June 17, 1892.] 



SCIENCE. 



343 



tbe Japanese are not liable to scarlet-fever, and the negroes 

 are equally exempt from yellow-fever, if we could ascertain 

 what condition it is that confers upon them this exemption, 

 ■we might be able to take a long step iu the direction of per- 

 sonal and general prophylaxis. There is no more vital 

 question, none more attractive to the most active minds of 

 the medical profession today, than this of immunity; and 

 in the direction of ethnic immunity there lies a wide avenue 

 for investigation promising to lead to results of the utmost 

 (Utility to the health and welfare of mankind. 



The Builders of the Great Zimbabwe Ruins. 



Among the auriferous reefs of Mashona-land, in south- 

 western Africa, about 20° south latitude, are found a number 

 of remarkable ruins of well-built stone cities, towers, and 

 forts, which have long been an enigma to archaeologists. 

 Needless to say, they were not constructed by any Austafri- 

 can people; no negro or negroid race ever built stone walls 

 voluntarily. The problem seems to be solved by the re- 

 searches of J. Theodore Bent, which are published in the 

 last number of the Proceedings of the Royal Geographical 

 Society. He visited and explored the ruins of the largest 

 .city, called the Great Zimbabwe. This being a word of the 

 ilocal dialect, meaning krall or town. 



His excavations show that these ruins were built and occu- 

 pied by a people engaged in gold-mining. Crucibles and 

 smelting furnaces were found, and in the vicinity " millions 

 of tons" of quartz have been worked over. The stone work 

 3s massive, very firm, the stones often carved and decorated, 

 .and the sites usually of great strategic strength. Many 

 images of birds, carved in stone, and also many phalli, in 

 the same material, were unearthed. Pottery was abundant, 

 ;the fragments often decorated with neat designs of animals, 

 plants, and scenes from life. No coins were exhumed, and 

 no inscriptions discovered, except some rude scratchings on 

 a bowl, which resembled Ogham characters. What is sig- 

 nificant, is the presence in the debris of Persian and Chinese 

 Celadon pottery, which is not of very ancient date. Bent's 

 conclusion is that the gold-seekers were Himyarites from 

 southern Arabia, and that their settlements were destroyed 

 by the savage Zenj from Abyssinia about the ninth century 

 of our era. 



Many consider this to be the Ophir of the Hebrews. An 

 interesting visit to it, not mentioned by Bent, is described in 

 the Verhandlungen of the Berlin Anthropological Society 

 for 1889, carried out by a young German named Posselt. 

 Both accounts present engravings of carved stones, figures of 

 birds, etc. ; but it is singular that neither explorer could find 

 B, single grave or skeleton of this ancient people. 



THE PROPER MOTIONS OF THE STARS.' 



BY W. H. S. MONCK, 



Some time since I pointed out in the columns of the 

 English Mechanic the great preponderance of proper mo- 

 tions in diminishing right ascension in certain catalogues 

 which I examined. I have now examined O. Struve's great 

 Pulkova Catalogue, which contains the proper motions of 

 nearly 2,500 siars, with a similar result. About two-thirds 

 of these motions are in decreasing right ascension. I sus- 

 pect that the sidereal year has been under-estimated by a 

 small fraction of a second, in consequence of which a star 

 whose proper motion is really insensible appears to have a 

 small motion in decreasing right ascension. The effect of 



' From the EuglUli Mechanic, May 2T. 



the sun's motion in space is very evident in the Pulkova 

 Catalogue. The right ascension of the apex of the sun's 

 way (the Americans use the shorter term, goal) may be 

 roughly taken at 18 h. The effect will be to produce an ap- 

 parent motion in diminishing right ascension on all stars 

 between 6 h. and 18 h., and an apparent motion in increasing 

 right ascension on all stars between 18 h. and 6 h. Dimin- 

 ishing right ascension predominates in both cases, while in 

 the latter the excess is only about 20 per cent. 



I noticed, however, a curious fact as regards the motions 

 in North Polar distance. The sun's motion produces an ap- 

 parent increase in North Polar distance in all parts of the 

 sky save the portions situated between the apex and the 

 North Pole on the one hand, and between the antapex and 

 the South Pole on the other. But taking the right ascension 

 of the apex at 18 h., as before, the motions in North Polar 

 distance ought to be symmetrically situated between 6 h. and 

 18 h. and between 18 h. and 6 h. But this is not the case. 

 Between 18 h. and 6 h. the proportion of increasing to 

 diminishing North Polar distances is two to one, while 

 between 6 h. and 18 h. it is only about four to three. It 

 occurred to me that this difference might arise from some 

 special drift in the stars of the Galaxy, of which a compara- 

 tively small number lie between 6 h. and 18 h. in the Pul- 

 kova Catalogue, which deals chiefly with northern stars. 

 I accordingly tried Mr. Stone's "Catalogue of Southern 

 Stars," which so far verified my conjecture. The great pre- 

 ponderance of increasing North Polar distances in it lie be- 

 tween 6 h. and 18 h., and the relative proportions are not 

 very different from those in the Pulkova Catalogue reversed. 

 Further examination will be necessary to clear up the ques- 

 tion; but I venture to suggest that the Galaxy has a south- 

 erly drift relatively to the majority of the non-Galactic 

 stars, and that we would obtain different goals for the sun 

 from the Galactic and the non-Galactic stars. 



May I add that in dealing with the fixed stars our present 

 unit of distance — a year's light-passage — seems to me incon- 

 venient. Besides the advantage of having a space unit 

 instead of a time unit, and the existence of some little un- 

 certainty as to the rate of propagation of light; we must 

 recollect that our standard of measurement is the distance 

 of the sun from the earth. The time occupied by light in 

 traversing this distance is uncertain to the extent of at 

 least two or three seconds, and the difference becomes consid- 

 erable when we are considering very remote bodies. I ven- 

 ture to suggest as a better unit the distance of a star having 

 an annual parallax of 1". This distance is 206,265 times 

 that of the sun. The distance of a Centauri on this scale is 

 about 1.33 and Sirius about 2.5 We should seldom, if ever, 

 have to use numbers as high as 1,000, and the reciprocal of 

 the parallax expressed in fractions of a second would in all 

 cases give the distance. 



THE PEAR-TREE PSYLLA. 



BY J. A. LINTNER. 



Until within a few years the pear-tree has been remarka- 

 bly free from insect attack, the amount of injury from such 

 source being probably less than five percent of that to which 

 the apple has been subjected. Recently two pests have 

 forced themselves upon the notice of pear growers, which 

 have already inflicted serious losses, and threaten, unless 

 arrested, greatly to interfere with the cultivation of this most 

 excellent fruit. Of these, the pear midge, Diplosis pyrivora, 

 which was introduced in this country about the year 1880, 



