436 SCIENCE PROGRESS 



H. S. White (Proc. Nat. Acad. Set. Washington, D.C., 191 5, 

 1, No. 8) shows that if seven points on a twisted cubic curve be 

 joined, two and two, by twenty-one lines, then any seven planes 

 that contain these lines will osculate a second cubic curve. 



The general form of the pseudo-sphere has not yet been 

 obtained, and there is therefore some interest in the determina- 

 tion of simple particular cases. Dr. J. R. Wilton (Proc. Lond. 

 Math. Soc. 191 5, 14, 339) writes on a pseudo-sphere whose 

 equation is expressible in terms of elliptic functions. 



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

 Greenwich. 



Dynamical Astronomy — Memoirs of the R.A.S., vol. Ix. pt. vi., 

 contains an important paper by H. Jeffreys, in which are dis- 

 cussed various hypotheses as to the internal structure of the 

 Earth and Moon. Some knowledge as to the internal density 

 and rigidity of the Earth can be derived from (i) the value of 

 the precessional constant, (ii) the Earth's superficial ellipticity, 

 (hi) the period of latitude variation, (iv) the velocity of earth- 

 quake waves, and (v) the lunar deflection of gravity. Using 

 the data which are provided in this way, and supposing the 

 Earth to consist of a homogeneous metallic core surrounded 

 by a rocky crust, it is found to be highly improbable that 

 either the shell or the crust can be permanently elastic, and 

 that owing to its plasticity, the Earth must have at present 

 the hydrostatic form to a high degree of accuracy. The only 

 possible distribution of density is then that originally given 

 by Wiechert, there being a homogeneous core of density 8*2, 

 and with a radius 0-78 of the outer radius, and a homogeneous 

 crust of density 3*2. The large ellipticities of the Moon are 

 then discussed, and are explained on the supposition that 

 the Moon solidified when much nearer the Earth than at 

 present, and when its period of rotation was only about six 

 of our present days. It is also found that the fact that the 

 Moon turns the same face always towards the Earth is not 

 necessarily explicable as an effect of tidal friction, as is gener- 

 ally thought : if the Moon's libration at the time of solidifica- 

 tion was small, it will have remained small whether or not 

 tidal friction has been operative ; if, however, the Moon 

 possessed a considerable libration when it solidified, tidal 

 friction must have since operated to reduce this to zero. 



