ino 



KNOWLEDGE. 



[July 2 le9l. 



but if the nebulous envelope was composed of a fog of 

 solid particles, each acted upon by repulsive forces from the 

 nucleus as well as by a repulsion from the sun, this test 

 for comparing the mass of the comet with the mass of the 

 sun fails us. Although we know from observation that in- 

 candescent gaseous matter generally exists round the head 

 of a comet, we have no evidence that the gaseous matter 

 forms an atmospheric envelope in equilibrium under 

 gravity: indeed, it seems probable, from polariscopic 

 observations, that a great part of the nebulous light is due 

 to solar light dispersed by small particles. 



Through the kindness of Mr. J. N. Cobb, of Phila- 

 delphia, I have obtained fi-om Mr. Alfred Eordame, of Salt 

 Lake City, Utah, a beautiful photograph of Rordame's 

 comet, taken on the 13th July. Its very rapid motion 

 amongst the stars is evidenced by the long trails they have 

 left on the plate while the camera was kept following the 

 comet in its motion amongst the stars. It will be seen 

 that, as in the photograph of Brooks' comet, reproduced 

 in the May number, the photograph of Rordame's comet 

 affords ample evidence that the matter of the tail was 

 driven away in clouds of varying density, and that these 

 clouds have a structure which suggests an analogy with 

 solar prominence forms, and seems to indicate that the 

 outrushing material has suffered resistance in passing 

 through a resisting medium. 



That these rapid changes of form and cloud-like masses 

 in comets' tails have only recently been noted seems to 

 point to the conclusion that the intermittent character of 

 the outflow of matter along the tail is more marked in 

 small comets than in the larger comets with which we 

 were previously familiar. It may be that the feeble gravity 

 that holds together the group of meteoric stones which 

 probably forms the nucleus of a comet, permits the form 

 of the group to be more easily disturbed by the ebullition 

 of vapour in the case of a small group than in the case of 

 a large group. 



Thus, in the case of a small comet, fresh meteors would 

 continually be brought to the outer part of the swarm, 

 where they can be more h-eely acted upon by the sun's 

 radiation, and with a given evolution of vapour these 

 changes would take place more rapidly in a small than 

 in a large group of stones, where" the gravitating forces 

 between the various stones of the swarm are larger compared 

 with the gaseous repulsion due to the vapour evolved ; and 

 in the case of a small comet, the changes of form within 

 the nucleus would probably be greater, and the evolution 

 of vapour more irregular, with a given rise of temperature 

 than in the case of a large comet. 



Hfttcts. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



♦ 



To the Editor of Knowledge. 

 Sii-, — In a letter in your May number (p. 116) Mr. 

 Skene states that he observed the zodiacal light " in day- 

 light, i.e., immediately after simset, of a pretty rose 

 colour," but I would venture to suggest that it is quite 

 impossible that what he saw really was the zodiacal hght 

 at all. Even m the tropics the zodiacal light is never 

 visible till at least half an hour after sunset, and even then 

 it can be seen only with difficulty. The mention of the 

 " rose-colour " suggests the probability that what was 

 really seen was a crepuscular ray, which would answer 

 vei-y well to the description given. During nearly twenty 

 years of careful observation of the zodiacal light I have 

 never been able to distinguish any trace of colour in it. 



My observations, indeed, have been almost entirely made 

 within the tropics, where the light makes a large angle 

 with the horizon, but this is the most favourable condition 

 for seeing it in its true form and colour. 



I would like to add that, whether or not there is any 

 connection between the intensity of the zodiacal light and 

 the sunspot cycle, there can be no doubt that during the 

 past three years the light has been more brilliant than 

 during several preceding years. At present the light is 

 very strong, and one and a half hours after sunset it can 

 easily be traced through an arc of from 60'^ to 70° from 

 the horizon. Yours truly. 



The Observatory, Madras, C. Michie Smith. 



30th May, 1804. 



.'SATURN'S DARK RING. 

 To the Editor of Knowledge. 

 Sir, — When, at the end of 1850, the now familiar 

 " crape ring " of Saturn was practically simultaneously 

 discovered in America by Prof. Bond, of Cambridge, U.S., 

 and in this country by Dawes, the question at once arose 

 as to whether this was, so to speak, a recent addition to 

 or development of the Saturnian ring system, or whether 

 it had merely escaped the attention of earlier observers. 

 When this question was first mooted, it was speedily 

 remembered that in the year 1838 Galle, in examining 

 Saturn, at Berlin, had noticed a perceptible shading off on 

 the inner edge of the interior bright ring, and that Encke 



had, m fact, published a des- 

 cription of this observation, 

 to which at the time but 

 scant attention would seem 

 to have been paid, possibly 

 owing to the somewhat 

 vague language in which 

 it was couched. Nay, ten 

 years previously, the dusky 

 ring would seem to have 

 been perceived by the 

 observers in Rome, who, 

 however, apparently did not consider the phenomenon 

 worthy of record, or, at all events, of publication. 

 Beyond this, in the outset, no one attempted to go, and 

 the advocates of the recent evolution of this strange 

 appendage of the planet entrenched themselves behind 

 the authority of Sir William Herschel, who, in his 

 innumerable observations of Saturn, must, they contended, 

 have seen it at one time or another had it been then 

 existent, or, at all events, as visible as it subsequently 

 became. Further research, however, once more showed 

 how utterly fallacious merely negative evidence may prove, 

 for Chambers, in his Descriptive Astronomy, speaks of a 

 passage in the Pliilosophical Transactions for 1723 which 

 " almost leads one to infer that he (Hadley) had seen the 

 dusky ring, though without being able to make up his mind 

 what it was." Moreover, we find Hind thus expressing 

 himself on page 32 of Vol. XV. of the Royal Astronomical 

 Society's Monthhj Xotices, in connection with this question : 

 " I have found among Picard's observations on the ring, in 

 the same work, a notice of that second dark belt, traversing 

 the globe at the interior edge of the rings, which, with every 

 appearance of probability, has been identified with the 

 obscure ring of modern observers. M. Otto Struve alludes 

 to observations of this nature made early in the eighteenth 

 centm-y; but probably Picard's had escaped his notice. . . . 

 We have, therefore, reason to suspect the existence of the 

 obscure ring, at least as far back as 1673, notwithstanding 

 the negative evidence which the subsequent observations 

 of Sir W. Herschel and others may be considered to afford." 



