THE ASTRONOMICAL DEDUCTIONS. 9 



when the inapplicability of this doctrine to natural cases was detected, 

 about a decade ago,* that there was a clear path opened and logical grounds 

 provided for developing a hypothesis of planetesimal accretion. It is 

 perhaps not too much to assume that the previous papers of this series 

 have shown that this hypothesis is even better fitted than gaseous con- 

 densation to give rise to the various rates of rotation actually presented 

 by the solar system. Under the planetesimal hypothesis, the primitive 

 rotation of the earth was not necessarily rapid, nor was the body of the 

 earth necessarily molten. Thus two of the primitive conditions, that were 

 formerly taken for granted on the basis of a nearly universal consensus of 

 opinion, have been brought into question and may now be fairly regarded 

 as being at best no more than working competitors with the alternative 

 of a solid elasti co-rigid earth, a view which is hampered by no compulsory 

 presumption as to any particular rate of primitive rotation, but is hospitable 

 to any rotational state which the direct evidences, astronomical, geological 

 and otherwise, may require. 



The speculative freedom relative to primitive rotations, which the 

 planetesimal hypothesis thus affords, directs attention anew to the actual 

 facts and to their unembarrassed implications. The most fundamental 

 case is that of the controlling body of the solar system itself. The present 

 rotation of the sun is relatively slow and its axis is incHned appreciably 

 to the common plane of the planetary system. When it is considered that 

 the mass of the sun is more than 700 times that of all the planetary deriva- 

 tives combined, this rate and this inclination assume radical importance. 

 This slow rotation and this inclination of axis are perfectly consistent with 

 the planetesimal hypothesis and have peculiar suggestiveness in that 

 relationship. On the other hand, they seem to me very difficult to reconcile 

 with any theory under which the outlying bodies are supposed to be derived 

 from a gaseous or quasi-gaseous spheroid by contraction, particularly 

 any theory which postulates that the derived bodies were discharged from 

 the central mass by the equatorial velocity of its rotation. Obviously 

 the planetary material thus separated should be accurately adjusted to 

 the sun's equatorial plane, and to the common plane of the system. Obvi- 

 ously also the great residual mass should have a rate of rotation appro- 

 priate to such a discharge. Having separated a succession of masses 

 from its equator to form the planets, and having further shortened its 

 radius some 36,000,000 miles after the last known planetary mass was 

 detached, the sun should have a rotatory velocity somewhat near that 

 requisite for another planetary separation. The velocity of rotation at 

 the equator of the solar nebula when it was supposed to have detached 

 the material for Mercury must have been, according to the Laplacian hy- 

 pothesis, about 28 miles per second. The equatorial velocity requisite to 

 bring the centrifugal and centripetal components of the sun's equatorial 

 motion into equality if the sun now had a radius of 1,000,000 miles is 176 

 miles per second; the velocity required to bring about this state at the 

 present surface of the sun is 270 miles per second. We should then expect, 



» Journal of Geology, vol. 5, 1897, pp. 668-669 



