174 



NA rURE 



{June 20. 1889 



about six miles' (Phi!. Trans. R.S., vol. clxviii. p. 289 ; quoted 

 on p. 308 of Darwin's ' Coral Reefs,' third edition). Here he 

 evidently refers to the shallow water extending on all sides to the 

 lOO-fathom line, where there is about that depth a sudden fall. 

 The fact is, such a character belongs to a large number of oceanic 

 islands, whether surrounded by fringing reefs or no reefs what- 

 ever, and is due to quite other causes than reef-building." 

 June 14. H. B. G. 



The Fireball of May 29, 1889. 



The fireball which I mentioned in Nature of June 13 (p. 150) 

 as having been seen at Leeds on May 29, was also observed at 

 Belfast by Mr. I. W, Ward, who recorded the latter part of its 

 course as from Vega to a Cygni. 



Comparing this path with that assigned by Mr. D. Booth at 

 Leeds, I find the radiant point at 214^° - 7°, which was in 

 azimuth li° (west of south), and altitude 28^° at the time 

 (loh. 45m.) of observation. 



When first seen at Leeds the fireball was situated over a point 

 in the Irish Sea, in lat 53° 58' N., long. 5° 22' W., and its 

 height was sixty-one miles. At its disappearance it was six miles 

 west-south-west of Stranraer on the coast of Wigtown, Scotland, 

 at a height of twenty-five miles. The earth point was ten miles 

 west of Troon, Ayrshire. The real length of path traversed 

 was seventy-five miles, and the velocity eight and a third 

 miles per second deduced from the estimated time of flight 

 (nine seconds) at Leeds. 



The radiant point at 214^° - 7° in Libra is situated near the 

 earth's anti-apex, and the motion of the fireball would therefore 

 be extremely slow, as it must have overtaken the earth in her 

 orbit. It is curious that several doubly- observed meteors which 

 have made their apparitions in the spring months have given the 

 same radiant point. Thus the large fireball of May 12, 1878, 

 seen in Scotland and the north of England had a radiant at 

 214° - 7° (Prof A. S. Herschel). The conspicuous meteor of 

 April 21, 1889, observed by Prof. Herschel at Croydon and the 

 writer at Bristol, had a radiant at 218° — 5°. The vernal months 

 appear to furnish us with a long-enduring shower from this 

 special region of the zodiac. W. F. Denning. 



Bristol, June 15. ■ 



Meteor. 



About 11.30 p.m. on the night of June 13, the sky being 

 partially covered with fleecy clouds slowly drifting from the 

 south-west, so that the full moon was frequently obscured, a 

 shooting-star appeared in the north, at an elevation of about 50° 

 to 60°, and descended obliquely towards the east. It was as 

 bright as a star of the first magnitude, and was visible during a 

 slightly zigzag flight of some 30°, leaving no trail. But the re- 

 markable thing was that the sky in that quarter was pretty 

 closely covered with the slowly-moving fleecy clouds, so that no 

 fixed stars were visible. The meteor, therefore, must have been 

 below the clouds, at least in the latter part of its course. 



Birstal Hill, Leicester. F. T. MoTT. 



spectively, and if C be the position of the screen for which the 

 spot disappears when viewed from the side towards A, and C 

 the corresponding position when viewed from the other side, it 

 is usual to say : Take the mean of AC and AC, and the mean of 

 BC and BC ; the squares of the means will be approximately 

 proportional to the intensities. 

 The relation 



I ^ A C . A C 

 r BC . BC' 



is more exact, as may be shown by the following : — 



Let a be the fraction of the light falling on unit area of the 

 spot from A which reaches the eye, and b the corresponding 

 fraction for the dry part ; and let c and d be the respective frac- 

 tions of the light falling on the other side of the paper, which, 

 after passing through, reach the eye. Then, since the spot and 

 the dry part in the position C are equally bright when viewed 

 from the side towards A, we have, equating the light per unit 

 area from the two p>arts — 



a -V c — b H- a, 



AC^ BC-^ AC-^ BC'-' 



Stationary Dust-Whirl. 



Yesterday morning, at 9.30, I was fortunate in witnessing 

 a stationary dust-whirl, about a hundred yards from where I 

 stood, on a dust-covered highway lying due east and west. The 

 morning was warm, 67° '5 in the shade, barometer at 30'o6, and 

 the sky clear, excepting a few isolated cumulus patches. The 

 air was still, the wind-vane indicating north-west. The ap- 

 pearance of the whirl presented a resemblance to a fountain 

 of water playing, only the base was broader than the upper part, 

 which was perfectly columnar. It remained, for nearly five 

 minutes, absolutely stationary, then suddenly ceased, recom- 

 mencing for a few seconds, on a much smaller scale, some ten 

 yards westwards. Its height, when at its best, would be about 

 25 feet, and its diameter, midway, 2 feet. I could not correctly 

 ascertain the spiral motions of the whirl, but judged the outer 

 spiral to move from west to east, and upwards. The wind 

 jumped round into the north-east shortly afterwards, with clear 

 sky, and the barometer steadily rising. No others were seen 

 during the day. J. Lovel. 



Driffield, June 17. 



Bunsen's Photometer, 



If we place the "grease- spot "'screen between two sources of 

 light, situated at A and B, whose intensities are I and I' re- 



AC 



i^ 



b) = _L_(^ 

 ^ BC- 



(1) 



If C be the position of the screen for which the parts appear 

 equally bright, as seen from the other side, we have — 



AC-^ + BC-" = AC'^''+BC-^' 



-^'-^Ja - b) ^ ^L [d - 

 BC- ' AC- 



(2) 



From (i) and (2) — 



AC^ 

 BC 



AC 

 BC" 



li a + c = b -\- d, v/Q see that C and C must coincide. This 

 condition implies that the light lost is the same for the spot and 

 for its surroundings. 



In the method of using the photometer, in which the two 

 lights to be compared are balanced successively against a third 

 light, and the spot in both cases is viewed from the same side, 

 the inequality of the portions of light lost by the two parts does 

 not disturb the result. D. M. Lewis. 



University College, Bangor, June 6, 



THE TUTICORIN PEARL FISHERY. 



A FTER an interval of more than twenty-seven years, 

 -^"^ the pearl-oyster {Avicula fucata, Gould) has pro 

 diiced pearls off the Madras coast of the Gulf of Manaar, 

 in sufficient quantities to be worth the expense of fishing. 

 The last fishery of the Tuticorin banks took place in the 

 years 1860-62, and resulted in a net profit to Government 

 of Rs. 3,79,297 (^37,929 at par). In olden times, when 

 Tuticorin was in the possession of the Portuguese and 

 Dutch, the fishery used to be carried on much more fre- 

 quently than it is at the present day, and a difficult prob- 

 lem, which remains to be solved, is, What are the causes 

 of the decline of the pearl fishery, and how can the 

 Tuticorin banks be made to yield a more frequent har- 

 vest ? Whether the baneful influence of the Molluscs 

 known locally as soorain and killikoy (Modiota sp., anc 

 Avicula sp.), the ravages of the file-fishes (Balistes) anc 

 Rays (Trygon, &c.), poaching, or currents, are responsible 

 for the non-production of an abundant crop of adull 

 pearl-producing oysters during more than a quarter ol 

 a century, it would be impossible to decide until ouf 

 knowledge of the conditions under which the pearl| 

 oysters breed, develop, and live, is more precise thai 

 it is at present. 



Superstition, as of old, still chngs to the native divers; 

 and I read, in a recent issue of the Times of Ceylon^ tha| 

 " at present there are said to be 150 boats, with their ful 



complement of men, all waiting at Kilakarai in readines^ 



