CONCLUSION. 601 



of stability, as it were the maintenance and duration of the 

 planets' existence, are dependent upon the condition of 

 mutual action with a separate circle. If by the entry of a 

 cosmical body coining from ivithout, and not previously be- 

 longing to the planetary system, that condition was disturbed 

 (Laplace, Expos, du Syst. du Monde, pp. 309 and 391), then 

 this disturbance, as the consequence of new attractive forces, or 

 of a collision, might certainly become destructive to the existing 

 system, until finally, after long conflict, a new equilibrium 

 was produced. The arrival of a comet upon an hyperbolic 

 orbit from a great distance, even when want of mass is made 

 up for by immense velocity, can excite apprehension only in 

 an imagination which is not susceptible of the earnest assur- 

 ances of the calculation of probabilities. The wandering clouds 

 of the interior comets are not more dangerous to our solar 

 system than the great inclination of the orbits of some of 

 the small planets between Mars and Jupiter. Whatever 

 must be characterized as mere probability, lies beyond the 

 domain of a physical description of the universe; science 

 must not wander into the cloud-land of cosmological dreams. 



