April 2Z, 1881] 



NATURE 



605 



picked up a good sub-triangular wedge-shaped implement. 

 Further search produced a second iuiplement, a good trimmed 

 flake, and a few simple flakes. The worked flints in the Hert- 

 ford gravel are however so rare that the search for them is the 

 most ho])eIess task conceivable. There i^ not more than one 

 flake in 500 tons, not one implement in 5000 tons of gravel. 

 The gravel from Ware is also brought to the east of London for 

 ballast, and I happened last year to mention the fact of my dis- 

 coveries to Mr. J. E. Greenhill, the Principal of a school near 

 Hackney Downs. Mr. Greenhill at once not only searched him- 

 self, but set his pupils to look over the Ware gravel, then laid 

 down in large quantities near Clapton, with the result that a 

 large broken ovate implement \\ a^ found and several flakes. I also 

 found a large and heavy '* slice " flake with numerous facets on its 

 worked side in the same gravel. Mr. Greenhill's success caused me 

 to lock cai'efully over a similar lot of gravel from Ware, laid down 

 near Victoria Park. In this I found a sub-triangular implement 

 and three flakes. I have also found a large greatly abraded 

 flake in the Amwell gravel at Amwell. El.-ie.vhere in east and 

 north-east London I have looked over thousands of tons of Hert- 

 ford and Ware gravel without decisive result A week or two 

 ag9, however, as my younger son \^■as returning home through 

 Finsbury Park, he picked up a good scraper-like implement in the 

 gravel (presumably from Hertford), just thrown on to the road 

 inside the park. On hearing of the discovery I at once went to 

 Finsbury Park and looked carefully over all the recently thrown 

 down material, butv\ith no further result. I have visited the 

 different pits at Hertford, Ware, and Amwell several times, but 

 there is never enough gravel exposed (considering the extreme 

 comparative rarity of the implements and flakes) to give one a 

 chance of findiu'^ an implement. 1 hav.^ found in the pits 

 several simple flakes, with the cone of percussion, and that is 

 all. At what depth the implements occur in the gravel I do not 

 know, but that implements really do come out of the high 

 gravels overlooking Hertford and Ware I think there can 

 be no doubt. Reference was made by me to these imple- 

 ments at the Anthropological Institute three years ago, when 

 two or three specimens were exhibited by me. 



WORTHING'ION G. SMITH 

 125, Grosvenor Road, Highbury, N. 



Sound of the Aurora 



With every respect f)r the ability and acuteness of the late 

 Sir John Franklin and his companions,' I do not think it con- 

 clusive, as Mr. House seemi to dn, that because they heard no 

 sounds " with the aurora borealis" (NATURE, vol. xxiii. p. 556), 

 no sounds are produced by it. 



All Indians, both on the shores of Hudson's Bay and near 

 Bear Lake, and the Eskimos on many parts of the coast, assert 

 positively that the bright, varying, flickering,' and rapidly- 

 nrjving aurora; do produce sound. The senses of hearing and 

 smelling in the Indian and Eskimo are far more acute than in 

 the civilised man ; and both sounds and smells which to the 

 latter are not perceptible are perfectly so to the more sensitive 

 auditory and olfactory organs of the savage. 



The theory that " the attractive force of the aurora is increased 

 within a certain limit as its rays proceed southward" is scarcely 

 borne out by my experience. 



When wintering at Fort Hope, Repulse Bay, in 1846-47 and 

 1853-54, '^f- 66' 32' N. , the result of my observations was, as 

 far as I can discover, exactly similar to that of Parry in 1824-25 

 at Port Bowen in lat. 73" 15' N., 400 miles further n orth and fifty 

 miles west of Fort Hope : at both no effect was produced on the 

 magnetic needle. 



At Repulse P.ay, and it may have been the same at Port 

 Bowen, the character of the aurora was perfectly different from 

 that generally seen at Great Hear Lake, which acted so power- 

 fully on the needle, the former being almost aUays of a uniform 

 pale yell )V.- or straw colour, with little rapid motion, whereas the 

 latter was generally flashing, flickering, rapidly moving, and of 

 diverse hues. 



One peculiarity of the aurora; observed at Repulse Bay may 

 be worthy of notice : they were chiefly seen to the magnetic 

 south — that is south 62° east true— usually in the form of an arch 

 rather low down — and I may add that in that direction at a 

 distance of thirty or f jrty miles fro.n our head-quarters a large 

 extent of sea is kept open all winter by strong currents. The 



t appropriate term frjm Prof. Stokes. F.R.S., &c., of 



Eskimjs of Repulse Bay do not say much about the aurora 

 beyond expressing a belief that it is the spirits of their dead 

 visiting each other in the heavens. 



It is probably a matter of little or no importance in a question 

 of this kind, but Mr. Rouse has given the latitudes of the 

 southern shores of Great Bear Lake from 90 to 200 miles too 

 far north. 



Fort Franklin, where Franklin made his chief observations, 

 is situated in latitude 65° 12' N. at the extreme south-west of 

 Greit Bear Lake, whereas Fort Confidence, where Sir J. 

 Richard on and I made ours with like results, is at the extreme 

 north-east of the lake in lat. 66° 54' N., the stations being 150 

 miles distant from each other. 



It is perhaps not being too sanguine to hope that in this 

 ]ierio 1 of marvellous discoveries, some instrument may be — if 

 not already — invented, with the aid of which one may be able 

 to decide the question satisfactorily as to whether the aurora in 

 any form doas or does not produce sjund. John Rae 



4, Addison Gardens, April 16 



THE SCIENTIFIC PRINCIPLES INVOL VED IN 

 ELECTRIC LIGHTING^ 



II. 

 Lectures III. and IV. 

 A LL machines for the conversion of mechanical work 

 ■'^ into electricity are founded on Faraday's great dis- 

 covery of the induced current derived from the relative 

 motion of a magnet and a coil of wire. They are either 

 continuous-current or alternate-current machines. From 

 the continuous-current machines of Pixii in 1832 and 

 Saxton and Clarke in 1835 and 1836, we pass to Wheat- 

 stone's introduction in 1845 of electromagnets in place of 

 permanent magnets to produce the magnetic field. In 

 1854 Werner Siemens and Halske introduced the Sie- 

 mens armature, in which the coil is wound longitudinally 

 in a groove. In 1S54 Hjorth patented an improved 

 magneto-electric battery, in which the currents induced in 

 the revolving armature pass round the electromagnets 

 and produce the mignetic field. This is the principle of 

 the dynamo-electric machine, which was afterwards re- 

 discovered by Siemens and by Wheatstone simultaneously 

 in 1 867, when on the same evening their two papers were 

 presented to the Royal Society. 



Then followed the Gramme armature, in which coils of 

 wire are wound in sections all in the same direction round 

 a ring ; each section, when a current is flowing through 

 it, may be regarded as an electro-magnet, and its principle 

 of action is clear at once from the principles of Arago 

 and from Lenz's laws for induced currents. 



In dynamo-electric machines the external work in the 

 electric arc is proportional to the square of the current, 

 and is also proportional to the number of turns of the 

 armature per minute. 



Any disturbance in the resistance of the arc reacts on 

 the electro-magnet, altering the strength of the magnetic 

 field, thereby increasing the disturbance ; this is the great 

 disadviintage of dynamo-electric machines as compared 

 with magneto-electric machines, where the magnet is 

 either a permanent magnet or is excited by means of a 

 separate current. Wilde, in 1863, employed a separate 

 continuous current machine to give a permanent magnetic 

 field, and made the armature of the second machine to 

 revolve in this magnetic field. In alternate-current 

 machines there is no commutator for making the current 

 continuous, but tlie currents from the coil are collected 

 and sent through the external resistance in opposite 

 directions for every half-turn of the armature. The 

 Alliance magneto-electric machine was the first of these, 

 which was converted by Holmes into a continuous-current 

 machine, and was by him first used in 185S to produce 

 the electric light for lighthouse illumination. He after- 

 wards again converted his marhine into an alternate- 



By Prof. W. Go'lls Ada 



ed from p. 582 



