60 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 17, 



§ II. Influence of Heat radiating from External Bodies. 



7. It has long been an opinion prevalent among philosophers, that 

 the sun, with the stars in general, has a proper motion of his own 

 from one part of space to another ; but it is only in recent times that 

 astronomers have been able to offer anything like demonstrative evi- 

 dence of such being the fact. At present no one, probably, who is 

 prepared to appreciate that evidence will entertain any serious doubts 

 as to the existence of the motion in question. M. Argelander, M. 

 O. Struve, and finally our countryman Mr. Galloway have all arrived 

 at results respecting the instantaneous direction of the sun's motion, 

 which, considering the nature of the problem, present a very striking 

 accordance with each other. M. O. Struve has also calculated the 

 velocity with which the sun is moving. His result is, that if the 

 sun were seen from the mean distance of stars of the first magnitude, 

 he would appear to describe an arc of about ^rd of a second in a 

 year. If this motion were rectilinear and uniform, it would require a 

 period of nearly seven hundred thousand years for the sun to move 

 over a space equal to the mean distance of the stars of the first 

 magnitude from the earth. This result of M. Struve' s is probably 

 liable to serious error, but it is not likely, I conceive, that the true 

 motion of the sun can be materially greater than his estimated motion. 



There can be no reasonable doubt that the stars are bodies of the 

 same nature as the sun of our own system, so far as regards the 

 emission of heat and light. Thus the stellar region is occupied with 

 innumerable centres of heat, and must at different points have very 

 different degrees of temperature, as measured by a thermometer 

 which should be exposed to the radiation from these different centres. 

 This consideration, combined with that of the proper motion of the 

 solar system from one part of space to another, led Poisson to the 

 opinion that the present internal temperature of the earth was not 

 due to its primitive heat, but to the circumstance of the whole solar 

 system having passed through a region of space of higher tempe- 

 rature than that now occupied by it. 



If we consider the subject with reference to unlimited periods of 

 time, it cannot be denied that this may have been a possible source 

 of terrestrial temperatures, although we may at the same time be 

 obliged to reject the theory in its application to the climatal changes 

 of the more recent geological periods. Let us examine how far it 

 will enable us to account for the comparative cold of the glacial epoch 

 in western Europe. 



The stellar system, of which our sun must be considered a consti- 

 tuent body, consists of stars generally at such enormous distances 

 from each other, that the region more immediately around each star 

 must derive an exceedingly small quantity of heat from all the other 

 stars, compared with that derived from the one placed in its centre. 

 The earth is placed in a region of this kind with respect to the sun. 

 The general question is — whether the arrangement of the stars with 

 reference to the sun may, at past geological epochs, have so far 

 differed from the present arrangement, as to have altered materially 



