272 



NATURE 



[October 27, 1921 



Aurora Borealis, Terrestrial Magnetic Disturbances, 

 and Sun-spots. 



In connection with the aurora borealis observed by 

 Major Lockyer on September 28-29 (Nature, Octo- 

 ber 6), there was a magnetic disturoance of consider- 

 able activity recorded at this observatory. It com- 

 menced at'i2h., September 28. There were minor 

 movements on the H magnet, D remaining quiet, 

 until a more active phase of the disturbance com- 

 menced about igh., September 28. The major move- 

 ments on all three magnets, D, H, and V, took place 

 between ih. 36m. and 4h. 50m., September 29. The 

 extreme ranges on the curves were D 30', H 827, and 

 V 857 (7=10-" C.G.S. units). The mean daily ranges, 

 for comparison, for the quiet days of. September were 

 D 7', H 367, and V 117. 



The only spot on the sun on September 28-29 was 

 of moderate size, in latitude +85 and longitude 560, 

 and it was approaching the sun's western limb. But 

 on September 28 the longitude of the central meridian 

 of the sun was 354- 1°- This gives the clue to the 

 probable origin of the magnetic disturbance which 

 accompanied the aurora, for the position is very near 

 the longitude of the following spot of the great group 

 of last May, namely, 358-8°, which on its passage 

 across the sun's disc was connected with the series 

 of magnetic disturbances of great violence. These, 

 with a lull on May 18, persisted from May 12 to 

 May 21. This spot -group was on the sun's equator, 

 and also crossed the central meridian on May 14-15 

 (Nature, June 2, p. 426). 



It appears to be most likely that this region of the 

 sun has remained magnetically active since the series 

 of violent storms of May 12-21. For we get the fol- 

 lowing sequence of magnetic disturbances, at intervals 

 of 27 or 28 days, corresponding to the period of the 

 sun's synodic rotation: — May 12-21, v. v. great; June 

 6-10, great; July 7-9, moderate, August 3-5, great; 

 September 2, v. great ; and September 28-29, v. great. 

 The magnets have also been considerably disturbed 

 on the early days of the present month of October, 

 especially on October 5 and October 8. Meanwhile, 

 the sun has been practically spotless. But here, 

 again, with regard at least to the disturbance of 

 October 5, there is a sequence of disturbances corre- 

 sponding to the synodic rotation period of the sun, 

 which probably has its origin in the later phases of the 

 violent storm of May. The sequence is: — May 21, 

 moderate; June 18, calm; July 15, moderate; August 

 II, moderate; September 8, great; and October 5, 

 v. great. It will be noticed that in this sequence the 

 magnets were quiet, and activity was in abeyance on 

 June 18. 



Since the violent storm of May there have been in 

 all, until October 8, 28 moderate, 2 great, and 4 very 

 great disturbances. All these disturbances, except 

 four marked moderate, fall into four series corre- 

 sponding to the synodical rotation of the sun, and of 

 these, again, 12 moderate, 2 great, and 3 very great 

 belong to the two series already discussed. It would, 

 therefore, be premature to conclude, from the absence 

 of sun-spots or other surface phenomena of the sun, 

 when a magnetic disturbance occurs, that there is 

 onlv a casual connection between sun-spots and terres- 

 trial magnetic disturbance. An area on the sun may 

 seemingly remain continuously or recurrently active 

 for several solar rotations, even after the disappear- 

 ance of the original solar disturbance. Or it may be 

 that clouds of electrons discharged from a very active 

 region on the sun remain undiffused for a consider- 

 able period. At the same time it is not evident why 

 the magnetic activity should sometimes actually in- 

 crease after a lull succeeding the original violent dis- 



NO. 2713, VOL. 108] 



turbance. Possibly six;ctro-heliograms in calcium 

 light may help to elucidate the subject, 



A. L. CORTIE. 

 Stonyhurst College Observatory, October 14. 



Sex-change in the Native Oyster. 



Dr. Orton's letter on the above subject published 

 in Nature of July 7, which 1 have just seen, touches 

 a matter not only 01 great biological interest, but also 

 of marked importance in the economy of oyster 

 fisheries. I can confirm the presence of sperm- 

 moru.ai in oysters which are functioning as " white- 

 sick " females, and also the observation mat on being 

 placed in sea-water the sperms appear to be fully 

 ripe. There are, however, a large number of oysters, 

 apparently the majority, in any fair sample which 

 may be examined at the breeding season which show 

 no advanced female elements, but are functioning 

 solely as males. These oysters, so far as I have been 

 able to notice, do not show signs of any rapid sex- 

 change. 



When one reflects that oysters are naturally found 

 in beds, and that fertilisation requires the free passage 

 of sperms through the water to impregnate function- 

 ing temales— if we are to discard Lacaze-Dutier's idea 

 of self-fertilisation— it seems inevitable that there is 

 an immensely greater loss amongst the male than 

 amongst the female elements, and the presence of an 

 excess of males seems explained. In the same way 

 the development of active sperms in the gonads of 

 oysters which are already bearing fertilised embryos 

 in their mantle cavities may be a provision to further 

 augment the supply of sperms. Tlie annual breeding 

 period— physical conditions being favourable— is 

 spread over a considerable interval in this country. 

 During the past summer, for instance, free-swim- 

 ming spat could be found early in June, yet I found 

 oysters with black spat on July 20, and " white-sick " 

 oysters as late as August 26. In that period, it seems 

 probable, from Dr. Orton's observations, that indi- 

 vidual oysters may have functioned first as female 

 and then as male shellfish. It would be very interest- 

 ing to learn, however, if in the Plymouth observa- 

 tions any ovsters functioning first as males showed 

 any signs of ripening into females. The annual 

 change of sex w^hich Dr. Orton refers to as possible 

 may be onlv in those oysters which are first pre- 

 dominantly female in sex.' W. L. Calderwood. 

 Edinburgh, October 14. 



A Relation between the Combined Atomic Volumes and 

 their Optical Refractivities. 



It has been shown (" Monograph on Molecular 

 Volumes,'^ Longmans, 1917) that there is a periodic 

 relationship between the atomic volumes of the com- 

 bined elements. The submultiple 36, which is the 

 atomic volume of combined hydrogen, has been found 

 significant. 



Table of Atomic Volumes 

 C A N A O 



14-8 (2-8) I2-0 (4*6) 7"4 

 A 15-2 15*0 161 



Si P s 



30-0 27-0 23-5 



F Mean 



6-5 37 

 15-1 

 CI 



21-6 



7'4 

 Br 



2g"o 



80 



I 



37-0 



I, N, and S, mean values. 



