ASTRONOMY. 205 



frictiou to be the principal agent of change with the former, but not 

 with the hitter. 



In the case of the contracting terrestrial mass we may suppose that 

 there was for a long time nearly a balance between the retardation due 

 to solar tidal friction and the acceleration due to contraction, and that 

 it was not until the planetary mass had contracted to nearly its present 

 dimensions that an epoch of instability could occur. If the contraction 

 of the planetary mass be almost completed before the genesis of the 

 satellite, tidal friction, due jointly to the satellite and the sun, will there- 

 after be the great cause of change in the system, and thus the hypothe- 

 sis that it is the sole cause of change will give an approximately ac- 

 curate explanation 'of the motion of the planet and satellite at any sub- 

 sequent time. It is shown in the previous papers of this series that 

 this condition is fulfilled with the earth and moon. 



The paper ends with a short recapitulation of those facts in the solar 

 system which are susceptible of explanation by the theory of the ac- 

 tivity of tidal friction. This series of investigations affords no grounds 

 for the rejection pf the nebular hypothesis, but while it presents evi- 

 dence in favor of the main outlines of that theory it introduces modifi- 

 cations of considerable importance. 



Tidal friction is a cause of change of which Laplace's theory took no 

 account, and although the« activity of that cause is to be regarded as 

 mainly belonging to a later period than the events predicated by the 

 nebular hypothesis, yet its influence has been of great and in one in- 

 stance of even jiaramount importance iii determining the present con- 

 dition of the planets and their satellites. 



THE SUN. 



The publication of Professor Young's book on the sun (International 

 Scientific Series), in 18S1, is important, as it is undoubtedly the au- 

 thority on the subject, and supplements the work of Secchi, and pre- 

 sents, beside, more philosophical and extended views. 



The first chapters deal with the dimensions and distance of the sun, 

 with the means and apparatus for studying its surface, with the i)he- 

 nomena of sun spots, their periodicity, etc. 5 with the phenomena of 

 solar ecliiises, the corona, etc. Perhaps the two most important chap- 

 ters are those relating to the sun's light and heat, and the different 

 theories relating to the constitution of the sun. The theory of Dr. 

 Hastings, spoken of in the last report of the Smithsonian Institution, 

 receives a discussion to which we must refer. In an appendix. Pro- 

 fessor Langley gives an account of his observations with the bolometer, 

 and the conclusions he has derived from them. To test these conclu- 

 sions still further, Professor Langley undertook an expedition to the 

 western ])art of the United States, which is anticipated by CHbbon, in 

 chapter 43 of the "Decline and Fall of the Koman Empire," in the follow- 

 ing words: " Their calculations may perhaps be verified by the astron- 

 omers of some future capital in the Siberian or American, wilderness.'' 



