which may influence Geological Phenomena. 295 



if at once attained^ to cause great local devastations in estuaries and confined 

 channels, but wliich would not account for any great diluvial phaenomena, 

 especially when it is considered that, the change taking place gradually, they 

 would become modified in their form by insensible gradations, and thus ac- 

 commodated to the altered circumstances ; a remark which may be extended 

 to the general outlines of coasts, which would be no doubt in some degree 

 altered. It does not appear, therefore, that any admissible extent of perturba- 

 tion produced by the sun's action on the lunar orbit can have materially in- 

 fluenced the geological state of the earth. 



Let us next consider the changes arising in the orbit of the earth itself about 

 the sun, from the disturbing action of the planets. In so doing it will be ob- 

 viously unnecessary to consider the effect produced on the solar tides, to 

 which the above reasoning applies much more forcibly than in the case of 

 the lunar. It is therefore only the variations in the supply of light and heat 

 received from the sun that we have now to consider. 



Geometers having demonstrated the absolute invariability of the mean di- 

 stance of the earth from the sun, it would seem to follow that the mean annual 

 supply of light and heat derived from that luminary would be alike invariable: 

 but a closer consideration of the subject will show that this would not be a 

 legitimate conclusion ; but that, on the contrary, the mean amount of solar 

 radiation is dependent on the excentricity of the orbit, and therefore liable to 

 variation. Without going at present into any geometrical investigations, it 

 will be sufficient for the purpose here to state it as a theorem, of which any 

 one may easily satisfy himself by no very abstruse geometrical reasoning, that 

 "the excentricity of the orbit varying, the total quantity of heat received by the 

 earth from the sun in one revolution, is inversely proportional to the minor 

 axis of the orbit." Now since the major axis is, as above observed, invariable, 

 and therefore, of course, the absolute length of the year, it will follow that, 

 the 7nean annual average of heat will also be in the same inverse ratio of the 

 minor axis ; and thus we see that the very circumstance which, on a cursory 

 view, we should have regarded as demonstrative of the constancy of our supply 

 of solar heat, forms an essential link in the chain of strict reasoning by which 

 its variability is proved. 



The excentricity of the earth's orbit is actually diminishing, and has been 

 so for ages beyond the records of history. In consequence the ellipse is in a 

 state of approach to a circle; and its minor axis being therefore on the in- 

 crease, the annual average of solar radiation is actually on the decrease. 



So far this is in accordance with the testimony of geological evidence, which 

 indicates a general refrigeration of climate ; but when we come to consider 



