Arr.DST, 1902.] 



KNOWLEDGE 



181 



Such a repulsive force we may tiud in an electrical action 

 of the sun, an action, the efficiency of which would depend 

 upon the surface of the body acted upon ; as contrasted with 

 that of gravitation, which depends upon its mass. Thus 

 whilst the nucleus is moviui^ in its orbit round the sun 

 under the iuHuence of gravitation, very minute ]>articles 

 in the envelope will find themselves practically less 

 strongly attracted towards the sun, or it may actually 

 be repelled from it ; in any case the effect is to 

 separate them more and more from the main body of 

 the comet. 



Of the particles thus driven away from the head, the 

 lightest would be those most strongly repelled ; and Prof. 

 Bredikhine found that several of the great comets of the 

 past century were distinguished by the possession of long 

 straight tails which must have been composed of particles 

 moving under an influence some twelve or fourteen times 



Fig. 3.— Prof. Bri'ilikbinc's aiialvsis of the tail of tlif Comot 

 on 1901, May 12. 



that of gravity. These long straight tails form an 

 exceedingly characteristic class, and have accordingly been 

 ranked by Prof. Bredikhine as composing his first ty]ie. 

 Such tails are obviously composed of the lightest material 

 enteiing iuto the composition of the comet, and assuming 

 this to lie hydrogen, then the average tail of the second 

 type might well be made up of hydrocarbons ; whilst iron 

 and the heavier metals would, from their molecular 

 weight, be suitable elements to form the third type. In 



the tails of the second type the effective repulsive force 

 does not diflfer greatly from gravity, and it is this typo 

 of tail, slightly, but not extravagantly curved, which is 

 most commonly observed in comets visible to the naked 

 eye. The third type is nsually sharply curved, short and 

 brush-like, and the repulsive force is considerably less 

 than that of gravity. 



The great comet seen in the southern hemisphere in 

 Ai)ril and May, 1901, has recently been analysed by Prof. 

 Bredikhine, in a paper from which the three accompanying 

 diagrams have been reproduced, and his results afford an 

 interesting example of his method of treatment, and of 

 the use which can be made of careful naked-eye observa- 

 tions of the positions of a comet's tail. 



On April '24th, before the perihelion passage, the comet 

 showed practically only a single tail, and that was of the 

 first or hydrogen type. After perihelion, the tails were 

 only of the second and third types, the matter composing 

 the first type tail having been apparently completely driven 

 away. On May 5th, the chief tail, which was distinguished 

 by a very well marked rift, showed this rift as of a conical 

 shape, the apex of which was occupied by the nucleus, and 

 not as usual of a eouoidal form. A drawing made by Mr. 

 J. Lunt at the Royal Observatory, Cape of Gooil Ho[)e, on 

 May 12th, was of especial interest It not only showed 

 the principal tail of May .5th ^vith its dark rift, but a long 

 broad faint tail some 25° in length, and a short tail 

 between the two. Prof. Brcdikhine's analysis of this 

 drawing is given in the accompanying diagram. The lines 

 13 and 14, the two branches of the principal tail, were 

 due to a repulsive force a little greater than unity ; the 

 line 15 is due to a force of about 0'65 ; the broad faint 

 tail, Prof. Bredikhine ascribes to substances of the third 

 type, and finds that when the particles which make it up 

 are traced back to the nucleus they indicate that a great 

 explosion took place on April 22nd ; a vast cjuantity of 

 matter of a wide range of density being driven oft' in a 

 single short-lived convulsion. The p)oints in the diagram, 

 a, j8, y and S, correspond respectively to values of the 

 repulsive force of 0-85, 065, 025 and 0'15. 



ILftttrs. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



MARKINGS ON JUPITER. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Last year there was a large dusky mass situated 

 between the south side of the equatorial belt and the 

 south temperate belt of Jupiter, aod joining the two. In 

 1901, June 18th, the longitude of the marking alluded to 

 was 258^, while on November 17th of the same year it was 

 176°. During five months it gained, therefore, 82°, its 

 rotation period being 9h. 56m. 185s., while that of System 

 II. of Mr. Ci'ommeliu's ephemerides was 9h. 55m. 40 63s. 

 This dusky mass, placed in the usually bright tropical zone 

 I'f Jupiter, is still conspicuously visible, and has greatly 

 distended itself since last year in a longitudinal direction. 

 On July 2ud I observed it with a 10-inch reflector, power 

 312, and found its p. side on the central meridian at 

 I3h. 51m., while its/, side came to the CM. at l(ih. ItJm., 

 or 2h. 25m. afterwards, so that the formation extended 

 over 87°0 of longitude. At the ])resent time it lies both 

 E. and W. of Ihe old red spot and hollow in the south 

 lielt. which is in longitude 43^. 



Though the dusky mass is now passing the red spot, 

 the latter is no darker than usual, and it seems highlv 



