Dec. 15, 1882.] 



KNOAVLEDGE 



469 



VENUS IN TRANSIT. 



IT has been noticed that when nearly the whole of the 

 disc of Venus is in the sun's face, the part of her out- 

 line which is still outside the sun is surrounded liy a fine 

 thread-like arc of light— light so bright that it is manifestly 

 not merely reflected sunlight, but sunlight itself. It is so 

 bright that in the mere instant of time during which the 

 photographic plate is exposed in taking solar photographs 

 this arc of liglit not only had time to record itself, but 

 even to record itself with an obvious increase of width 

 due to over-exposure. It was then sunlight brought 

 into view round the convexity of Venus's globe by the 

 bending power of the planet's atmosphere. This observa- 

 tion is well worth repeating, and that fine arc of light is 

 well worth studying with the spectroscope, for such a 

 thread-like line of light would tell the story of the atmo- 

 sphere through which it had passed in a most cftective 

 fashion. No slit would be required for the sifting of 

 the solar rays, seeing that the line of light must be as 

 fine as the finest slit would make it. Its breadth is, 

 in fact, no other than that of the atmosphere of 

 Venus, seen at a distance of some twenty-six millions of 

 miles, and the part of that atmosphere efleotive in bending 

 the sun's rays cannot be deeper than twenty or thirty miles, 

 a thickness which at that enormous distance is practically 

 evanescent. The other peculiarity, which is worthy of 

 careful study, is the ring of light seen round Venus when 

 she is on the disc of sun. It is commonly stated that this 

 ring of light is brighter than the background — the solar 

 photosphere — on which it is seen ; but this, it need hardly 

 be said, is absolutely impossible. Mr. Brett, the artist whose 

 imagination enclosed Venus in a glass envelope, showed how, 

 in his opinion, the brightness of sunlight, coming through 

 the \itreous atmosphere, might be strengthened by internal 

 reflection, and so forth, until it was brighter than its 

 source. But although this idea is of course utterly in- 

 consistent with optical laws, about which, unfortunately, 

 many excellent painters know very little, the seeming 

 brightness of tliis arc of light is very well worth studying. 

 It is an optical illusion, no doubt, but an optical illusion 

 often leads the way to the knowledge of a very real physi- 

 cal law. Moreover, by interpreting optical illusions, we 

 learn how, in future, illusions of the same sort may be 

 prevented from misleading us. 



But now it may be asked how the science of astronomy 

 is to be benefited by that which, after all, must be regarded 

 as the most interesting matter associated with the observa- 

 tions of Venus in transit — the determination of the sun's 

 distance? Newton used to say that, so far as the law 

 of gravitation is concerned, it matters nothing whether 

 the sun is a hundred millions of miles away, or a 

 thousand millions, or only one million. All the move- 

 ments witliin the solar system would take place in just 

 the same manner whether the sun were near or far oti', or 

 rather (for tliat is the real question at issue) whatever the 

 dimensions of the solar system. Precisely as a clock's 

 liands may be shortened or lengthened, and yet show time 

 as well as before, so our ideas as to the diuuMisions of the 

 solar system may vary without the slightest changes in our 

 ideas as to the movements within the .system, or as to the 

 law on which those movements depend. All tliis is true 

 so far as mere motion is concerned — the kinetics of 

 tlie solar system would not be changed in the least by 

 a change doubling or halving our estimate of the 

 -sun's distance. But it is diUerent with the pliysics of the 

 solar system. Any change in our estimate of tlie sun's 

 distance involves a change in our estimate of his physical 

 power, his might as ruler of the solar system, his capacity 



as tlie source of all the supplies of light and heat by which 

 it is nourished, the time during which these forms of energy 

 can be supplied, tlie scale on which those mighty processes 

 take place, whose existence we infer from spots and facul.-c, 

 cyclonic storms, eruptive prominences, and even, in all pro- 

 bability, in the changes aflecting the solar corona itself. — 

 Standard. 



EARTHQUAKES IN HEREFORDSHIRE. 



By T. W. Webb. 



MAY I beg permission to add a few particulars to your 

 truly remarkable account of the earthquake felt in 

 Herefordshire in the early morning of Oct. 6, 1803. My 

 parish of Hardwick lies between the upper end of the Golden 

 Valley, to which you have referred, and the town of Hay, 

 and many of us heard the roar and felt the concussion 

 with the greatest distinctness. One cottager compared 

 its approach to the. sound of a carriage rolling over 

 boards, and when it came up it rattled the tiles on the 

 roof progressively, as far as I recollect, from one end to 

 the other, which, if his impression was correct, would bring 

 it from the west. Feeling a great interest in such pheno- 

 mena, and never having witnessed one, I thought myself 

 very unfortunate on the occasion. During the first half 

 of the night I had been examining nebuL-e and other 

 faint objects with my 51-inch object-glass, and with 

 some success. I had made out for the first time the 

 longer of Bond's canals in the great Andromeda nebuhe, 

 and had readily detected the nebula in the Pleiades, and 

 had perceived Otto Struve's Pons Schroteri in the nebula 

 of Orion. The increase, however, of moonlight and cold 

 induced me to discontinue my employment a short time — 

 perhaps three-quarters of an hour — before the arrival of 

 the shock, which I should otherwise have had such an 

 admirable opportunity of obser\-ing. As it was, I was 

 partially roused from my first sleep by a concussion 

 which rattled violently the doors of a locked wardrobe, and 

 probably the pictures on the walls, but passed away leaving 

 me too drowsy to think of anything more than a furious 

 gust of wind, which I had forgotten on awaking next 

 morning. The servants, however, felt their beds lifted up. 

 The only remaining evidence in my house was the throwing 

 on the ground of a small unframed picture that stood upright 

 against a wall ; but the chimney of an old cottage about a 

 mile distant was believed to have lieen cracked at that time. 

 But I think it was more extensive than is intimated in the 

 account you have given. It was felt at Liverpool, and 

 announced the previous evening, unless my memory deceives 

 me, by the singular movements of some linen hanging on 

 a line. At Cheltenham, where my wife was then 

 staying, its eUects were not less marked than in tlie distant 

 valley "of the Wye and Monnow. It awoke her roughly 

 from sleep ; the sound, as she described it, resembled that 

 of great rocks falling one over another into a deep chasm ; 

 everything in the room was shalvcn ; the Venetian blinds 

 quivered ; and the night-light was extinguished. Several 

 persons wi>re made ill. One of them, a respectable trades- 

 man, descril)ed the sensation as that of being heaved up 

 from lieneath in bed, and having a light in tlie room he 

 saw the books on a partially-filled shelf tilted from side to 

 side. A baby was rolled out of its cot ; and a. policeman 

 stated that he could see the undulation of the houses in 

 the moonlight, and was obliged to steady himself by liold- 

 ing on to a lamp-post 



Since that time we have felt two distinct shocks at my 

 house, though of less severity. In the former instance a 

 little dog leaped in terror from a lady's lap, though none of 



