March 19, 1885] 



NA TURE 



46; 



not dream of. We have no right to assume that there 

 may not be condensational vibration in the luminiferous 

 ether. We only do know that any vibrations of this kind 

 which are excited by the reflection and refraction of light 

 are certainly of very small energy compared with the 

 energy of the light from which they proceed. The fact 

 of the case as regards reflection and refraction is this : 

 that, unless the luminiferous ether is absolutely incom- 

 pressible, the reflection and refraction of light must gene- 

 rally give rise to waves of condensation. Waves of 

 distortion may exist without waves of condensation, but 

 waves of distortion cannot be reflected at the boundary 

 surface between two mediums without exciting in each 

 medium a wave of condensation. When we come to the 

 subject of reflection and refraction we shall see how to 

 deal with these condensational waves, and find how easy 

 it is to get quit of them by supposing the medium in- 

 compressible. But it is always to be kept in mind to be 

 examined into: Are there or are there not very small 

 amounts of condensational waves generated in reflection 

 and refraction ; and may, after all, the electric force not 

 depend on the waves of condensation ? Suppose that 

 we have at any place in air, or in luminiferous ether, a 

 body that, through some action we need not describe, 

 but which is conceivable, is alternately positively and 

 negatively electrified : may it not be that this will be the 

 cause of condensational waves ? " It is then supposed 

 that two spherical conductors are connected to the 

 terminals of an alternating dynamo machine, and the 

 quotation proceeds: — 



" It is perfectly certain, if we turn the machine slowly, 

 that in the neighbourhood of the conductors we will have 

 alternately positively and negatively electrified elements 

 with reversals perhaps two or three hundred per second 

 of time, without a gradual transition from negative 

 through zero to positive, and the same thing all through 

 space ; and we can tell exactly what is the potential at 

 each point. Now, does any one believe that, if that revo- 

 lution was made fast enough, the electrostatic law would 

 follow ? Every one believes that, if that process be con- 

 ducted fast enough several million times, or millions of 

 million times per second, we should be far from fulfilling 

 the electrostatic law in the electrification of the air in the 

 neighbourhood. It is absolutely certain that such an 

 action as that going on would give rise to electrical 

 waves. Now it does seem probable to me that electrical 

 waves are condensational waves in luminiferous ether, 

 and probably it would be that the propagation of these 

 waves would be enormously faster than the propagation 

 of ordinary light waves. I am quite conscious, when 

 speaking of this, of what has been done in the so-called 

 electro-magnetic theory of light. I know the propagation 

 of electric impulse along an insulated wire surrounded by 

 gutta-percha, which I worked out myself about the year 

 1 CS54, and in which I found a velocity comparable with 

 the velocity of light. . . . That is a very different case 

 from this, and I have waited in vain to see how we can 

 get any justification of the way of putting it in the so- 

 called electro-magnetic theory of light." 



In those parts of the lectures which deal with wave 

 propagation in an isotropic medium, by far the most 

 interesting parts are those which treat of the conditions 

 at bounding surfaces, whether these surfaces be reflecting 

 and refracting surfaces or surfaces of radiating molecules, 

 or surfaces of absorbing molecules. Lord Rayleigh's in- 

 vestigations and his own on the likelihood of the density 

 or the rigidity of the substance composing a molecule 

 differing from that of the ether are also full of interest 



Mm h of this part of the subject has been thoroughly 

 worked out before, but here the treatment is so original, 

 the language is so suggestive, and I need hardly say that 

 the whole course of lectures is so pregnant with useful 

 ideas, that every one who reads this part will be well 

 repaid. 



Having now roughly indicated the novel points and the 

 general mode of treatment of the problem in molar 

 dynamics, I propose in the next notice to give some 

 account of the problem in molecular dynamics, which 

 occupied half of the time. George Forbes 



{To be continued.) 



THE LONG DURATIONS OF METEORIC 

 RADIANT POINTS 



IT is unfortunate that the observation of shooting stars 

 is associated with difficulties of no common order. 

 The very large number of distinct showers visible at the 

 same epoch, their extremely attenuated character, and the 

 many impediments to accurate determinations of the 

 flights of the individual shooting stars proceeding from 

 them, exercise an unfavourable influence on the work and 

 deter many observers from grappling with a subject which 

 is admittedly beset with such perplexing details. Apart 

 from this, there exists the great necessity for observations 

 to 'be sustained during the whole night, and this is rarely 

 practicable either by amateur or professional astronomers, 

 who generally have other important work in hand. In 

 fact, meteoric astronomy requires the almost exclusive 

 attention of the observer, and must be closely pursued for 

 a long period of time if anything like comprehensive 

 results are to be obtained. The voids occasioned either 

 by moonlight or cloudy weather in a short series of 

 observations are only to be filled up by prolonged watches 

 extended over many consecutive years. 



The long visible duration of a large number of radiant 

 points of shooting stars is, it must be confessed, a fact 

 which defies satisfactory explanation. The ingenious 

 theory which had attributed to meteor streams an identity 

 with cometary orbits, required that the visibility of such 

 streams should be of very brief character, though in the case 

 of an abnormally wide system or of a shower directed from 

 a point near the earth's apex the duration might be 

 longer than usual, but the radiant point could not 

 maintain a perfectly fixed position amongst the stars. 

 This general view of the subject is, however, not accord- 

 ant with the results of recent observations, for while there 

 are undoubtedly some cometary showers which display 

 all the peculiarities taught by theory, there are many 

 other streams which continue visible for several months 

 and retain a stationary position in the firmament. It is 

 evident therefore that these streams are presented to us 

 under totally different circumstances as regards orbit to 

 the true planetary showers, and are amenable to con- 

 ditions and laws which form a problem the solution of 

 which is arrested by no ordinary difficulties. 



The multiplicity of streams would naturally originate a 

 false appearance of long duration in certain radiant points, 

 but observations of very precise character would soon 

 show that the point of radiation, as successively deter- 

 mined, differed considerably, being not, in fact, confined 

 absolutely to the same point, in the sky. But it is now 

 proved that there are no differences, other than those in- 

 troduced by small unavoidable errors of observation, in 

 the centres from which shooting stars continue to fall 

 during several months. Indeed, it seems a probable 

 inference from the observations that some showers exist 

 all the year round, though not visible during the epoch 

 when they are very near to the sun. 



That such long enduring radiants of meteors can have 

 a community of origin and belong to physically associated 

 streams in the same degree as the true cometary meteor 

 showers is very difficult to understand. But the fixity of 

 the radiant over so long an interval would yet seem to 

 indicate some bond of close affinity existing between 

 them. At any rate we have no reason to suppose that a 

 large number of showers, distinct in themselves, can occur 

 consecutively from the same points of the sky owing to a 

 common peculiarity of grouping. 



