

 similarity sufficient to establish a bond of connection, and to afford a 

 basis for speculation ; but the dissimilarity remains so great as to ren- 

 der 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 appearances 

 which we can reproduce upon the earth ; but we can not safely apply 

 empirical formulae (like that of Dulong and Petit, for instance), de- 



