Aprils, 1881] 



NATURE 



529 



Induction Current from Leyden-jar Discharge 



I WAS led to try the following simple experiments with the 

 current induced from Leydeii jar discharge, from the knowledge 

 of what is the case in the Ruhmkorf's coil induced cvnrents. 



I have since fuund the exjieriments have often been tried 

 before ; but all may not be aware how simply and easily the 

 results are shown. 



After trying flat coils I found it more convenient to coil stout 

 insulated wire round glass tubes. A few yards of stout wire 

 rouhd a tube A one foot long and i,J inches diameter gave me 

 my primary coil ; a .'similar tube B of less diameter to put 

 inside A formed my secondary ; and a third coil c enabled me to 

 examine the current induced in B, with respect to its magnetising 

 power, at a distance, away from the influence of the primary A. 

 An electric-il machine, a Leyden jar or two, a number of un- 

 magnetised knitting-needles marked with paper at one end, and 

 a compass needle to test polarity, are also required. 



The experiments are as follows : — 



(i) With coil A alone show that a needle is magnetised accord- 

 ing to Ampere's law ; if we regard the direction of current to 

 be from the + coat of the jar to the - coat. 



Ltydi 



(2) Now arrange the coils as shown, A as primary, B as 

 secondary, c in the circuit of B, and a needle in c. Let there be 

 no break in the circuit of B and c. 



Then the magnetism induced in knitting-needle shows by (i) an 

 action as of a current inverse to that in A. 



(3) Now interpose a break in circuit of B and c, so that induced 

 current gives a spark. We now get the needle oppositely mag- 

 netised, showing a direct induced current ; and the magnetisation 

 is far stronger than in (2). 



This points to the two opposite induced currents both passing 

 when there is no air-break ; and the inverse having the strongest 

 magnetising action ; but the result weak. An air-break stops 

 the inverse current, and the far greater effect of the direct 

 current un-neutralised by the inverse is very marked ; it can 

 easily be shown to a class, for example, 



I may add that I have been unable to try what results one 

 gets with galvanometers from lack of instruments. 



Cheltenham W. Lai;den 



Classification of the Indo-Chinese and Oceanic Races 



It is surprising that Mr. Murton should have had any difTicidty 

 about the characters used by me to indicate the word Papi'nvah. 

 The form in question is identical with his own, the apparent 

 difference being due to the two dilTercnt characters respectively 

 employed by us to express the labial/. In the Arabic alphabet 

 there is no sign for this letter, because the sound does not occur 

 in the Arabic phonetic system. Hence other Muhammadan 

 nations using the A' abic alphabet supply the missing letter in 

 various ways, the Persians, Afghans, and Indians by the form 



v,„^ , the Malays usually by 



Hence the two apparently 



different but really identical forms 



which have puzzled Mr. Murton and his Malays of Singapore. 



A. H. Keane. 



Crabs and Actinia 



The account of the actinia on the claws of a crab in your 



last number (p. 515) is of interest as raising the point as to what 



benefit the crab cerives from its friends. In the Annals of 



Natural History, many years ago, I wrote that, having for a long 



period kept Adamsia palliata with/'. Prideauxiim confinement, 

 I had no doubt but that the white tentacles of the anemone were 

 a bait which attracted various small animals within reach of the 

 crab's claws, though it did not, as Mobius seems to think, in 

 any way aid tlie crab in capturing its prey. 



In the .South Sea Islands I saw a splendid specimen of a crab 

 carrying a large actinia. The habit of the crab was to conceal 

 itself entirely in the sand, leaving the actinia waving its tentacles 

 on the surface. No sooner however did a small crab, annelid, 

 or other creature come within reach than the crab, .shooting its 

 elaw out of its concealment in the sand, struck out, and in most 

 cases captured it. Here there was no doubt of the use of the 

 actinia as a decoy. II. Stuart Wortley 



Patent Office Museum, South Kensington, W , April 4 



Migration of the Wagtail 



With reference to the statement of Ilerr Adolf Ebeling (apud 

 the correspondence from the New York livening Post, quoted in 

 your issue of February 24, vol. xxiii. p. 387), that the fact that 

 wagtails in their wintering "came to Africa, and especially to 

 Nubia and Abysssinia, was then unknown to us," permit the 

 remark th,it as "then" must refer to a date not earlier than 

 1850 (when Heuglin went to Egypt), the appearance of wagtails 

 on the w est coast of Africa, as far south as Cape Verd, had 

 been observed more Ihan half a century before. 



In the Annalts de Chimie for July, 1793, M. Prelong, one of 

 the lieutenants of Stanisla'--, Chevalier de Boufflers, and director of 

 the hospital at Goree, records the arrival on .Se|)tember 14, 1788, 

 of a flight of bergeronnettes from the north. In connection with 

 this M. Prelong remembered that Adanson h.-'d seen swallows 

 at Senegal October 9, 1750 (?), while he himself could testify to 

 their leaving his native pays (the Hautes-Alpes) towards the 

 end of September. Prelong took ship for home in the middle 

 of May, 17S9, and was accompanied by his feathered friends the 

 vi'agtails. N. J. 



New York, March 1 5 



Sound of the Aurora 



Upon this subject it may not be out of place to recall the fact 

 that the passage of large meteors is not uncommonly described 

 as acconqianied by a hissing sound. I have met with state- 

 ments of this kind in the case of meteors which were proved to 

 have been twenty, thirty, or forty miles distant from the observer, 

 and the sound of which, therefore, if it had reached him at all, 

 must have reached him after such an interval of time that he 

 would have been very unlikely to connect the two phenomena. 

 Moreover the sound described in these cases is of a totally 

 different character from the true sound of meteors, which is 

 spoken of by those who have heard it as a heavy roaring or 

 rumbling sound. 



The explanation of the alleged "hissing" is not difficult when 

 we remember that the untrained observer of a bright meteor 

 (although it may be distant fifty or a hundred miles from him) 

 invariably regards it as a near object, falling, it may be, into the 

 next field, or behind a neighbouring hill. Regarding it in this 

 light, he attributes to it, by a well-known mental process, a 

 sound such as a firework at the same distance might be expected 

 to produce. 



May not the "rustling" of the aurora be equally a subjective 

 phenomenon ? George F. Burder 



Earthquake Warnings 



In Comptes rendus, Ixxxi. October, 1S75, I find it stated, on 

 the authority of M. Rivet, Telegraph Superintendent at Fort de 

 France, Martinique, that when that island was visited by 

 repeated earthquake shocks in September, 1875, each shock was 

 preceded by a very marked disturbance of the electric telegraph 

 needles. M. Rivet suggested that in this way warnings other- 

 wise unattainable of impending shocks might be obtained. 



It would be interesting to learn whether this observation has 

 been confirmed by recent experience on the Continent. 



Such v/arnings might often be the means of averting loss of 

 life and property, and in cases like that at Ischia would, by their 

 occurrence or non-occurrence, afford fome additional clue to the 

 real nature of the forces at work. H. M. C. 



Charlton, March 31 



