ON EARTH TREMORS. 1!87 



Earth Tremors. — Report of the Committee, consisting of Mr. Gr. J. 

 Symons, Mr. C. Davison {Secretary), Sir F, J Bramwell, Pro- 

 fessor Gr. H. Darwin, Professor J. A. Evving, Dr. Isaac Egberts, 

 Mr. Thomas Gtray, Sir John Evans, Professors J. Prestwich, 

 E. Hull, Gr. A. Lebour, R. Meldola, and J. W. Judd, Mr, M. 

 Walton Brown, Mr. J. Gtlaisher, Prof. C. Gr. Knott, Prof J, H. 

 PoYNTiNG, and Mr. Horace Darwin . {Draton up by the Secretary). 



Appendix — Account of Observations made with the Horizontal Pendulum. 

 Bij i>r. E. VON Eebeur-Paschwitz, p. 309. 



The Committee were appointed to consider the advisability and desirability 

 of establishing in other parts of the country observations upon the pre- 

 valence of earth-tremors, similar to those now being made in Dui-ham in 

 connection with coal-mine explosions. 



In the present report descriptions are given of several instruments 

 which have been used in the study of earth-tilts and earth-pulsations. 

 The list is not a complete one, some having already been included in the 

 reports of the Committee on the Lunar Disturbance of Gravity for 1881 

 and 1882, drawn up by Messrs. G. H. and H. Darwin.' The first of 

 these valuable reports also contains an account of the pendulum with 

 double-suspension mirror, used in the well-known experiments in the 

 Cavendish Laboratory, Cambridge. The extraordinary sensitiveness of 

 this pendulum led Mr. Horace Darwin to design another form of the 

 instrument, smaller and simpler in construction, but capable of measuring 

 smaller angles than are required in these experiments. The method of 

 determining the angular value of the scale-divisions has also been altered. 

 The new pendulum was made some months ago by the Cambrido-e 

 Scientific Instrument Company, and Mr. Darwin, being unable at the 

 time to give much attention to it, has lent it to the Committee for trial. 

 The preliminaiy experiments have been made by the Secretary at 

 Birmingham, chiefly under Mr. Darwin's guidance, and the results are 

 described below. The Committee think it most desirable that the 

 observations should be continued in Birmingham, and also made in other 

 parts of the country. 



One of the most delicate instruments that have been used for the 

 observation of earth-tilts is the horizontal pendulum of Dr. E. von 

 Rebeur-Paschwitz, who has described the results obtained with it in 

 several valuable memoirs. The Committee considered that an account 

 of these observations in English would be of great use, and at their 

 request Dr. von Rebeur-Paschwitz kindly consented to write the very 

 luteresting and valuable summary which forms an appendix to this 

 report. 



Nadirane of M. G. Wolf. — The nadirane erected in 1863 by M. 

 d'Abbadie at Abbadia, near Hendaye, is described in the reports for 

 1881 and 1 882,- as well as some of the results which have been obtained 

 with it. 



' Brit. Assoc. Reports, 1881. pp. 93-126 ; 1882, pp. 9.5-119. In referring to these 

 reports in the following pages they will be quoted as the Reports for 1881 and 1S82. 



2 Brit. Assoc. Reports, 1S81, pp. 116-118 ; 1882, pp. 102, 103. The following is a 

 list of M. d'Abbadie's papers on this subject : — 



