570 



NATURE 



[January 23, 19 13 



with a drop of turpentine, to act as an insulator, 

 between them. On this was laid a thin cover-glass, 

 as used for covering objects when mounted for the 

 microscope. Unintentionally the potential between 

 the two tinfoil strips rose high enough to permit a 

 spark to pass through the oil between them, and 

 when this occurred a small piece was blown out of 

 the centre of the cover-glass, being about 1/16 in. in 

 diameter on the upper side and about half this on the 

 lower, the piece of glass having the appearance of a 

 small truncated cone. The cover-glass was only held 

 down by the film of oil separating it from the strips 

 of tinfoil ; yet the fragment of glass was ejected with 

 considerable force. 



The cause of the effect may be explained as follows : 

 the energy liberated from the spark was sufficient to 

 cause the pressure to rise rapidly to a high value in 

 the confined space, either by decomposing the oil and 

 heating the liberated gas, or by forcing away the oil 

 along its path, and so compressing it. (We are, 

 however, not concerned here with the question as to 

 whether the pressure on the glass cover had its seat 

 in the oil alone or in the gas liberated from the oil, 

 but only with the fact that the passage of the spark 

 through the oil gave rise to a series of events which 

 culminated in the forcing out of a plug of glass from 

 the cover.) As the time during which the spark lasted 

 was almost negligible, the rate at which the pressure 

 rose near the spark was great enough to send out 

 a pulse of pressure through the oil. This pulse strik- 

 ing the thin cover-glass had sufficient energy stored 

 in it to cause the small piece of glass to be removed 

 from it. 



This effect is the converse of that noted by Mr. 

 J. Y. Buchanan during the voyage of the Challenger 

 in 1873, and repeated by him while on board the yacht 

 of H.S.H. the Prince of Monaco, the Princesse Alice, 

 in the summer of 1902 (see Proc. Roy. Soc, 1903, 

 vol. Ixxii., p. 88; or Nature, 1903, vol. Lxviii., p. 334). 

 I will quote from Mr. Buchanan's paper : — "The brass 

 tube (Figs, i and 2, plate i.) above referred to was 

 the case for holding a piezometer which was accident- 

 ally broken. With it I repeated the experiment which 

 I had made in the Challenger, with this difference, 

 that I used only one sealed glass tube. It was an 

 ordinary pipette of 50 c.c, sealed up at both ends close 

 to the body. It was wrapped in a piece of muslin and 

 loosely packed with cotton waste so as to occupy the 

 middle of the brass tube. 



"The length of the brass tube was 33 cm., and its 

 diameter 4"i3 cm. Its weight without the cover was 

 350 grams. Both the top and the bottom are pierced 

 with many holes so as to allow passage to the water. 



"Thus charged, it descended on the sounding line 

 to a depth of 3000 metres, and when it came up it was 

 evident from its appearance that the experiment had 

 succeeded, .^s in the experiment on board the Chal- 

 lenger, the glass had been converted into a snow-white 

 powder. The external effect also was confined entirely 

 to that part of the brass tube which had been occupied 

 by the sealed glass tube. Above and below it there 

 was no disfiguration." 



In this case it was easier for the water outside to 

 distort the brass tube than to flow through the per- 

 forated caps covering the ends, and so fill the space 

 lately occupied by the glass bulb. In the case of the 

 punctured cover-glass the pressure rose so suddenly 

 on the spark passing through the oil that there was 

 not sufficient time to raise the glass as a whole, or 

 to push away the film of oil lying between it and the 

 glass slide, with the result that a minute piece of glass 

 was forcibly blown out. Had the cover-glass 

 possessed the ductility of brass, there would perhaps 

 NO. 2256, VOL. 90] 



have been a bulge formed instead of a piece being 

 bodily removed. 



On another occasion I had a practical demonstration 

 of the power given out by a spark. It was in the 

 early days of wireless telegraphy, and I had con- 

 structed an oscillator of a simple type, consisting of 

 a pair of brass balls immersed in paraffin oil, the oil 

 and balls being contained in an inverted bottle from 

 which the bottom had been removed. The bottle was 

 about 2^ in. in diameter, and about 4 in. deep, and 

 the balls were situated at the centre, one above the 

 other, and i in. apart. I had not passed more than 

 about a dozen sparks between the balls when suddenly 

 the glass was shattered. The large end of the bottle 

 was open, and the free surface of the oil was about 

 2i in. in diameter. We have in this case direct 

 evidence of a pressure being transmitted in the form 

 of a pulse, or single w'ave, to the glass containing 

 vessel of an intensity sufficient to cause it to break. 

 The cause of this pressure was the spark passing 

 from ball to ball through the oil, and while passing 

 pushing away the oil on all sides with a rapidity which 

 gave rise to a pulse of pressure. This pulse travelled 

 outwards w-ith great velocity, and contained such a 

 store of energy that on striking the sides of the vessel 

 it was sufficient to rupture the glass. The potential 

 energy of the original electric charge was converted 

 into the kinetic energy of the spark, and this in turn 

 was transformed into the energy of the pulse, which 

 was finally transferred to the glass. As the amount 

 of energy was too great for the glass to hold, it found 

 an outlet in shattering the vessel. 



The "pressure in an electric spark" is a term by 

 no means uncommon in scientific literature, yet but 

 little attention is paid to the effects which this pressure 

 exerts on surrounding objects, as, for example, when 

 a tree or house is struck by lightning. They all be- 

 long to the type mentioned above. 



In conclusion, I would recommend a careful study 

 of the paper bv Mr. J. Y. Buchanan referred to above 

 to those interested in the subject of the sudden relief 

 of great pressure. W. G. Royal-Dawson. 



17 Pembridge Gardens, London, W. 

 January 8. 



The Halo in the Ricefield and the Spectre of the 

 Brocken. 



In connection with the curious Japanese 

 phenomenon of the halo seen around the head of the 

 shadow of a person standing in a ricefield in early 

 morning (Nature, p. 419, December 12, 1912), it may 

 be of interest to recall that some recent balloon 

 voyagers have reported observations of a bright halo 

 surrounding the shadow of the car thrown upon a 

 horizontal cloudfield by oblique solar rays. Coloured 

 diffraction rings are sometimes seen surrounding the 

 head of the "spectre of the Brocken," but for these to 

 be visible theory requires that the drops constituting 

 the mist should be of uniform size. In an article in 

 the Meteorologische Zeitschrift (p. 282, June, 1912 ; 

 see also Science Abstracts, p. 574, December, 1912), 

 by Prof. F. Rirharz, discussing the theory of the sub- 

 ject, reference is made to an observation by Dr. Bieber 

 from the balloon Marlinrgoi a halo around the shadow, 

 and also to other verbal communications of a similar 

 character. Prof. Richarz's article is followed by 

 another describing a photograph taken by Dr. 

 Wegener of a series of three diffraction rings seen 

 around the shadow of the same balloon, the Mar- 

 burg, on another voyage. The centre of the rings 

 was the point corresponding to the shadow ^ of the 

 eye, or of the camera objective. On calculating the 



