182 Mary Somerville. 



from the reciprocal attraction of the planets and 

 satellites are periodical, whatever the length of the 

 periods may be, so that the stability of the solar 

 system is insured for unlimited ages. The pertur- 

 bations are only the oscillations of that immense 

 pendulum of Eternity which beats centuries as ours 

 beats seconds. 



La Place, and all the great mathematicians of that 

 period, had scarcely passed away when the more 

 powerful Quaternion system began to dawn. 



