344 



NATURE 



{Feb. 9, 1888 



1821," ix, + no pp. i6mo. In the preface Mollweide says 

 that von Prasse was his predecessor in the Chair of Mathematics, 

 as stated by Prof. Virchl. This work is entered under both 

 the title-names in Poggendorff's " Biographisch-literarisches 

 Hand wort erbuch," a circumstance that might have given a clue 

 to the authorship of the " Demonstratio." Strangely enough, 

 this is not the only instance in which von Prasse omitted his 

 name in essays written by him for academical celebrations. _ I 

 can only surmise that this was done with a view to republication 

 in his " Commentationes Mathematicas/'and that the name was 

 written on the copies distributed as invitations to the celebra- 

 tions. Whatever the reason, it has in this instance obviously 

 added greatly to the trouble ordinarily experienced when dealing 

 with this class of academical essay, the bibliography of which 

 is so complicated, and at the same time often so important. 



Ralph Copet.and. 

 Lord Crawford's Observatory, Dun Echt, January 25. 



A New Historic Comet ? 



At a recent meeting of the Asiatic Society of Japan, a paper 

 was read by Mr. W. G. Aston, H.B.M. Consular Service. 

 This paper will certainly rank high amongst historic papers re- 

 lating to Japan and Korea. Briefly described, it is a comparison 

 between the ancient records of these two countries and China, 

 and its aim is to establish the relative credibility of these various 

 records. Mr. Aston has so far confined his attention to the 

 period preceding A. D. 500; and his general conclusion is that, 

 as historic writings, the Korean and Chinese chronicles are 

 far superior to the Japanese of the same date. 



In the Tongkan, as the ancient records of the Korean king- 

 doms are called, there is a notice, of which the following is a 

 translation : " Summer, fourth month, Pekche ; comet visible ; 

 day-time." The fourth month began on May 14 or 15. At 

 the request of Mr. Aston, I tried to find out if any such 

 comet had been observed elsewhere. The only list of historic 

 comets obtainable in Japan was the list given in Faye's 

 " Astronomy" ; and I am not sure if this is meant to be com- 

 plete. According to Mr. Aston, the Pekche comet appeared 

 in May or June, a.d. 302. The nearest date in Faye's list is 

 A.D. 295. If this is the same comet, then one at least of the 

 dates must be wrong. It is quite possible, however, that both 

 are correct ; in which case we shall be indebted to Mr. Aston 

 for having added one more to our list of historic comets. In 

 coming to a conclusion, we must know to what source we owe 

 the knowledge of the 295 comet, and whether this source has 

 greater claims to chronological accuracy than have the Korean 

 records. Not having the references at hand for studying these 

 points, I have written this note to Nature, in the hope that 

 someone interested in the matter may be able to come to a 

 decision on this question of a possibly new historic comet. 



Cargill G. Knott. 



Imperial University, Tokio, Japan, December 19, 1887. 



"Is Hail so formed?" 



Under the above heading in Nature of January 26 (jd. 295) 

 there is a short paper by Cecil Carus-Wilson, in which the 

 writer assumes that under certain conditions, drops of water, 

 whilst falling from the upper branches of a tree, become converted 

 into ice before reaching the ground, whilst other drops falling from 

 the same tree, but at 10 feet less altitude, came to the ground in a 

 fluid state. There is, I think, a simpler solution of this question 

 than the one given. Suppose the following conditions — namely, 

 a frost sufficiently severe as to lower the temperature of the 

 leaves and branches of a tree to a few degrees below the freezing- 

 point ; after which a very gradual thaw comes on, accompanied 

 by a fine rain or Scotch mist which freezes on the tree. 



Where the leaves and smaller branches hang downwards, small 

 beads of ice would form on their points. As the air became warmer 

 the ice would thaw, and fall to the ground either in the liquid 

 form, or the beads at the ends of the leaves and twigs would 

 become detached in their solid state, and reach the ground as 

 ice-pellets. 



Sometimes these ice pellets extend in length, and assume the 

 form'of small icicles. J. Kae. 



4 Addison Gardens, January 28. 



MODERN VIEWS OF ELECTRICITY.^ 



Part III. {continued). 



VII. 



First Representation of the Field due to a Current. 



RETURN now to the consideration of,a simple circuit, 

 or, say, a linear conductor, and start a current through 

 it ; how are we to picture the rise of the lines of force in 

 the medium ? how shall we represent the spread of mag- 

 netic induction "i First think of the current as exciting 

 the field (instead of the field as exciting the current, 

 which may be the truer plan ultimately). 



Fig. 



If we can think of electricity in the several molecules 

 of the insulating medium connected like so many cog- 

 wheels gearing into one another and also into those of 

 the metal, it is easy to picture a sideways spread of rota- 

 tion brought about by the current, just as a moving rack 

 will rotate a set of pinions gearing into it and into each 

 other (Fig. 34). But then half the wheels will be rotating 

 one way and half the other way, which is not exactly 

 right. 



How is it possible for a set of parallel whirls to be all 

 rotating in the same direction ? 



000 

 000 



Fig. 35. 



If there is any sort of connection between them they 

 will stop each other, because they are moving in opposite 

 directions at their nearest points ; and yet, if there is no 

 connection, how can the whirl spread through the field ? 



Well, return to the old models by which we endea- 

 voured to explain electrostatics, and think whether they 

 will help us if we proceed to superpose upon them a 

 magnetic whirl in addition to the properties they already 



Fig. 36." 



-Rows of cells alternately positive and negative, geared together; 

 free to turn about fixed axles. 



possess. Looking at Figs. 5, 6, and 13, we remember we 

 were led to picture atoms and electricity like beads 

 threaded on a cord. And these cords had to represent, 

 alternately, positive and negative electricity, which always 

 got displaced in different directions. 



We are forced to a similar sort of notion in respect of 

 the wheels at present under discussion : in order that 



' Continued from p. 323. 



