574 THE POPULAR SCIENCE MONTHLY. 



not fatal, to the views which it has been so frequently adduced to sus- 

 tain. The superficial character of the examination which Laplace has 

 given to this subject is betrayed by two statements which he makes in 

 regard to it in two different parts of his writings. In setting forth the 

 nebular hypothesis in his " Systeme du Monde," he asserts that the 

 matter separated from a contracting nebula would take and maintain an 

 annular figure, if there were a complete uniformity in its entire circuit 

 and in its rate of cooling ; but in the "Mecanique Celeste," in treating 

 on Saturn's rings, he concludes that their preservation would be impos- 

 sible without some decided irregularities in their structure. It is scarce- 

 ly necessary to say that the annular appendage could not be of long 

 duration if the conditions necessary for its existence or security in one 

 age were fatal to it at another. On examining the alleged history of 

 its birth, also, we feel at a loss for some cause of intermission in the 

 work of detaching matter from the cooling nebula. It is difficult to 

 imagine why, after the outer ring was completed, the separation of mat- 

 ter from Saturn, after a long continuance, should have ceased for a 

 while ; and why, after the completion of the inner ring, the centrifugal 

 force again became weak, and that it has declined steadily until the 

 present time, when, at the planet's equator, it is scarcely one-sixth of 

 the force of gravity. Since the period when Saturn is supposed to have 

 launched forth the zone of matter circulating nearest to him, his move- 

 ments could be but little retarded by tidal action ; and there seems to 

 be no cause which could reduce his angular velocity of rotation during 

 a contraction from the loss of primitive heat. 



Kant, who regarded the rings as composed of aeriform matter sepa- 

 rated from Saturn, was led to the natural inference that the time in 

 which the planet turns once on his axis must be equal to that which the 

 nearest annular zone requires to make a circuit around him. From such 

 considerations, the eminent savant was induced to assign for the rotation 

 of the planet the period of six hours, twenty-three minutes, and fifty- 

 three seconds. But this theoretical or predicted length of Saturn's day 

 is only about three-fifths of the actual value which was first revealed by 

 the observations of Sir William Herschel, and lately determined with 

 more precision by Prof. A. Hall. From the difficulties which the facts 

 present in this case, Laplace endeavors to extricate his doctrine of 

 planetary evolution by maintaining that it requires only that Saturn's 

 day should be shorter than the period of revolution due to the inner 

 ring, supposed to be one unbroken solid mass. But the basis on which 

 this conclusion is founded has been exploded by modern researches, 

 which show the impossibility of the existence of such vast solid annular 

 structures ; and Prof. Kirkwood, though long a supporter of the views 

 of the great French astronomer, has lately pronounced the evidence ob- 

 tained from the Saturnian system and from the inner moon of Mars as 

 adverse to the nebular hypothesis. 



While all scientific researches are exposed to uncertainty in propor- 



