366 



Njl TURE 



[Feb. 17, 1 88 1 



and Heath Hou-e, and about 550 yards south-weit of the 

 first hole ; and still later in that month, on the 19th, 

 a third subsidence made its appearance, this time about 

 100 yards to the south- east of the fir^t subsidence, and 

 nearer to All Saints' Church. 



The Astronomer-Royal and other inhabitants of the 

 district being anxious to know how far other subsidences 

 were probable, asked the Metropolitan Board of Works, 

 who have jurisdiction over the Heath, and who had 

 fenced in the sinkings, to investigate their cause. This 

 however they declined to do, though giving to the Astro- 

 nomer-Royal permission to do so ; this authority he handed 

 over to a newly-formed society, called the Lewisham 

 and Blackheath Scientific Association, who formed a 

 committee of investigation, including members of the 

 West Kent Natural History Society, for which end sub- 

 scriptions are now being sought, and operations will 

 shortly be commenced, as announced in our columns. 



The surface of the chalk is estimated by one member 

 of the committee, Mr. T. V. Holmes, as probably occurring 

 at about 100 feet from the surface at or about the Ordnance 

 datum line. The investigations so far made show the third 

 sinking to consist of an oval vertical shaft 7 feet 8 inches 

 diameter by 6 feet 9 inches, with a depth of 18 feet, open- 

 ing into a cavity extending in both directions, and partly 

 choked with fallen earth, giving a total diameter, as far 

 as e.xamined, of 14 feet. The upper part of the shaft is 

 described by Mr. Holmes as consisting of sand and clay 

 resting on sand, overlying pebbles, in which the cavity 

 below is formed. The material carefully removed from 

 the bottom of the pit is found by Mr. H. W. Jackson to 

 be of the same material as the upper beds of the shaft, 

 proving the sinking due to removal of material from 

 below. The first sinking is filled up and cannot be inves- 

 tigated ; the second is not fully examined for want of 

 funds, but is wholly in gravel, and also extends under- 

 ground in two directions. 



Various theories have been suggested by different 

 observers to account for their origin, soine consider- 

 ing them artificial, Admiral Hamilton that they are 

 caused by the abstraction of water caused by the main- 

 drainage works, which tapped powerful springs in the 

 Lower Woolwich Road; others connect their appearance 

 with removal of chalk, and water in the chalk, by the Kent 

 Waterworks, who lift daily about nine million gallons a 

 day from their wells in the neighbourhood, whilst others 

 connect them with excessive rainfalls, the first subsidence 

 having taken place after the great floods in the Ravens- 

 bourne, caused by the rain of the night of the iiLh and 

 morning of the 12th of April, 1878. 



The height of the chalk water-line {journal. Society of 

 Arts, 1877J at Woolwich Dockyard well is about 15 feet 

 below the Ordnance datum line before pumping, at the 

 Kent Waterworks, Plumpstead, i foot 4 inches below, but 

 at the Kent Waterworks wells at Ueptford it is pumped 

 down to nearly 70 feet below, rising 50 feet after pumping, 

 or about 20 feet below Ordnance datum. The surface of 

 the chalk at Bromley, at the Shortlands pumping-station, 

 has risen to 70 feet above the datum, the water rising 

 after pumping to 122 feet above it. This district is on the 

 south side of a synclinal axis ranging east-north-east 

 through Eltham, described by Mr. Whitaker, which 

 throws in a trough of London clay, that cuts off this 

 supply, from the chalk water entering at the Greenwich 

 Park escarpment. 



The water-level under Blackheath is at, or about, 

 Ordnance datum, trending south towards the London 

 clay synclinal, corresponding, under the site of the 

 subsidences, to the surface of the chalk beneath the 

 Thanet sands, and if there is no great quantity of chalk 

 above the water-level it appears improbable that the sub- 

 sidences are due to pipes descending vertically into the 

 chalk, but it is quite possible that the drainage works, 

 removing the waters held by the pebble beds above, 



disturbed their stability, and caused their subsidence. On 

 the other hand it is not impossible that drift levels may 

 have been driven into the chalk from the ancient chalk- 

 pits a mile distant, ceasing when they reached the outcrop 

 of the chalk against the Thanet sand, and which is imme- 

 diately under the site of the subsidences. 



C. E. De Range 



MERCADIER'S RESEARCHES ON THE 

 PHOTOPHONE 



A N elegant series of researches in photophony have 

 -^~*- lately been published by M. E. Mercadier of Paris, 

 who has very carefully examined the phenomenon dis- 

 covered by Graham Bell and Sumner Tainter, that an 

 intermittent beam of light may generate a musical tone 

 when it falls upon a thin disk. By way of distinguishing 

 this phenomenon and its applications from the pheno- 

 menon of sensibility to light exhibited by annealed sele- 

 nium, which constitutes 'the essential principle of the 

 articulating photophone, M. Mercadier adopts the name 

 of radioplwny for the subject of his research : a name 

 which appears moreover to have the advantage of not 

 assuming iJ priori what kind of radiations, luminous, 

 calorific, or actinic, are concerned in the production of 

 the phenomenon. It is agreed by all who have experi- 

 mented in this direction that the pitch of the note emitted 

 by the disk corresponds precisely with the frequency of 

 the intermittent flashes of light : but it has been disputed 

 whether the effect is due to light or to heat. Prof. Bell 

 found that the beam filtered through alum water to 

 absorb the calorific ultra-red rays produced tones ; and 

 that even when a disk of thin ebonite rubber was inter- 

 posed, the beam robbed of both heat-rays and light-rays 

 could still generate tones. On the other hand, from the 

 list of substances given by the original discoverers, it was 

 evident that since dark and opaque substances with dull 

 surfaces, and those which, like zinc and antimony, have 

 high coefficients of thermal expansion, produce, ceteris 

 paribus, the best results, the effects must probably arise 

 from heating effects due to absorption of radiations of 

 some kind and their degradation into heat of low tem- 

 perature. 



M. Mercadier has summarised his results in an article 

 in the Comptcs rciidits, from which the substance of this 

 article is translated freely. The chief conclusions are as 

 follows : — 



L Radiophony does not appear to be an effect due to 

 the vibration 0/ the receiving disk vibrating transversely 

 in one mass as in an ordinary vibrating elastic plate. — 

 This conclusion appears to be justified by the following 

 observations : that, given a thin plate of any kind, under 

 the conditions necessary for the production of the pheno- 

 menon, it produces equally well tones of all different 

 degrees of pitch from the lowest audible up to the 

 highest that can be generated experimentally by optical 

 intermissions, and which in M. Mercadier's apparatus 

 attained to a frequency of 700 vibrations per second. 

 Moreover it was found that these changes of pitch were 

 accomplished without any defect in the continuity of the 

 phenomenon ; which would seem to indicate that it was 

 not necessary for the plate to vibrate in any particular 

 nodal or partial mode. Also the receiving disk will pro- 

 duce chords equally well in all possible tones from the 

 highest to the lowest, the chord being complete no matter 

 whether the fundamental pitch be raised or lowered by 

 altering the speed of the lotating apparatus by which the 

 interniittences are produced. M. Mercadier's apparatus 

 consisted of a glass wheel carrying on its surface a paper 

 disk pierced with four series of holes, numbering respec- 

 tively 40, 50, 60, and 80. Through any one of these series 

 of holes a small pencil of rays could be passed, and, by 

 raising or depressing the a.xis of rotation of the wheel, 

 could be sent successively through each of the four, thus 



