ON. EARTH TUEMOltS. 289 



"With this apparatus it is possible to measure an angle of 0"05 ; with 

 M. d'Abbadie's nadirane, one of 0"-03. 



M. Wolf informs me that the only result of his observations, which 

 have been carried on for several years, is that thei-e has been no permanent 

 change of level in the floor of the observatory cellar. 



Tromometer of H. T. Bertelli. — This instrument being now well known 

 a brief account will be sufficient here. In its original form it consists of 

 a mass of 3 kilogrammes suspended by a copper wire 1 mm. in diameter 

 and 3 m. long. In the ' normal tromometer,' which is the form generally 

 iidopted throughout Italj', a mass of 100 grammes is suspended by a 

 very fine copper-wire l^m. in length from a stout arm projecting from 

 the column which forms the stand of the instrument. From the centre 

 of the bob of the pendulum there projects downwards a short style, the 

 point of which, after reflexion in the hypotenuse-face of a right-angled 

 ])rism, is observed with a microscope. In some instruments the style 

 ends in a small disc, on which are ruled two fine lines at right angles to 

 one another. The mirror is provided with a glass micrometer-scale ruled 

 to tenths of a millimetre, and tenths of a division may be estimated by 

 the eye. It is therefore possible with this instrument to measure an 

 earth-tilt of about 1^". The scale can also be rotated so as to detemiine 

 the direction of the movement. 



Tremor-recorder of Professor J. Milne. — The pendulum consists of a 

 weight of 7 lb. suspended by a fine iron wire 3 feet 3;^ inches long. The 

 upper end of this wire is soldered into a small hole in a plate which forms 

 the top of a tripod-stand about 5 feet high. From the base of the bob a 

 spike projects downwards, and is kept by a spring in contact with the 

 end of a long light vertical pointer made of a strip of bamboo. The length 

 of the pointer is about 16^ inches, the short arm, which is in contact with 

 the spike of the pendulum, being about yLth of the length of the longer 

 arm. The instrument is made recording by discharging a spark every 

 five minutes from the end of the pointer, and thus perforating a band of 

 paper which is moved by clockwork beneath the end of the pointer. A 

 second band of paper moves at right angles to the former, in order to 

 avoid the loss of a record in case the pointer should move parallel to 

 the first band. Though designed independently, this instrument, as 

 Professor Milne remarks, is similar to one previously made by M. Bouquet 

 de la Grye.' 



To avoid the friction arising from the movement of the pointer, M. 

 Chesneau suggests that the bob of the pendulum should be a lens, a point 

 of light close to it forming an image at a greater distance, which should 

 record on a strip of photographic paper the movements of the pendulum.* 



The tromometer now used by Professor Milne is described in the 

 'Report of the British Association' for 1892 as well as in the two last 

 papers in the following list, in which he has given the results of his work 

 on this subject : — 



1. ' Earth Tremors,' ' Japan Seism. Soc. Tran.'!.,' vol. vii. pt. 1, ISS.'?, pp. 1-15. 



2. ' Earth Pulsations,' 'Nature,' vol. xxviii. 1883, pp. .367-372. 



3. ' Earth Tremors,' ' Nature,' vol. sxix. 1884, pp. 450-459. 



' Paris, Acad. Sci., Comjrt. Rend., vol. xcvi. 1883, p. 1857; &\so Bajjport Annue. 

 iirVEtat de V Ohservatoire de Paris jyuur VAnnt'e 1885 (Paris, 1886). pp. 24-26. 



• M. B. de Chancourtois,/ Sur un Moyen de constater par Enregistrement continu 

 les petits Mouvements de I'Ecorce Terrestre,' Paris, Acad. Sci., Compt.liend., vol. xcvi. 

 1883: pp. 1857-1859. 



1893. u 



