i8 SCIENCE PROGRESS 



W. R. D. Shaw, Computation of Aerodynamic Support (for circular arc camber), 



Aer.Journ. xxii. 1918, 109. 

 Sir G. Greenhill, The Rankine Trochoidal Wave, Proc. Roy. Soc. 94, A, 191 8, 



238-49. 

 T. H. Havelock, Periodic Irrotational Waves of Finite Height, Proc. Roy. Soc. 



95, 191 8, 38-50. 

 G. Green, On Ship Waves and on Waves on Deep Water Due to the Motion 



of Submerged Bodies, Phil. Mag. vi. 35, 1918, 48-63. 

 H. Jeffreys, Problems in Denudation, Phil. Mag. vi. 35, 191 8, 179-90, in which 



the author concludes that surface water motion is affected by gravity and 



friction, but only to a negligible extent by hydrostatic pressure and inertia. 



ASTRONOMY. By H. Spencer Jones, M.A., B.Sc, Royal Observatory, 

 Greenwich. 



The Present Position of the Nebular Hypothesis. — An article 

 by Mr. J. H. Jeans bearing this title appeared in a recent issue 

 of Scientia, in which are discussed the arguments for and 

 against the famous nebular hypothesis of Laplace. It is re- 

 markable that this hypothesis, put forward by Laplace more 

 than a century ago to explain the origin of the solar system, has 

 not yet definitely been either proved or disproved. It may 

 be recalled that Laplace supposed a hot, nebulous mass of gas 

 in rotation to be gradually cooling and shrinking, the velocity 

 of rotation correspondingly increasing in accordance with the 

 principle of conservation of momentum. The increase in the 

 speed of rotation causes a flattening of the mass, and, after a 

 certain stage, Laplace considered that a ring of matter would 

 be thrown off from the equator which would break up and 

 collect into a number of finite masses. A continuance of the 

 shrinkage would result in these smaller masses similarly break- 

 ing up, and in this way he supposed that the existence of the 

 planets and planetary satellites in the solar system could be 

 accounted for. 



To test this theory by mathematics is a matter of extreme 

 difficulty, and the first attempts were limited to the study of 

 the behaviour of a rotating, homogeneous and incompressible 

 mass of matter. The evolution was traced by Darwin and 

 Liapounoff to the well-known pear-shaped figure. Jeans has 

 recently finally proved that the pear-shaped figure is unstable, 

 and that the furrow rapidly deepens until the mass divides 

 into two portions, which probably approximate to equality in 

 mass. Each of these masses will itself be rotating and shrink- 

 ing, and Russell has shown that the larger mass will break 



