June 5, 1913] 



NATURE 



543 



strokes would be unaccompanied by the pheno- 

 mena of slicing- or pulling or rising- above the 

 natural gravitational path. The dynamical con- 

 dition for the production of underspin is clearly 

 pointed out by Tait in his various articles. The 

 resultant blow must be delivered so as to act in 

 a line which passes beneath the centre of mass. 

 For this purpose the blow need not be horizontal ; 

 but a horizontal stroke with a lofted club will 

 produce underspin if the club-face hits the ball 

 below the height of the centre. In this sense loft 

 is an important factor in the production of under- 

 spin. In all cases, whether the ball is hit with a 

 downward or a practically horizontal stroke, the 

 production of underspin depends on the existence 

 of a pronounced tangential component of impulse, 

 and this requires that the direction of the blow 

 must be inclined to the face of the club. 



-Mr. Yaile expresses the same idea when he 

 speaks of the ball being hit with a glancing blow. 

 In his explanation of the manner in which the 

 wind-cheater is produced he is indeed quite sound ; 

 and it is a matter of regret that a book so admir- 

 able in many respects should be marred, not only 

 by faulty dynamics, but by an inability to follow 

 the dynamical reasoning of a master like Tait. 

 Mr. Yaile sneers at the mathematician and physic- 

 ist as having gone on utterly fallacious lines. He 

 misquotes, and when he quotes aright he not 

 unfrequently misunderstands. Nevertheless, on 

 the more practical aspects of the game, when he 

 is speaking for himself, and not piling up adjec- 

 tives of denunciation against the mathematician, 

 Mr. Vaile has done no small service in removing 

 some of the clouds of mystery with which popular 

 writers have obscured the soul of golf. 



C. G. Kxott. 



THE AGE OF THE EARTH. 

 The Age of the Earth. By A. Holmes. Pp. xii + 

 196. (London and New York : Harper and 

 Brothers, 1913.) Price 2s. 6d. net. 



THE question "For how long has it been 

 possible for organic beings to have lived 

 on the earth? " must always be one of supreme 

 interest ; and it is good to find a book which 

 states shortly but quite clearly how far we have 

 proceeded towards an answer. 



No great weight ought ever to have been given 

 to the argument from the lengthening of the day, 

 because it assumes that the ratio of the polar 

 and equatorial diameters of the earth now is the 

 same as when the earth ceased to be liquid ; in 

 spite of the fact that great forces are acting tend- 

 ing to change this ratio. Kelvin's argument from 

 the temperature gradient downwards in the earth's 

 NO. 2275, VOL. 91] 



crust ceased to be of value when it was shown 

 that greater conductivity, in the interior led to 

 an enormously larger answer ; for whether such 

 greater conductivity is or is not probable, it 

 could not be said to be impossible. We are sorry 

 that Mr. Holmes should refer to the work of 

 Mr. Clarence King as if it affected the question. 

 He, following Kelvin, assumed that there could 

 be no greater conductivity inside the earth than 

 in the crust. But all earth-cooling arguments have 

 been set aside by Mr. Strutt's measurement of 

 radium in rocks, and they are now of historical 

 interest only. 



Three of the old arguments still hold the field 

 — one from the sun's energy, a second from the 

 amount of salt in the oceans, and the third from 

 observations of rates of erosion and deposit of 

 sediment. To these a fourth is now being added 

 which is likely to have great weight in settling 

 the matter — the increase in the proportion of lead 

 to uranium in rocks as time goes on. Mr. 

 Holmes has himself devoted much time to the 

 laboratory study of radio-active minerals, and of 

 the creation of lead and helium from uranium ; 

 in the present state of our knowledge we can 

 suggest no modification of his figures. He ought, 

 however, we think, to be prepared to accept a less 

 age for the earliest sedimentary rocks than 1300 

 million years. 



Consideration of the amount of sodium in the 

 ocean gives less than a quarter of this age, as 

 does also the consideration of the accumulation 

 of carbonate of lime. These two methods of 

 study are on a much less certain basis than the 

 calculation from the rate of accumulation of sedi- 

 ment, which, however, gives about the same age. 

 With this last method Mr. Holmes, as a geologist, 

 is very familiar. The suggestion that erosion 

 used to take place more slowly because all con- 

 tinents were smaller and lower in level than now, 

 would lead to a better agreement between the 

 two methods which he favours. If the 

 average slopes were 60 per cent, of what 

 they are now, we are led to multiply the 

 age by four. On the whole, we feel with Mr. 

 Holmes that the question is in a fair way towards 

 settlement, but, unlike him, we still see a difficulty 

 due to the age of the sun. A person who has not 

 made the calculation will scarcely believe in the 

 liberality with which Kelvin treated his opponents 

 in regard to the argument based on the sun's 

 heat. Assuming that the whole mass of the sun 

 was once scattered through space, and by mere 

 gravitation the stuff came together as we now 

 have it, and considering that it is denser in its 

 central part, the total amount of energy given 

 out as heat cannot be much greater than 25 million 



