274 



NA TURE 



\7an. 20, 1 88 1 



cradle of the human race. Observing that the anthro- 

 poid apes of equatorial Africa— gorilla and chimpanzee- 

 are dolichocephalous. while those of Malaysia — orang-utan 

 and gibbon — are brachycephalous, certain polygenists 

 have suggested that the former may be the progenitors 

 of the dohchocephalous Negroes, the latter of the brachy- 

 cephalous Negritos. But we have seen that the Papuans 

 of the extreme east (New Hebrides, Fiji, &c.) are also 

 dolichocephalous, and even of a more pronounced type 

 than the natives of Sudan. On the other hand, the 

 Obongos, Akkas, and other pigmy tribes of Central 

 Africa appear to lae brachycephalous," so that the theory 

 fails at both extremes, Fiji and the Gaboon. Assuming 

 however that mankind may have been evolved in the 

 Eastern Archipelago or in some now submerged adjacent 

 lands, and bearing in mind the relative value attached to 

 the idea of race, as implied in our definition of species, 

 the present conditions might still admit of explanation. 

 In the Andamanese Islanders, whom Prof. Flower justly 

 regards as of an " infantile type," and in the Javanese 

 Kalong, whose features von Rosenberg describes as the 

 most decidedly ape-like he had ever seen, we would have 

 still in situ the' earliest extant representatives of primeval 

 man. Migrating westwards across a now lost " Lemuria." 

 this primitive Negrito race may have reached equatorial 

 Africa, where it is stdl represented by Du Chaillu's 

 Obongo, Lenz's Abongo or Akoa, Schweinfurth's .Akka, 

 and where it may under more favourable conditions have 

 become differentiated into the Negro of Sudan. Migrating 

 eastwards across a continent of which the South Sea 

 Islands are a remnant, the same Negritos may have 

 similarly become slowly differentiated into the present 

 Papuan or Melanesian peoples of those islands. Mi- 

 grating northwards, before the severance of the Archi- 

 pelago from the mainland, they reached Malacca and 

 the Deccan, where they may still be represented by the 

 Maravans and other low castes of that region. Moving 

 thence over the Asiatic continent, they became under 

 more temperate climes differentiated, first probably into 

 the yellow Mongol, and then through it into the fair 

 Caucasian tvpe. But however this be, the subsequent 

 migrations of the Mongols and Caucasians to the Archi- 

 pelago, as above set forth, was probably, after all, but a 

 return under new forms to their old homes. Here their 

 mutual interminglings have again evolved fresh types 

 and sub-tvpes, producing a chaos of races whose true 

 affinities I have endeavoured in these papers to elucidate, 

 while fully sensible that in all such inquiries the last word 

 still must be, fdix qui potuit rcnim cognosccre causas. 



A. H. Keane 



THE PHOTOPHONE 

 'X'HE following calculation, made with the view of 

 *- examining whether the remarkable phenomena 

 recently discovered by Prof. Bell could be explained on 

 recognised principles may interest the readers of NATURE. 

 I refer to the un-clcctrical sounds produced by the simple 

 impact of intermittent radiation upon thin plates of various 

 substances. 



It has been thought by some that in order that a body 

 exposed to variable radiation may experience a sensible 

 fluctuation of temperature its rate of cooling must be rapid. 

 This however is a mistake. The variable radiation may be 

 divided into two parts — a constant part, and a periodic 

 part — and each ot these act independently. Under the 

 influence of the constant part the temperature of the body 

 will rise until the loss of heat by radiation and conduction 

 balances the steady inflow; but this is not appreciable by 

 the ear, and may for the present purpose be left out of 



*_ The Akkas certainly ; but Lenz seems to think that the Abongos are 

 dolichocephal:>us, so that this point remains still to be settled. Dr. Barnard 

 Davis however in his Ttusaurns Craniortan recognises brachycephaly in 

 equatorial .\frica, four out of eighteen skulls in his collection fro.ni this region 

 being distinctly brachycephal ms. 



account. The question is as to what is the effect of the 

 periodic part of the whole radiation, that is, of a periodic 

 communication and abstraction of heat which leaves the 

 mean temperature unaltered. It is not difficult to see 

 that if the radiating power of the body were sufficiently 

 high, the resvflting fluctuation of temperature would 

 diminish to any extent, and that what is wanted in order 

 to obtain a considerable fluctuation of temperature is a 

 sloii' rate of cooling in consequence of radiation or 

 convection. 



If 6 denote the temperature at time /, reckoned from 

 the mean temperature as zero, q be the rate of cooling, 

 E cos p t the measure of the heating effect of the incident 

 radiation, the equation regulating the fluctuation of tem- 

 perature is — 



'^^ + q6= Eco%pt. 



<it 



Thus — 



Q ^ E cos (//+ e) 



showing that if / and E be given, 6 varies most when 

 q = o. 



Let U5 suppose now that intermittent sunlight falls 

 upon a plate of solid matter. If the plate be transparent, 

 or absorb only a small fraction of the radiation, little 

 sonorous effect will be produced, not merely because the 

 radiation transmitted is lost, but because the heating due 

 to the remainder is nearly uniform throughout the sub- 

 stance. In order that the plate may bend, as great a dif- 

 ference of temperature as possible must be established 

 between its sides, and for this purpose the radiation should 

 be absorbed within a distance of the order of half the 

 thickness of the plate. If the absorption be still more 

 rapid, it would appear that the thickness of the plate may 

 be diminished with advantage, unless heat conduction in 

 the plate itself interferes. The numerical calculation 

 relates to a plate of iron of thickness d. It is supposed 

 that (^ is negligible in comparison with p, i.e. that no 

 sensible gain or loss of heat occurs in the period of 

 the intermittence, due to the fluctuations of temperature 

 themselves. 



If the posterior surface remains unextended the exten- 

 sion of the anterior surface corresponding to a curvature 



p 'is — , and the average extension is — Let us in- 



p " -P 



quire what degree of curvature will be produced by the 

 absorption of sunlight during a time t, on the supposition 

 that the absorption is distributed throughout the substaiice 

 of the plate, so as to give the right proportional extension 

 to every stratum. 



If Nt denote the heat received in time / per unit area, 

 c the specific heat of the material per unit volume, e the 

 linear extension of the material per degree centigrade, 

 then 



j__ 2e//t 



p C.d'' 



In the case of sunshine, which is said to be capable of 

 melting 100 feet of ice per annum, we have approxi- 

 matelv in C .C. S measure 



Ht= -ooS /. 



ct 



= .016'' 

 cd- 



Thus 



P 



For iron e = '000012, c = '86. 

 Thus if / = 5I (of a second), </ = '02 cents. 



- = I -12 X 10 \ 

 P 



This estimate will apply roughly to a period of inter- 

 mittence equal to ^-A-^th of a second, /.<■. to about the 

 middle of the musical scale. If the plate be a disk of 

 radius r, held at the circumference, the displacement at 



