August 8, 191 8] 



NATURE 



449 



account of the reduction in tempjerature caused 

 by the release of pressure ; solidification would 

 occur more slowly. The origin of the asteroids 

 presents greater difficulties. They may have 

 started as ijndependent pJanets of exceptionally 

 small size ; but the fact that none of their mean 

 distances is greater than that of Jupiter, and only 

 one is less than that of Mars, indicates a closer 

 relationship between their origins. Several pos- 

 sible explanations can be advanced ; the writer in- 

 clines to the belief that they were formed by the 

 close approach of a primitive planet to Jupiter, 

 leading to tidal disruption. 



The large size of the moon relative to the earth 

 suggests a fundamental difference of origin be- 

 tween it and the other satellites. It seems likely 

 (indeed, on the theory of a formerly heated earth 

 it is almost certain) that it was once much nearer 

 the earth than it is now, and has receded on ac- 

 count of the friction of the lunar tides. It is natu- 

 ral to think that just before this state of motion 

 with a comparatively small separation between 

 their surfaces the earth and moon formed one 

 body. The rotation would then be so rapid that 

 the longest free period of the mass was nearly 

 equal to the period of the semi-diurnal solar tide, 

 which was consequently enormously magnified by 

 resonance; and it is highly probable that the de- 

 formation became so great that the mass 

 separated into two- parts. This is not the only 

 conceivable origin of the moon that would be con- 

 sistent with the tidal theory ; but several peculiar- 

 ities in our sub-system suggest that it is the most 

 likely. No other satellite in the system can have 

 been formed in this way. 



As has iust been remarked, lunar tidal friction 

 has probably been the predominant cause in the 

 evolution of the earth and moon. No other satel- 

 lite can raise tides of such importance ; but those 

 raised in Mercury by the sun must have been much 

 more effective in reducing the rotation of this 

 planet. Now from the fact that Mercury has no 

 satellite we may infer that it never rotated so fast 

 as the earth did before the moon was formed ; and 

 therefore the solar tides will have been able to 

 reduce its rotation so as to make it always keep 

 the same face towards the sun, which again agrees 

 with observation. 



Every satellite except the moon has probably 

 been more influenced in its orbital motion by the 

 resisting medium than by tidal friction. The most 

 striking effect of the medium being to reduce the 

 eccentricities of orbits, this accounts for the almost 

 perfect circularity of the orbits of most satellites, 

 tspecially those nearest their primaries, where the 

 density of the medium was probably greatest. The 

 effect of tidal friction on the eccentricities is not 

 certain, depending on certain unknown jAysical 

 quantities. 



It may be said finally that at every point where 

 the tidal hypothesis has been tested it agrees with 

 dynamical theory and with observation. Several 

 facts otherwise unaccounted for are explained 

 by it, and nothing has yet been discovered to be 

 definitely opposed to it, though a few difficulties, 

 NO. 2545, VOL. lOll 



such as the origin of comets and meteor swarms, 

 still remain. 



References. 



J. H. Jeans: "The Part Played by Rotation in 

 Cosmic Evolution," Monthly Notices of Royal Astro- 

 nomical Society, vol. Ixxvii., 1917, pp. 186-99; 

 " Note on the Action of Viscosity in Gaseous and 

 Nebular Masses," \oc. cit., pp. 200-4; "The Motion 

 of Tidally Distorted Masses, with Special Reference 

 to Theories of Cosmogony," Memoirs of R.A.S., 

 vol. Ixii., 1917, pp. 1-48. 



Harold Jeffreys: "The Resonance Theory of the 

 Origin of the Moon," Monthly Notices, vol.' Ixxviii., 

 1917, pp. 116-31; "On the Early History of the Solar 

 System," loc. cit., pp. 424r-4i. 



Harold Jeffreys, 



GRASSLAND AND FOOD SUPPLIES. 

 '^T^HE persistent criticism with which the 

 -»- " ploughing-out policy" of the Board of 

 Agriculture has been assailed has been intensified 

 of late with the evidence of failure, total or partial, 

 of some of the crops grown this year on newly 

 ploughed grass land. The columns of the daily 

 Press have been freely used, and have revealed 

 much division of opinion amongst practical • agri- 

 culturists as to the measure of success or failure 

 in different areas. A new note has been intro- 

 duced into the discussion by a letter from the Duke 

 of Marlborough in the Times of July 30, in which 

 he endeavours to demonstrate from the publica- 

 tions of the Board that the policy is fundamentally 

 unsound. • 



Basing his criticism upon a leaflet issued by the 

 Board in the spring of last year, his Grace argues 

 that, so far from the data there given proving that 

 an increase of food supply can be expected from 

 the ploughing-up of grass and growing corn, they 

 demonstrate rather that the chances of securing 

 such an increase are very speculative and scarcely 

 likely to be realised. The facts are not disclosed 

 that the leaflet in question deals with the produc- 

 tion of winter food for cows, and therefore only 

 indirectly for the human population ; and, further, 

 that the dairy farmer is advised that a much 

 greater return of milk-producing food can be ex- 

 pected from growing root crops rather than corn. 



The basis of comparison of the feeding values 

 of the crops adopted in the leaflet is quite inap- 

 plicable to the assessment of the relative returns 

 for human feeding, since for the latter purpose 

 grass is worthless until converted into other forms 

 which represent only a fraction of the weight of 

 grass harvested, whereas the corn crops, by the 

 simple and expeditious process of milling, yield 

 anything from 60 per cent, upwards of the weight 

 of the grain in a form directly usable for human 

 consumption, whilst, in addition, the accompany- 

 ing straw, according to its nature, when used for 

 food production purposes, will be roughly equal 

 to one-half its weight of hay. The Duke of 

 Marlborough anticipates this objection by pointing 

 out that, so far as the oat crop, at any rate, Is 

 concerned, only a small proportion of it has, in 

 actual fact, been directlv utilised for human con- 



