484 



NA TURE 



[March 26, 1858 



(3) Thegrea; i of Krakatoa on August 26 and 



27, 1883, was ai 1 1 by subterranean noises which 



mbling the rolling of cannon 



or f tn 1 description from the Sunda 



Islands er from that from CaVman-Brac. 



of the Krakatoa eruption 

 have had id !lave been detected 



at a distance never heard of before. As is well known 

 {vide Nature, vol. xxx. p. 10), the explosions were heard 

 over a circle of 30 radius, i.e. 3300 kilometres. It is 

 indeed only the quarter of the length of the earth's dia- 

 meter ; if the hypothesis is true, we would have here a 

 considerable extension of the propagation of the sound 

 through the earth. 



(5) Ca'iman-Brac lies very near the antipodes of Kraka- 

 toa. The exact position of Krakatoa is 105 30' E. long, 

 and 6° S. lat. ; Caiman-Brae, 79 30' W. long, and 19 30' 

 N. lat. The antipodes of Krakatoa is also 4 30' more 

 towards east, and 13" 30' more towards south; it is in 

 the middle of the United States of Colombia, on the 

 Magdalena River, between the towns Antioquia and 

 Tunja. 



(6) The time at which the noises have been heard at 

 Cai'man-Brac corresponds sufficiently to what we know 

 about the time of the eruption of Krakatoa. From the 

 report of R. D. M. Verbeek (Nature, vol. xxx. p. 10) the 

 explosions of the volcano have been noticed in the Sunda 

 Islands on August 26 and 27, and especially on the morning 

 of the 27th. The noise reached its maximum at Buitenzorg 

 on the 27th at 6.45 a.m. ; at Batavia at 8.30 ; and at Telok- 

 Betong at 10 o'clock. From the difference of longitude 

 August 27, S.30 a.m. at Batavia is the same time as 

 August 26, 8.5 p.m. at Cai'man-Brac. If we admit that 

 the propagation of the sound through the 12,000 kilo- 

 metres of the earth's diameter would take about one 

 hour, the maximum detonations must have reached the 

 Caimans on August 26 at 9 p.m. Unfortunately the 

 letter of Mr. Roulet does not give us the exact time of 

 day at which the sounds were heard at CaTman-Brac ; I 

 have asked my correspondent to complete, if possible, his 

 observation on that point. 



I do not wait for the reply before publishing the pre- 

 sent communication for the following reasons : — I believe 

 it is very important to call attention without further delay 

 to this fact, and to beg of the inhabitants of the coast and 

 the islands of the Carribean Sea to collect all that can be 

 remembered about these events ; perhaps they heard also 

 the noises described at the Caimans, and they can con- 

 firm, or complete, or correct the observation given by 

 Capt. Woodville. 



In case the correlation between the noises at Ca'i'man- 

 Brac and the Krakatoa eruption would be ascertained, it 

 would be a fact of uncommon interest which would equal 

 and surpass the other astonishing phenomena to which 

 the cataclysm of the Sunda Strait gave rise : the transmis- 

 sion of the atmospheric waves to the barometers of the 

 whole earth, the propagation of the marine waves to the 

 maregraphs of Europe and America, the crepuscular and 

 auroral glows of the autumn of 1833, the solar corona of 

 1884 (which is still apparent, and can be observed every 

 day in February and March, 1885), the abnormal polar- 

 isation of the sk'y (A. Cornu), &c, &c. F. A. FOREL 



Morges, Switzerland, March 8 



REMARKS ON OUR METHOD OF DETER- 

 MINING THE MEAN DENSITY OF THE 

 EARTH 



T N NATURE for March 5 (p. 40SJ Prof. Mayer suggests an 

 ■*• improvement in our method of determining the mean 

 density of the earth, from which it appears that our plan 

 has not been properly understood. This misunderstand- 

 ing, no doubt, has arisen from the incomplete description 



of our method given in the Nature (Jan. 15, p. 260) report 

 of the Proceedings of the Berlin Physical Society, which 

 report was probably the only source of information access- 

 ible to Prof. Mayer. We are led therefore to give a short 

 description of our method. 



Let H 1 k l represent a section of a cubical block of 

 lead, about two metres in the edge, and weighing 100,000 

 kilos. The balance A B c is placed in the middle of the 

 upper horizontal surface. It bears the scale-pans d and E. 

 Under these scale-pans the block is bored vertically 

 through, and two other scale-pans, F and G, are suspended 

 below the block, attached to the balance by means of rods 

 passing through these openings. 



A weight in D is brought into equilibrium by weights in 

 G. The weight in d is acted upon by the earth's attrac- 

 tion + that of the block, and that in G by the earth's 

 attraction — that of the block. The weights in G are then 

 greater than that in D by twice the attraction of the 

 block. The weight in D is now removed to F and 

 counter-balanced by weights in E. The weight in E will 

 be less than that in F by twice the attraction of the block. 

 The difference of the two weighings gives therefore four 



A A 



times the attraction of the block. A correction must be 

 introduced for the variation in the earth's attraction due 

 to the different heights of D, E, and F, G. 



In order to obtain as great a deflection of the balance 

 by the method suggested by Prof. Mayer, each of the 

 mercury spheres must exert the same attraction as our 

 lead block. This would require spheres having radii of 

 about one metre. The length of the beam of the balance 

 would be necessarily at least two metres. Besides each 

 mass of mercury would exert, some attraction on the 

 weight on the other side, and thus lessen the deviation of 

 the balance. 



The method given by Prof. Mayer, except for the 

 suggested employment of mercury, is then no improve- 

 ment on ours. If we should use mercury, we would con- 

 struct a cubical vessel to contain it, and use it as we 

 propose to use the lead block. The advantage of using 

 mercury is, however, counterbalanced by the difficulty of 

 obtaining it in such large quantities as would be 

 necessary. ARTHUR KONIG 



Franz Richarz 



Berlin, Physical Institute of the University, 

 March 15 



