Jan. 17, 1884] 



NATURE 



261 



figures ; and from subsequent experiments it would seem that the 

 resin server the purpose almost as well as ebonite as far as clear- 

 ness of definition is concerned. A melal plate, which may or 

 may not be insulated, formed a base for the resin. I mention 

 these details since the ebonite rods and plate are not so well 

 within every one's reach, on the score of greater expense and the 

 necessity of having them specially constructed for the experi- 

 ments. A plate machine of some size (iS-inch plate) seems neces- 

 sary, as J find that, un'ess the Leyden jar is charged to rather 

 high potential, no shadow is formed, and, further, that the sharp 

 definition of the shadows increases with the charge of the jar. 

 The screen used was a design, cut out in cardboard and tinfoil 

 pasted over it, very similar in shape to that given in Fig. 2 in 

 Prof. Thompson'.s paper, and the shadows obtained were sub- 

 stantially similar to that in Fig. 3. But here a small point not 

 before recorded cime out : — If the pin, from whose point the 

 discharge is made to take place, be slanted in any direction, 

 which is easily done with the sealing-wax holder by simply 

 heating, the shadow of the object then lengthens out curiously, 

 just as do the shadows formed by an object intercepting light rays 

 as the obliquity of incidence is increased. 



The new feature, however, which appeared from my experi- 

 ments, and which is not recorded by Prof. Thompson, although 

 very likely the experiment may have been done before, is as 

 follows : — Instead of starting with the resin plate in a neutral 

 condition, T gave it a rather strongs negative charge by nibbing 

 it vigorously w ith a fox's brush and disch-irging the Leyden jar 

 as before on to the pin, using precisely the same object to cast the 

 shadow as before. Its character now, however, was completely 

 altered, appearing as I have endeavoured to represent it in the 

 figure. A simple cross, having little resemblance as to outline 



with the object, was the result. The red-lead of course was 

 picked out by the negatively-charged resin under the object and 

 piled up to form the cross, which was much more strongly red, 

 as one would expect, than the former shadow. There was also 

 a rather wide neutral region around the cross, considerably more 

 than in the former experiments. It seems to me that this effect 

 is something more than the attenuation of the shadow spoken of 

 by Prof. Thompson, where the screen is electrified independently. 

 Since the subject is one of considerable interest, perhaps it may 

 be useful to show that any one having access to a fairly good 

 electrical machine can repeat and possibly extend Prof. Right's 

 investigations. \V. F. Smith 



17, Colville Mansions, W. 



Cosmic Dust 



I FOUND in the Nietiws van dm Dag of December 28, 18S3, 

 that a violet sand had been found in the dunes (probably near 

 Scheveningen). The paragraph runs as follow s : — When seen 

 imder the microscope (feeble magnifying) the ordinary yellow 

 sand seemed to be composed for the greater part of almost w hite 

 transparent grains, among w hich were a few light yellow, and 

 pink, and single black grains The violet sand, however, 

 showed almost all the grains imbibed by a light violet tint, and 

 moreover it contained a very great number of black glittering 

 ^ains. An idea which occurred to me made me take up a 

 small magnet, and on stirring with it in a glass full of the sand, 

 the ends w ere covered by feathers formed by the black grains quite 

 the same as the feathers which are formed on putting a magnet 

 into filed dust. Proliably I had there grains of a combination of 

 iron ; of the latter there was a great deal in it. Now this is the 

 question : Are these grains of the same kind as those which the 



naturalists have found and gathered on the snow-fields in the 

 Polar regions, thus called cosmic dust ? 



Stuttgart, January E. Metzger 



Diffusion of Scientific Memoirs 



I THINK it would promote scientific information if it were 

 more the custom for those who need copies of papers to make 

 direct application for them. Authors are usually provided with 

 separate impressions for distribution, but .ire often much in the 

 dark as to how to turn them to the best advantage. The bulk of 

 such copies usually find their way to men of established scientific 

 position who have worked at the subject of the paper in past 

 years, but have perhaps ceased to take interest in it ; while 

 those who are actively engaged upon the subject, if they do not 

 happen to have already published matter of importance, are left 

 unprovided for. 



1 believe that most authors would willingly send copies of 

 their memoirs to younger men, known to be engaged in scientific 

 work, who should make application. But there is one rule 

 which must be observed with the utmost stringency — otherwise 

 I should feel that the evil of the present suggestion outweighs 

 the good — viz. t/ie applicant must never expect a ti'Hlten aitstuer, 



Cambridge R. 



^A^eather on Ben Nevis and Snowdon 



I WAS much interested with the account of a visit paid to the 

 Ben Nevis Observatory on December 26, 1883, described in 

 Nature of January 3 (p. 219), more particularly as the weather 

 experienced on the summit was almost identical with that on 

 .Snowdon at the same time. I ascended Snowdon on December 

 23, 25, and 26 from the wet, east, and north, and a neighbour- 

 ing mountain, Glyder F.Tch, on the 24th. The views from the 

 summit on the 25th and 26th can be best described by the fol- 

 lowing quotation from NATURE (p. 219), referring to Ben 

 Nevis : — " The view from the summit was magnificent. All round 

 there floated a billowy ocean of white mist " (extending from the 

 slopes of the mountain to the horizon north, south, east, and 

 west), " through which rose here and there black mountain 

 peaks." " Overhead the sky w'as blue,'' and the sun shone bril- 

 liantly. The upper surface of the ocean of clouds was on the 

 25th about 2COO feet, and on the 26th icco feet, above sea-level. 



On the 24th I ascended Glyder Fach through about 2500 feet of 

 mist, and, to again quote from Nature (p. 219), on reaching the 

 ridge '* suddenly emerged from the gloom of the mist into the 

 brightest of daylight. Overhead the sky was blue, a fresh light 

 breeze was bluwing" from the north-vest. I here noticed a 

 curious phenomenon. I became suddenly aware, w-hilst standing 

 in the sunlight on the ridge, that the air was full of an exceed- 

 ingly minute dust driven by the w ind from the north-w est and de- 

 scending at .in angle of about 40°. The fall ceased quite suddenly 

 one or two minutes after I noticed it. The impression left on my 

 mind was that anything popularly spoken of as dust would be 

 exceedingly coarse compared with it. There was no snow on 

 the ground. 



The phenomenon known under the name of the " Brocken 

 Spectre," mentioned by Mr. Chrystal, may frequently be seen 

 from the summit of .Snowdon by any ore not afraid of a little 

 mist. T. SiNGTON 



Kersal Moor, Manchester, January 7 



Teaching Animals to Converse 

 J. S. B. seems to h.ive -misunderstood Sir John Lubbock's 

 idea. It would be no great test if draw ings were made, as the 

 dog would see so little difference. Thus a dog of mine knows 

 instantly whether he may go out with my housekeeper or not 

 according to whether she wears her hat or her bonnet. In the 

 first instance he knows she is going where he may go, and he is 

 on his feet barking with joy as soon as she appears. If she has 

 the bonnet on, he knows it to be church, or a visit to friends in 

 the country, where he cannot go, and, like the " eldest oyster '' 

 (I quote from memory), he "winks his eye, and shakes his hoary 

 bead." If dr.iwings of hat and" bonnet were m.ade, he would 

 know them at once. 



Some years since I had a remarkably clever Skye terrier, 

 whose wisdom was at the time shown in a letter to the Times. 

 This dog I taught as follows. When I went out it was quite 

 sufficient to say " Ves " or "No" in an ordinary tone; but 

 wanting to take him beyond that, I taught him very quickly to 



