180 



KNOWLEDGE. 



[August, 1902. 



as|iO(( weri' proliablv not in the spot itself, hut were 

 bi-oii^'lit about l)v the contact of tbc more swiftly-moving' 

 bolts ou the southern bouniliiries of the object. Tlie dark 

 end of a short belt or condensation (of which there are 

 nnmy examples in this ref,'iou), when just [n-eceding or 

 foUowin-; the spot, might readily induce the appearance of 

 a tapering extremity. 



The following selected observations of the hollow, in 

 which the faint relic of the red spot is situated, will 

 sufficiently prove that during the last two years the longi- 

 tude has not changed relatively to the zero meridian, ba.sed 

 on a rotation period of 9li. 55m. 'lOfiSs. The first two 

 transits were obtained with a 4-inch Cooke refractor, the 

 remainder with a 10-inch reflector, power 312, by the writer 

 at Bristol : — 



ASTRONOMY WITHOUT A TELESCOPE. 



By E. Walter Maunder, f.k.a.b. 

 XVI.— THE STRUCTURE OF COMETS. 



In the chapter on " Morning and Evening Stars," I 

 pointed out that the systematic observation of heliacal 

 risings and settings offered a chance — a rare one, it is true, 

 but still one not to be despised — of making the first dis- 

 covery of a comet. Unfortunately comets, bright enough 

 to be visible to the naked eye, have been but very scarce 

 visitants, nor can we reasonably expect that they will 

 be more numerous in the future. Still, when one does 

 come, it justly attracts universal attention; and the 

 "astronomer without a telescope " will naturally be anxious 

 to know if there is any work within his power to effect 

 upon it. 



There is. He cannot of course expect to make useful 

 determinations of the comet's place ; nor can he scrutinise 

 the changes which take place in the minute details of its 

 head. But the shape, extent, and precise form and posi- 

 tion of the comet's tail are better observed by the naked 

 eye than with the telescope ; since the eye can embrace a 

 far wuder field, and is the fitter instrument for dealing 

 with great extensions of faint light. To map out, night 

 by night, the precise position of the tail or tails with 

 reference to the neighbouring stars, to trace its limit and 

 to determine its exact form, are by no means unimportant 

 tasks. 



And for this reason. The last thirty years have seen the 

 development and gradual acceptance of a theory which 

 explains the origin and structure of those far-stretching 

 wisps of light which our forefathers found so mysterious 

 and awe-inspiring. The first step towards the elaboration 

 of this theory was made by Olbers nearly a century ago in 

 a memoir on the great comet of 1811 ; but in its present 

 shape we owe it to Prof. Bredikhine, lately the Director of 

 the Poulkova Observatory. 



It was very early noticed that the tails of comets are 

 in general directed away from the sun, and the instance of 



certain comets, which passed at perihelion very close to 

 the solar surface, was sufficient to prove that we must not 

 regard a comet's tail as forming a body coherent with the 

 head. Thus the great comet of 1843 swept round some 



^'",;''!'l'|l;Vli 



Fig. 1.— The Comet of 1901 on April 24 and May 5. 



180° of longitude at perihelion in something like eighteen 

 hours of time. The tail which had been seen before 

 perihelion, pointing away from the sun in one direction, 

 could not possibly have been composed of the same 

 material as made up the tail, lying in the opposite direction 

 after perihelion. But if it were supposed that the sun 

 were capable of exercising a repulsive force upon some 

 portion of the substance of the comet, driving it off in a 

 continuous sti-eam, then the general Ijehaviour of cometary 

 tails would be accounted for. The tail, seen at any 



12 .Mxti 



Fig. 2.— Tlie Comet of 1901 on Mar 12. 



particular time, would be the summation of particles which 

 had left the comet at different siiccessive instants, just as 

 the trail of smoke from the funnel of a locomotive, as seen 

 at any particular moment, is composed of particles that 

 came off from it at successive instants, and is not a body 

 coherent with the engine. 



