20 On the Causes of Change in the 



The temperature of any point in stellar space is that which 

 would be indicated by a thermometer at that point receiving the 

 heat radiating from all the stars composing the universe. The 

 temperature of all bodies must necessarily be affected by this 

 radiation, and in different degrees, according to the positions in 

 space which they may occupy. Hence Poisson was led to adopt 

 the notion that the actual temperature of the earth, whether super- 

 ficial or internal, is due to the circumstance of the solar system 

 having passed through a warmer region than that which it now 

 occupies, in the course of that motion by which astronomers gene- 

 rally believe it to be constantly moving from one part of space to 

 another. What may have been the possible effect of this cause in 

 the lapse of indefinite time, it is impossible to say ; but I cannot 

 understand how it could be very considerable without a totally 

 different distribution of the group of stars to which the sun should 

 belong, or the near approach of the solar system to some indivi- 

 dual star. The latter hypothesis, however, would be inconsistent 

 with the integrity of the solar system as it now exists, if we sup- 

 pose the proximity to any single star to become such as to produce 

 any material modification of terrestrial climate ; and perhaps it 

 may be difficult to conceive how the first hypothesis should escape 

 a similar objection. At all events, it maybe regarded as certain, 

 that according to neither of these hypotheses can any considerable 

 effects have been produced by this cause on terrestrial temperature 

 within the later tertiary period, and that we cannot thus account 

 for the cold of the glacial epoch. 



In considering the influence of the third cause, — that of the 

 configuration of land and sea, — I have endeavoured to ascertain 

 approximately what would be the climatal conditions, more espe- 

 cially in western Europe, in the four following hypothetical 

 cases : — 



1. The configuration of land and sea the same as at present, 

 but without the Gulf Stream. 



2. The Gulf Stream the same as at present, except that its pro- 

 gress into the North Sea is supposed to be arrested by a barrier of 

 land, extending from the North of Scotland to Iceland, and thence 

 to the coast of Greenland. 



3. The basin of the Atlantic from the Tropic to the North Sea 

 converted into land, uniting the old and new continents. 



4. Large portions of the continents of Europe and North Ame- 

 rica submerged beneath the surface of the ocean, and the Gulf 

 Stream directed into some other course. 



By a study of Dove's admirable Map of Isothermal Lines, we 

 easily recognise the masses of land in the northern parts of the 

 old and new continents, and the Gulf Stream as the principal 

 causes of the abnormal forms of the isothermal s in the higher 

 latitudes of the northern hemisphere. In like manner the irre- 



