198 



NATURE 



[June 26, 1890 



mention now one simple way which I illustrated by experi- 

 ment at the meeting referred to. It is fairly well recognized 

 that sheet lightning is the reflection of a flash on a cloud, for j 

 example ; but if there happens to be the presence of a cloud with \ 

 a small opening in it somewhere between the actual flash and ; 

 the distant surface of clouds, then, instead of " sheet " lightning 

 appearing on the latter, there will be "projection " lightning — that I 

 is, the image of the flash, whose shape will depend upon the shape | 

 of the cloud on which it is cast. 



In speaking of zigzag representations of lightning flashes, it is 

 important to make some distinction between the artistic zigzag 

 and a common pictorial type such as is seen on the covers of 

 electrical books, in dissolving views, in scenic eflects, and even | 

 in street advertisements. It is hardly fair to saddle the artists j 

 with the latter class. A good specimen of an artistic zigzag I 

 flash, and one which shows an observance of nature, can be j 

 seen in Wilson's famous picture of "Celadon and Amelia." 



It certainly seems at first sight strange that the "projection " 

 flash should not be included in the photographs of lightning 

 flashes. Its non-appearance may be due ( i ) to the photographic 

 plates not being sufficiently sensitive to register a flash of 

 diminished brilliancy, for the projected image of any source of 

 light has not the same intensity as the source itself. (2) The 

 "projection" flash being of rarer occurrence, the number of 

 photographs yet taken may not have included it. If the type is 

 rarer, it may be objected that it is not likely that artists would 1 

 generally depict a rare type in preference to the more common j 

 one ; but the less dazzling nature of the " projection" would be | 

 sufficient to account for its adoption, rendering the form of the 

 flash more distinct to the average eye. To take an illustration, 

 if an electric arc light is suddenly flashed before our eyes, we 

 fail to distinguish the form of the white-hot carbon points, but if 

 its image were flashed upon a screen, their form would be dis- 

 tinctly visible. ' 



It is worthy of note that some painters have chosen to repre- 

 sent other types than what I have termed the "projection" 

 flash. See Turner's " Stonehenge," where "streaming" light- 

 ning is pictured. Eric Stuart Bruce. 



10 Observatory Avenue, Kensington, W., June 16. 



The Bagshot Beds of Essex. 



In the second part of the paper on the Westleton beds, by 

 Prof. Preslwich, recently published in the Quarterly Journal of 

 the Geological Society (vol. xlvi. p. 152), a section of the Brent- 

 wood railway cutting is given, which is, if possible, of more 

 interest from the Eocene beds described than from its bearing 

 on the questions dealt with in the paper. 



Reading the new section together with what we already know, 

 we get the following succession of beds at Brentwood : — 



(1) Pebble beds, capping the plateau up to 15 feet thick. 



(2) Bagshot beds, about 50 feet, consisting of — 



{a) Yellow or white sands (bed 6 of Mr. Whitaker's section, 

 " Geology of London," i. 274). 



(b) The green sands and clays with fossils of the railway 

 cutting. 



(c) Yellow sand with seams of clay of the railway cutting. 



(3) London clay, about 435 feet, the upper part consisting of dark 

 grey clay, with one or more beds of loam and yellow sand, the so- 

 called "passage beds "exposed in the brick-fields near the station. 



The fossils which Mr. Herries and I found near Frierning 

 (Whitaker, "Geology of London," i. 276) came from white 

 sand probably answering to bed 2a. 



This section seems to show pretty clearly that the Bagshot 

 beds of Brentwood are more nearly allied to the marine Brackles- 

 ham (Middle Bagshot) series than to the Lower Bagshots of the 

 Bagshot Heath district, which are probably freshwater. If 

 this be so, the masses of pebbles which overlie them may well be 

 the remains of the pebble beds which so often mark the base of 

 the Upper Bagshot (Barton) beds, and the parallel drawn 

 by Mr. Herries and myself between the pebbles which cap the 

 Wailey and Brentwood plateau in Essex and those which cap 

 Hook Heath and other hills in the Bagshot district becomes the 

 more marked (Proc. Geol. Assoc, vol. xi. pp. 13, 16, 20). 



I Hare Court, Temple. Horace W. Monckton. 



Electro-Magnetic Repulsion. 



Those who have not the means of showing the striking effects 

 produced by Prof. Elihu Thomson may be glad to know a 

 simple illusti-ation of the same principles. 



NO. 1078, VOL. 42] 



A top consists of a soft iron disk with a brass axis put through 

 it. A small magnet is held over the edge whilst spinning ;. 

 each elementary sector as it moves up to and away from the 

 poles of the magnet has currents induced which are repelled by 

 the magnet ; as the rotation dies out, the currents at a certain 

 point become too feeble to overcome the attraction of the soft 

 iron by the magnet. I bought the top two or three years back 

 of M. Manet, 49 Rue Lourmel, Paris. W. B. Croft. 



Winchester College, June 21. 



A Remarkable Appearance in the Sky. 



There was an appearance in the sky last night, so remark- 

 able that I am tempted to describe it, in case, our situation 

 being high, it should have been better seen here than elsewhere. 

 Along the horizon, from north to about north-east, a faint bank 

 of cloud extended, above which was a space of light like that of 

 the early dawn or of the rising moon. There was no quivering, 

 or shooting upwards of rays, as in the ordinary northern lights ; 

 the light was steady,- white tending to yellow, brighter at the 

 lower part. Above it hung a purplish haze, through which the 

 stars shone brightly, and occasional strips of dark cloud. It 

 did not happen to be observed till 10.30 p.m., and it was hardly 

 altered at 1.30 a.m., when it was still bright enough to marlc 

 the window-frame through a white blind, like moonlight. 

 Besides the position, the fact of a solar eclipse occurring that 

 day proved the moon to have nothing to do with it. 



Sussex, June 18. M. E. 



PROBLEMS IN THE PHYSICS OF AN 

 ELECTRIC LAMP.^ 



IV/rORE than eighty years ago Sir Humphry Davy 

 ^^^ provided the terminal wires of his great battery 

 of 2000 pairs of plates with rods of carbon, and, bringing 

 their extremities in contact, obtained for the first time a 

 brilliant display of the electric arc- The years that have 

 fled away since that time have seen all the marvellous 

 developments of electro-magnetic engineering, have placed 

 in our possession the electric glow-lamp, and brought the 

 art of electrical illumination to a condition in which it pro- 

 gresses each year with giant strides. In addition to the 

 importance attaching to their ever-increasing industrial 

 use, there are many questions of purely scientific interest 

 which present themselves to our minds when we proceed 

 to examine the actions that take place when a carbon 

 conductor is rendered incandescent in a high vacuum, or 

 when an electric arc is formed between two carbon poles. 

 It is to a very few of these physical problems that I desire 

 to direct your attention to-night, but more especially to 

 one which is particularly interesting from the bearing 

 which it has on the general nature of electric discharge. 



We know as a very familiar fact that if we attempt to 

 raise the temperature of a carbon conductor inclosed in a 

 vacuum beyond a certain limit, not far removed from the 

 melting-point of platinum, the carbon begins to volatilize 

 with great rapidity. If an electric glow-lamp has passed 

 through its carbon more than a certain strength of current, 

 the glass bulb speedily becomes darkened by a deposit of 

 this volatilized carbon condensed upon it ; and experience 

 shows us that we cannot raise the temperature of that 

 carbon beyond a definite point without causing this waste 

 of the conductor to become very rapid. In the highly 

 rarefied atmosphere within the bulb of a glow-lamp, the 

 carbon, when at its normal incandescence, must be con- 



' Friday Evening Discourse delivered at the Royal Institution by Prof. 

 J. A. Fleming, M.A., D.Sc, on February 14, 1890. 



^ Sir Humphry Davy laid a request before the managers of the Royal In- 

 stitution on July II, 1808, that they would set on foot a subscription for the 

 purchase of a large galvanic battery. The result of this suggestion was that 

 a galvanic battery ot 2000 pairs of copper and zinc plates was set up in the 

 Royal Institution, and one of the earli,est experiments performed with it was 

 the production of the electric arc between carbon poles, on a large scale. 

 It is probable, however, that Davy had produced the light on a small scale 

 some six years before, and, according to Quetelet, Curtet observed the arc 

 between carbon points in 1802. See Dr. Paris's " Life of Sir H. Davy." 



