72 



KNOnA^LEDGE 



[Feb. 2, 1883. 



COMETS' TAILS. 



HAVING rend your admiralilo article in the Cmifnm- 

 /loriiri/ Ri'i-ifir on Coinots, 1 venture to oli'cr to your 

 readers a new theory on comets' tails [not altogether 

 new. Bredichin's theory is similar ; and in its general 

 features the tlieory was suggested long since hy Prof. 

 Norton, of Yale (first), and l>y myself independently, later. 

 — R. A. P.], which, I think, if worth nothing more, is 

 at least as good as anything yet devised, and therefore 

 equally worth demolition at your hands. 



I venture to think that the repulsive force which acts is 

 nothing more than that which, on a smaller scale, acts in a 

 Crookes' radiometer. That space is more or less tilled with 

 greatly attenuated matter, many considerations will lead to. 

 Only last issue of Nature contained an account of re- 

 seaiches by Professor Aubrey, in which he claims to have 

 discovered an absorption of radiation in the space between 

 us and the sun, and to have traced this to the presence of 

 hydro-carbons. Struve's researches on the distribution of 

 the stars, showing that the stars of the lower magnitudes 

 are less numerous than they should be, if stars were 

 equally distributed in space and there were no absorption 

 nf li'jht, points in the same direction. Again, what is to 

 prevent the outer edges of all atmospheres flying off in the 

 same form, as the unlimited expansibility of a gas would 

 lead us to suppose 1 Here, then, we have one of the con- 

 ditions in space that also exists in the glass bulb of the 

 radiometer. 



Next, I wish to show that any rery small particles of solid 

 matter would be strongly acted on by this repulsive ten- 

 dency, and the more strongly the smaller they were. 

 The conditions of repulsion are simply that the repelled 

 side should be slightly warmer than the other ; the radia- 

 tion of the sun, so rapidly received, so rapidly parted 

 with in the cold interstellar space, would ensure this. 



Now let the repelled particles be assumed to be spheres 

 — the worst possible shape for the hypothesis — and let the 

 repulsive tendency per unit of surface directly exposed to 

 the sun =(/) ; then the repulsion on each particle will 

 be (;)7rr- (resolving the lines normal to the sun), and 

 the attraction of gra\itation will be |(/u!r5''. Hence, 

 the total action on the particle towards the sun will 

 be 7rr-(|grpr— -(/j). Now, the smaller the particle the 

 smaller r will be, and, if small enough, this ex- 

 pression must have a minus value, and the total 

 resultant action on the particle will be changed from attrac- 

 tion to a violent repulsion, within certain limits depending 

 on the velocities of the molecules of the attenuated gas 

 acting on the particle, as violent as we please by making r 

 as small as we please. What, then, do I suppose 1 That 

 the comet approaching the sun — stopping radiation — be- 

 comes, at its surface, lost ; hence, the violent jets seen 

 there, directly they get a certain distance from the heated 

 surface, are condensed as fog, in small particles, and beaten 

 back liy this interstellar molecular " laonibarding " action, 

 into the shape of these envelopes — a line of envelope mark- 

 ing a line of equal temperature ; but this envelope itself 

 shuts in the heat within, and radiation out ; and another 

 envelope is formed within that — the edges of these envelopes 

 sweeping back under the repulsive action to form the 

 tail. 



I conceive the successive striations in the tail to be " the 

 winnowing out " of the different elements, according to 

 size, ifec, as you mention in the Contemporary. 



Next, although I conceive the tail to be made up of 

 small ])artic/es, probaVily solid, yet the specti'oscopic obser- 

 vations would surely not deny this. The luminous clouds 

 seen a few weeks back in conjunction with the aurora 



were probably made up of minute liquid or solid particles, 

 and yet would most probably have given a spectrum of a 

 few bright lines. [They did not, however. — Ed.] 



Fred. F. Geensted. 



THE WEATHER PROPHECIES. 



I HAVE just read in Knowledoe Sir Edmund 

 Beckett's strictures on " Weather Prophecies," and 

 agree with him as to the desirability of testing and record- 

 ing the tests, though my observations do not accord with 

 his. It is true that I have only made them when at the 

 seaside, and exclusively in reference to the wind and the 

 warnings. I have generally found the warnings correct, 

 though often unpunctual, i.e., the gale has come later than 

 was expected. 



On one occasion especially, sailing from London to 

 Dublin, and calling at several places on the South Coast, 

 we starti d from Cornwall in spite of the cone, the weather 

 being exceptionally fine. We soon repented, and the 

 captain, a man of much experience, stated that he had 

 found the warnings generally reliable. 



It would be interesting if, say, three independent 

 observers in each of the districts named would make a 

 similar record to that of Sir Edmund Beckett, and send 

 these to Knowledge. The observer should write his notes 

 of the day's weather hefoie rea<H»r/ the offieial prophecy, 

 and make no alteration afterwards, " litera scripta mnnet." 

 If on any day the three observers disagreed materially, the 

 observation for that day should be erased as worthless. If 

 two agreed closely and one was discrepant, the order of that 

 day's record should be rated accordingly. 



If some unprejudiced person would act as umpire, 

 making the comparisons, and expressing numerically 

 the weights pro and con, a general summary of the 

 results might be so made as to express numerically the 

 percentage of success and failure of the British clerks of 

 the weather. My prophecy concerning the prophecies is 

 that they will be found very successful as regards the wind, 

 but doubtful otherwise. I have theoretical reasons for 

 this anticipation, over and above the before-named experi- 

 ences. These reasons I will explain if my prophecy prove 

 correct. If wrong, like a true prophet, I will say no more 

 about it. W. Mattieu Williams. 



The Electric IIesist.\nce of Glass. — Mr. G. 

 Faussereau has been experimenting in this field, and says 

 (in Comptes Rendus) : — For common glass, of specific 

 gravity 2-539, expressing the resistances per cubic centi- 

 metre in millions of megohms, we obtain the following 

 results : — 



Temperatures. Resistances. 



-(- 61-2°C 0-705 



-I- 20 91-0 



— 17 79700 



To form an idea of the magnitude of this latter resistance 

 we may remark that it represents about twice the resistance 

 of a copper wire of one square millimetre in section, ex- 

 tending from the earth to Sirius. The resistance varies 

 about by one-ninth of its value per degree of temperature. 

 Hard glass of Bohemia, of specific gravity 2 131, is from 

 ten to fifteen times more conductive than ordinary glass at 

 the same temperatures. The crystal glass used has the 

 specific gravity 2-033. It is 1,000 to 1,500 times less con- 

 ductive than ordinary glass at the same temperatures. Its 

 conductivity is only manifested above 40° C. At 46-2° 0. 

 its resistance = 6182 ; at 105° = 11-6. 



