Mar. i6, iSSS.] 



SCIENTIFIC NEWS. 



245 



inaudible tread of one of its natural foes. The first 

 intimation to a glow-worm of its peril would be its pass- 

 age down the gullet of its enemy; no withdrawal of its 

 light could possibly avail it then. And therefore, the 

 fact that the exhibition of the light is not involuntary 

 adds only to the mystery which surrounds it. 



The luminosity does not entirely cease upon the death 

 of the insect. The writer, upon one occasion noticing 

 the familiar greenish light upon the floor of a room in 

 which several glow-worms had been confined, found on 

 examination that it was due, not to one of the living 

 insects, but to the dead body of one which had evidently 

 been accidentally trodden upon and crushed, and which 

 had remained there sufficiently long for its corpse to 

 have withered and shrivelled. Yet the light was still 

 fairly powerful, although the liquid from which it pro- 

 ceeded was dry. 



We have a second glow-worm in Great Britain, known 

 only to entomologists, and restricted to one or two 

 localities upon the south coast ; but this, as the female 

 has not yet been discovered, is not available for purposes 



Neither can we say what causes the light in the centi- 

 pede, the jelly-fish, or the animalcule. Again, we know 

 nothing of the source whence it proceeds, or the func- 

 tion which it may fulfil. The whole subject is one of 

 intense interest, but of equally intense difficulty. It is 

 not a mere question of anatomical detail, but one which 

 very probably involves some force unknown as yet to the 

 zoologist, unknown, very possibly, to man, but of far 

 more importance than any of us suppose or imagine. 

 And to him who devotes his time and his energies to the 

 work of investigating the source and the nature of this 

 most mysterious light, the results of his labours, although 

 they may not be conclusive, cannot but prove instructive 

 in the highest degree. 



THE INDUCTION TOP. 



THE induced currents developed in metallic masses 

 in motion, in magnetic fields, can be very strikingly 

 displayed by the well-known experiment of Foncault's disc. 



of comparison. In the fire-flies and lantern-flies of the 

 tropics, however, we find the luminosity greatly intensi- 

 fied, just as is the poison of the venomous snakes, and 

 the scent of the many creatures which depend upon 

 their offensive odour as a means of protection. Capture 

 a fire-fly, and by passing it backwards and forwards 

 along the lines one can see to read a book or a news- 

 paper upon the darkest night. Yet still we are in the 

 same condition of absolute ignorance concerning the 

 nature of the light, its origin, and its uses. It does not 

 appear to be in any way due to the operation of heat — 

 a difficulty for those who insist that fight and heat are 

 almost interchangeable terms. It is certainly more 

 intense if its producer be plunged into oxygen ; but that 

 is probably due to the increased vitality which naturally 

 ensues from the experiment. Not the most susceptible 

 of instruments will detect any alteration in the bodily 

 temperature when the light is turned on or turned off'. 

 Not the most delicate of chemical tests can show us that 

 this light is in any sense the product of combustion. The 

 bodily functions go on just as in any other insect, and 

 there is not even any direct communication between the 

 secretory glands and the organs of digestion. 



But this instrument is both complicated and expensive ; 

 hence La Nature thinks it useful to call attention to 

 the much simpler apparatus represented in the ac- 

 companying figure. This instrument demonstrates in a 

 very simple manner the existence of these induction 

 currents, paradoxical at first sight, but susceptible of an 

 easy explanation. 



The apparatus as constructed by M. Marnet consists of 

 a disc of sheet-iron mounted on an axle, and set in rapid 

 rotation by means of a string like the common humming- 

 top. When at rest, the iron disc is attracted by a horse- 

 shoe magnet presented at any point of its surface, but 

 when the disc is spinning rapidly, if we present either 

 pole of the magnet, or both together at its surface, there 

 instantly ensues repulsion, and the top takes an inclined 

 position, which it retains as long as its velocity is 

 sufficient. When the speed falls below a certain rate, 

 depending on the thickness of the disc, on the magnet, 

 its distance, position, etc., the repulsion ceases ; the 

 disc is then attracted, it clings to the magnet, and the 

 magnetic properties of soft iron are resumed. 



The explanation of the phenomenon is very simple. 

 On approaching the magnet to the revolving disc, this 



