November 20, 19 19] 



NATURE 



315 



Variations of Refractive Index. 



Experiments conducted in the research laboratory of 

 the firm of Adam Hilger, Ltd., by Mr. G. M. Fleming 

 show that, in certain circumstances, distinct differ- 

 ences of refractive index may occur in certain liquids 

 at the separating surface between the liquid and a 

 polished glass surface. In a few exceptional cases the 

 differences are very great ; in the case of ether, for 

 instance, they may amount to as much as 002 in the 

 refractive index. 



These results appeared to me of considerable import- 

 ance, and it was intended that the investigations 

 should be continued here. 



As a first hvpothesis, I proposed to assume that the 

 effect was due to variations of pressure in the neigh- 

 bourhood of the interface, such variations of pressure 

 being due to cohesion, and occurring according to the 

 intimacy of contact between liquid and glass. Very 

 attractive lines of thought suggest themselves when 

 the phenomenon is contemplated from this point of 

 view. 



Unfortunately, more urgent preoccupations inter- 

 vened, and the results have therefore been communi- 

 cated to the director of the British Scientific Instru- 

 ment Research Association, in the hope that he may 

 find a place in the programme of work for further 

 studv in this direction. Meanwhile, I should be 

 grateful if anv of your readers could refer me to any 

 prior observations of the kind. F. Twym.an. 



Research Department, Adam Hilger, Ltd., 

 75.A Camden Road, N.W.i, November 7. 



The Audibility of Thunder. 



From reading a recent letter in Nature (October 16) 

 discussing the distance that thunder can be heard, I 

 am induced to send you the following observation : — 

 On the evening of February 26, 1912, when camped 

 on North Chincha Island (off the west coast of South 

 America), a brilliant display of lightning in the distant 

 high interior to the east attracted our attention. The 

 cloud-stratum from which the storm evidently issued 

 lav far behind the clear coastal zone and the lower 

 foothills, but hid from my camp the upper regions of 

 the Cordillera. Both I and a Peruvian friend heard 

 quite clearly the low distant peals of thunder. As I had 

 been told that thunder was an almost, if not a quite, 

 unknown phenomenon on the coast — this was the first 

 thunderstorm, indeed, that my companion, a man of 

 more than forty years of age, had experienced — I pur- 

 posely made a record, during the best part of an hour, 

 of the intervals elapsing between the flashes and the 

 peals, and from my journal I find the average to have 

 been 320 seconds. Henry O. Forbes. 



Beaconsfield, Bucks, November 7. 



Linkage in the Silkworm: A Correction. 



In referring to Tanaka's work on silkworms I made 

 (Nature, November 6, p. 216) a mistake which 

 should be corrected. His discovery was not that two 

 characters linked in the male were not linked in the 

 female, but that in a case of linkage common to both 

 males and females it is only in the males that crossing- 

 over occurs. Since, on the analogy of Abraxas, the 

 female is presumably in the silkworm the heterozygous 

 sex, this observation is complementary to and con- 

 sistent with Morgan's evidence that in Drosophila 

 there is no crossing-over in the male, which in that 

 animal is heterozygous in the sex-character. The 

 paper is in Journ. Coll. Agr., Tohoku Imp. Univ., 

 vii., 1916, pt. 3. Also the forms found by Patterson 

 associated with males and females should have been 

 called "asexual," not "inter-sexes." W. Bateson. 



November 14. 



NO. 2612, VOL. 104] 



THE PREHISTORY OF SOUTH AFRICA.^ 



"THE bulk of Dr. McCall Theal's book is as 

 •^ valuable now as it was when first issued, 

 twenty years ago. But though " illustrated and 

 enlarged," it is not "improved" so much as one 

 would have expected. Dr. Theal does not make 

 much use — though he alludes to its publication 

 in 191 1 — of Dr. Peringuey's important study of 

 the Stone age in South Africa, though the 

 theories of Peringuey and Shrubsall would have 

 materially helped him in his attempts to picture 

 the first peopling of South Africa by Man. Also, 

 in the scanty evidence he has gathered together 

 of the origin and wanderings of the Bushman 

 race he — as do most other historians of Africa — 

 overlooks the statement of the Italian traveller, 

 Ludovico di Varthema, who in his 1508 voyage 

 across the Indian Ocean stopped at Mozambique, 

 and, journeying a short distance inland to some 

 table-topped mountain, described a short-statured 

 savage people living on the mountain-top whose 

 language consisted largely of "clicks," "like the 

 sounds used by Sicilian mule-drivers." I have 

 myself gathered up and recorded legends in South 



('<) 



W 



Fig. I.— Drawings of the skulls of two Strandlooper types : (a) the oldest 

 and most like to the Hamite or the Cro-Magnon of Europe; (^) a 

 Strandlooper skull that is very Bushman-like. The originals are 

 approximately the same size. 



Nyasaland of a yellow-skinned, Bushman-like 

 tribe that lived down to a few hundred years ago 

 on the inaccessible upper parts of Mts. Mlanje 

 and Chiperone. 



So far as we can trace the race movements in 

 .'\frica south of the Zambezi prior to the definite 

 entry of South Africa into recorded history, we 

 find them to be something like this : At a com- 

 paratively remote period — say, thirty to twenty 

 thousand years ago — there was living in southern- 

 most Africa a human type now named or nick- 

 named the .Strandloopers (" shore-runners "), 

 whose skulls show a slight resemblance to the 

 Bushman type, but whose brain capacity was much 

 higher (1600-1500 c.c. in the male, compared 

 with an average of 1200 c.c. in the Cape 

 Bushman, and an average of 1480 c.c. in the 

 Bantu-speaking Negroes). The higher type of 

 Strandlooper skull {a in Fig. i) in fact reminds 

 one of the Hamitic skulls of North-east Africa or 

 of the Cro-Magnon type of Europe thirty thousand 



^ "Ethnography and Condition of South Africa before A. D. 1505." By 

 Dr. (Jeorue McCall Theal. Second Edition in the Present Form (Il!ui- 

 trated), Enlarged and Improved. Pp. xx + 466. (London: George Allen 

 and Unwin, Ltd., 1919.) Price Ss. bd, net. 



