November 21, 1912] 



NATURE 



)35 



a parent as one who develops it at a later age? I 

 think there is not a doubt as to the answer to be 

 given ; those who become insane before twenty-five, 

 even if they recover, are far less likely to become 

 parents than those who become insane at late ages — 

 many, indeed, of them, considering the high death- 

 rate of the insane, will die before they could become 

 parents of large families. Now Dr. Mott took 50S 

 pairs of parents and offspring, "collected from the 

 records of 464 insane parents whose 500 insane off- 

 spring had also been resident in the County Council 

 Asylums," and ascertained the age of first attack. 

 As at present advised, it seems to me that his data 

 must indicate a most marked antedating of disease in 

 the offspring, but an antedating which is wholh' 

 spurious. There is, I think, a further grievous fallacy 

 involved in this method of considering the problem ; 

 but before discussing that I should like to see if my 

 criticism of this method of approaching the problem of 

 antedating can be met. Karl Pe.^rson. 



Biometric Laboratory, University College, London, 

 November 11. 



Is the Earth Shrinking? 



I H.-\VE carefully looked at this question from every 

 point of view which presented itself to me, and doubt 

 very much whether any direct evidence will ever be 

 forthcoming on this subject, unless it should one day 

 be established that the changes of magnetic declination 

 are associated with a slight difference of rotation 

 between the core of the earth and its crust, for such 

 a movement would have to be explained by a difference 

 of rate of contraction between the two. 



The foldings and crackings of the earth's surface 

 have been attributed to variations in the rate of cool- 

 inp- of the earth. Thus whenever this rate is acceler- 

 ated, the surface cools faster than the core, and should 

 crack like a drying ball of clay ; whenever the cooling 

 rate is diminishing, as assumed by Lord Kelvin, the 

 core should shrink faster than the skin, like a drying 

 apple, and folding should occur. But to my mind, as 

 recently explained in " Unity in Nature," such effects 

 would be entirely masked by such foldings and crack- 

 ings as are slowly progressing even to-day, for the 

 sediment which is being constantly deposited on the 

 floors of the oceans must cause the underlving strata 

 to grow warmer and to expand in every direction, 

 resulting in slight local risings, which are most 

 marked near the mouths of large rivers, and 

 in distant bulgings and foldings of the weakest 

 lines of the earth's crust, which are the 

 mountain ranges. On the other hand, the gradual 

 wearing away of the surfaces of the con- 

 tinents and mountain ranges must cause the underlying 

 strata to cool, to shrink, and to crack. This sugges- 

 tion would certainly more than account for all the 

 foldings, faults, and cracks to be found in the earth's 

 crust, even if a considerable allowance be made for those 

 oases in which the expansions and contractions occur 

 in the same direction, and partly balance each other. 

 C. E. Stromeyer. 



" Lancefield," West Didsbury, November 7. 



THE HARDNESS OF COINS. 

 TTARDNES.S is a word which is used in 

 -L J- various senses. In dealing with metals, it 

 sometimes means the cutting or scratching hard- 

 ness, but is more often defined briefly as the 

 resistance to permanent deformation, a property 

 which is of tjreat importance to all users of metals. 

 It is this kind of hardness with which those 

 NO. 2247, "VOL. 90] 



engaged in minting are chiefly concerned. When 

 a blank is struck in a coining- press, the metal is 

 compressed and at the same time forced to flow 

 into the recesses of the dies, and the ease with 

 which this can be done depends on the amount of 

 resistance offered by the metal to a force 

 momentarily applied and tending to deform it. 

 The hardness should therefore be measured by 

 the effects of a sudden blow, and falling-weight 

 machines, such as Shore's scleroscope, offer a 

 ready means of doing this. 



The hardness numbers given below are sclero- 

 scope readings, about w-hich it may be said that 

 a piece of metal giving a higher reading is cer- 

 tainly harder than a piece of similar metal giving 

 a lower reading, but that the readings cannot be 

 taken as proportional to the hardness, except as 

 a rough approximation. It cannot be admitted, 

 for example, that a specimen with a hardness 

 number of 40 is exactly twice as hard as one with 

 a hardness number of 20. 



The application of hardness tests to the coins of 

 the realm has resulted in some curious and in- 

 teresting data being obtained. 1 It is found, 

 naturally enough, that the blow of a coining press 

 does not raise soft metal to a state of maximum 

 hardness. A sovereign blank after annealing has 

 a surface hardness of 25-5, and this is raised to 

 50-53 on being struck in an ordinary press, the 

 maximum hardness of standard gold being about 

 76. Silver coins of similar size are hardened to 

 much the same extent ; but while sixpences, for 

 example, have a hardness number of about 50, 

 florins are only 37. These are the hardnesses of 

 the "table" or flat portions of the coins, but the 

 raised portions of the designs are much softer, 

 especially the highest parts of large thick coins 

 in high relief. Thus in George V. florins the 

 centre of the effigy has a hardness of only 31, 

 that of the annealed blank from which the coin 

 is struck being 27^5. Such coins will evidently 

 wear very differently from coins made in low 

 relief, such as the modern French coins, in which 

 the surface hardness is higher and more uniform. 



The hardness of the surface of coins, however, 

 differs widely from that of the interior. The 

 force of the blow seems to be expended chiefly 

 on the surface layers. When these are carefullv 

 removed, the hardness of the underlying metal is 

 found to be considerably less. The hardness 

 rapidly falls off with depth, and near the centre 

 even sixpences are almost as soft as annealed silver. 

 Old worn coins are similarly soft. 



It is clear, therefore, that a freshly-minted coin 

 has a hard skin and a soft core, and that after the 

 removal of the skin by wear, the loss of weight in 

 circulation will proceed very much as though the 

 coin had been annealed before it was issued. 

 That this is a matter of some importance is illus- 

 trated by the fact that the loss by wear of the 

 coinage, which falls on the State, amounts to 

 30,000?. per annum for gold, and somewhat more 

 for silver. 



Annealing, one of the oldest processes prac- 



1 Memorandum on " The Hardness of Coins," 42nd Annual Report of 

 the Deputy Master of the Mint, 1911. pp. 107-112. 



