660 THE PRINCIPLES OF SCIENCE. [cHAr. 



that this effect is due to a thin film of oxide of iron, ^hich 

 forms upon the surface of the iron and protects it.^ 



The law of gravity is so simple, and disconnected from 

 the other laws of nature, that it never suffers any disturb- 

 ance, and is in no way disguised, hut by the complication 

 of its own effects. It is otherwise with those secondary 

 laws of the planetary system which have only an em- 

 pirical basis. The fact that all the long known planets 

 and satellites have a similar motion from west to east is 

 not necessitated by any principles of mechanics, but 

 points to some common condition existing in the nebulous 

 mass from which our system has been evolved. _ The 

 retrograde motions of the satellites of Uranus constituted 

 a distinct breach in this law of uniform direction, which 

 became all the more interesting when the single satellite of 

 Neptune was also found to be retrograde. It now became 

 probable, as Baden Powell well observed, that the anomaly 

 would cease to be singular, and become a case of another 

 law, pointing to some general interference which has taken 

 place on the bounds of the planetary system. Not only 

 have the satellites suffered from this perturbance, but 

 Uranus is also anomalous in having an axis of rotation 

 lying nearly in the ecliptic ; and Neptune constitutes a 

 partial exception to the empirical law of Bode concerning 

 the distances of the planets, which circumstance may 

 possibly be due to the same disturbance. 



Geology is a science in which accidental exceptions are 

 likely to occur. Only when we find strata in their original 

 relative positions can we surely infer that the order of 

 succession is the order of time. But it not uncommonly 

 happens that strata are inverted by the bending and 

 doubling action of extreme pressure. Landslips may carry 

 one body of rock into proximity with an unrelated series, 

 and produce results apparently inexplicable.^ Floods, 

 streams, icebergs, and other casual agents, may lodge 

 remains in places where they would be wholly unexpected. 

 Though such interfering causes have been sometimes 

 wrongly supposed to explain important discoveries, the 

 geologist must bear the possibility of interference in mind. 



' Experimental Researches in Electricity, vol. ii. pp. 240--2J5. 

 * Murchison's Silurian System, vol. ii. p. 733, &c. 



