6o4 



NA TURE 



[October 19, 1905 



the nerve-annoying tremors incident to the usual recipro- 

 cating engines, the Vu^iniah has proved far and away 

 the quietest steamship I have ever voyaged on. Excellent 

 evidence of this, I think, lies in the exceptionally large 

 number of passengers who dined comfortably in the saloon 

 at the, roughest period of our entire passage. There was 

 a fairly ' heavy sea on, and the ship was by no means free 

 from wave-origined motion. So I am quite of the opinion 

 that sea-sickness and all its train of discomforts must be 

 greatly aggravated by the fengine-borne tremors of the 

 ordinary steamship, and that many people who are delicate 

 sailors under ordinary conditions might take ocean journeys 

 with comparative comfort in a turbined ship. , 



So unostentatious are the ' rotary engines of the 

 Virginian, let alone their occupying but one-fourth the 

 space of the usual expansion engines, that the quietness 

 of their powerful and effective working, in every part of 

 the ship, was continually deceiving one into thinking that 

 the vessel had lost headway, or might have come to anchor 

 altogether. Especially was this true in the dining saloon, 

 that most critical of all spots, where one could rarely 

 detect so much as a ripple on water in a glass, although 

 going ahead at full speed of 15 knots. 



To my mind the Virginian seerried to behave all the 

 voyage quite as if her motive power w-ere entirely without 

 her ; in fact, she could scarcely have ridden more smoothly, 

 or with less of that exasperating vibration (the unceasing 

 action of which, I am convinced, is a prominent factor in 

 inducing mal de mer), if she had been towed at the 

 identical speed bv a huge hawser. David Todd. 



R.M.S. Virgin'uin, Straits of Belle Isle, October 4. 



A Parasite of the House-fly. 



Reverting to the recent correspondence under this head- 

 ing between Mr. Davenport Hill and Prof. Hickson 

 (Nature, August 24 and 31), I recall that a few years 

 back many house-flies with Chelifers attached were sent 

 to me at the Natural History Museum for determination of 

 the species and explanation of the phenomenon. The first 

 task was as easy as the second was difficult. The Chelifer 

 was in most, nay in all, cases, so far as my memory 

 serves, Chernes nodosui. But those who suggest that 

 the explanation is to 'be sought and found in the value 

 of the habit as a means of securing dispersal hardly 

 realise, I think, the ditTitulties in the way of' its 

 acceptance. Chelifers are' minute,' active, and, for arthro- 

 pods, not exceptionally prolific. Hence the sufficiency of 

 " elbow-room '* for the survivors of a family of, say, forty, 

 on the site choSen by the female for her progeny does not 

 coincide with the' view that they have special need of 

 transportation. Moreover, when we remember that a 

 Chelifer attached to a fly is exposed to the danger of being 

 killed by the enemies of that insect, and also to the great 

 chance of being landed in a W'holly unsuitable environ- 

 ment, it can hardly be maintained that the advantage 

 derived from this method of dispersal has been a sufficiently 

 important factor in survival to preserve and foster an 

 initial instinct to g'^ab and hang on. to the legs of flies. 

 That the aerial porterage thus secured, whether fortuitously 

 or " intentionally," must be a means of dispersal is too 

 obvious to dispute ; but I do not think more than that 

 can be claimed for it, since it is as likely to end in failure 

 as in success. 



Chelifers may be found not uncommonly beneath the 

 wing-cases of large beetles. Presumably this habitat has 

 been adopted for the sake of the food supplied by the 

 parasitic mites infesting- the beetles. This fact, I think, 

 suggests a line of investigation w'hich mav lead to a more 

 satisfactory e.xplanation of the association between Chelifers 

 and flies than that put forward in Prof. Hickson 's letter. 



Zoological Gardens, October 14. R. I. PococK. 



Incandescence of Meteors. 



It is with great diffidence that I approach this difficult 



subject, but the thcoi-y that the incandescence of meteors 



is due to the heat generated by the friction between these 



bodies and the molecules of gas composing our atmosphere 



NO. 1877, VOL. 72] 



I have ■ aHvays found diflicult to believe. The following 

 theory- is one which has occurred to me, and seems quite 

 a- plausible one. Meteors are usually of a metalliferous 

 nature, and consequently will have a comparatively low 

 electrical resistance. When they approach the earth they 

 will enter a magnetic field, and they will cut the lines of 

 force of this field at a high velocity. A high electrical 

 potential Will be generated, and consequently electric 

 currents which will be inversely proportional to the resist- 

 ance. The electrical energy thus produced will be dissi- 

 pated in heat, and if of sufficient intensity will raise the 

 meteor to incandescence. The truth or otherwise of this 

 theorv could, I believe, be calculated, as the data necessary 

 for doing so will, be' at the disposal of readers of N.atvre 

 who make this branch of astronomy their study. This 

 theory may have already been advanced, as I am not in 

 touch with the latest developments of the science. 



Coatbridge, September 5. George A. Brown. 



The electric currents which the author of the above lettir 

 regards as possibly constituting an efficient source of the 

 luminosity of meteors must no doubt arise, and play .i 

 certain part in the heat and light development. But the 

 measure in which they can be supposed to contribute to 

 it must clearly be extremely small ; or rather, it must be 

 incomparably subordinate to the intense ignition of the air 

 produced, not at all by friction,' but by the air's adiabatic 

 compression against the front surface of the meteorite . 

 which is certainly quite competent, by itself alone, to develop 

 what may be said to approach pretty nearly to fabulous 

 degrees of temperature. If the kinetic energy of translation, 

 in foot-pounds {v'l2g), of i lb. of the air propelled (at, say, 

 30 miles per second) with the meteor's speed (t;' feet/sec.) 

 on its front face, be divided by 330; the- number thus 

 obtained (1,180,620° C, in the case .supposed) will be the 

 number of centigrade degrees through wh|ich it will be 

 heated by the pure process of cjompreijsion, supposing that' 

 the air can continue to subsist at all 'with .its ordinary 

 mechanical deportment and thermody'namical^ prop'erties un- 

 affected at that enormously high tepiperature.' In the 

 further forward, gradually advancing layers; and in the 

 laterally escaping currents of the air,' on' which'the high 

 forward speed of the' meteor is dnly.'pdrtially impressed, 

 and which move more slowly on their various courses, the 

 compressions are cor'refepondinglv 'less,- and the lower but 

 still exceedingly high tem'perafurefe 'can be 'Similarly calcu- 

 lated 'from any fair estimates ■■ of the 'air's collective or 

 absolute velocity of translation in those different positions. 



It is in the different rates of transport .of these heated 

 air-streams, all of them, as well as the highly attenuated 

 motionless atmosphere around, affording very e^y passage- 

 ways to electricity, across the earth's .magnetic 'field or 

 system of lines of magnetic .force, that fitting circuits can 

 certainly be found (either passing through, or else entirely 

 omitting the meteorite itself), in which, in the way 

 suggested in the above .letter, electric currents may be quite 

 certainly concliided to be .magneto-electrically induced. 

 For ^vhile one part of a closed air-circuit resting against 

 the meteorite's front surface, and another part of it situated 

 inthe still atmosphere in front of or behind it,' would be 

 journeying towards or from each other' with full meteor- 

 speed, the circuits so composed would be most suitably 

 conditioned for developing induced currents round them by 



^ .Mthough a very general belief, it is as yet an en lirely mistaken supposition 

 that the high speed of impact of a meteorite into the rarer regions of the 

 atmosphere reduces the air, by givins it no time to dissipate itself in front of 

 the meteorite, to a state of granulation, or to a wedged throng of. molecules 

 producing heal by friction infi-r sr and against the surface of the meteorite. 

 Just the reverse of this condition is, however, really true, that the air 

 remains a perfectly and frictionlessly elastic fluid, however much it is com- 

 pressed ank] intensely heated by the impact. The speeds of sound-waves in 

 the heated air which perform the office of transmitting and maintaining the 

 orderly array of pressures in the streaming flows, at length differ in defect, 

 in fact, from Ihe air's speeds themselves in proportions which, as those 

 mount up to meteor-speeds of many miles per second, only decline asymp- 

 totically to about the ratio i:\^5, or nearly 1:2!- Sifice, then, these 

 sound-waves, which convey the strokes and shocks of the collision to and 

 fro between "the meteor-cer.tre and the surrounding air. arise and travel 

 in the moving field of the compressed air ds if it were at rest, it is easy to 

 perceive that by their extremely rapid actions a most^ exceptionally perfect 

 elastic-fluid relation, or steady disposition of the lines, or lanes of air-flow 

 and blast-pressure, must really he established and maintained in evenly 

 persistent shapes and contour, in the swirl of incandescent air which forms 

 the meteor's head. 



