ox* THE MEASUREMENT OF THE LUNAR DISTURBANCE OF GRAVITY. 97 



through Bologna, and it was arranged that at 30 meters from the Palazzo 

 Malvasia they should break into a trot. The pendulum, situated only 6 

 meters from the street, was observed to be unaffected by this, and continued 

 its oscillations in the B.W. azimuth. A pool of mercury was violently 

 agitated, and it was concluded that the motion communicated to the 

 ground by the artillery was exclusively vertical. 



At page 5 of the ' Bulletino ' for 1876 (January to May), Rossi 

 writes a ' Guida pratica per le osservazioni sismiche.' This article con. 

 tains a description of the instruments which have been used by the 

 Italian observers. 



Bertelli used a pendulum protected from the air, with a microscope 

 and micrometer for. evaluating the oscillations. The upper part of the 

 support of the pendulum consisted of a spiral spring, so that vertical 

 movements of the ground could be recorded. This instrument he calls a 

 tromo-seismometer. 



Professor Kgidi, of Anagni, proposed to use the reflection from mer- 

 cury. The object observed was to be a mark fixed on a wall, and the 

 reflected image of the mark was to be observed with a telescope. The 

 deviation of the vertical was to be evaluated by noting the amount of 

 movement required to bring the cross-wires of the telescope on to the 

 mark. This instrument has not, I think, the advantages of M. d'Abbadie's, 

 because the light was incident at about 45° on the mercury, and thus 

 the mark and telescope were remote from one another ; whereas in the 

 arrangement of M. d'Abbadie the mark and microscope are close together, 

 and only a micrometer wire in the microscope is movable. 



Cavalleri used ten pendulums of graduated length, and found that 

 sometimes one of the pendulums was agitated and sometimes another. 

 Rossi observed the same with his pendulums at Rocca di Papa. It thus 

 appears that the free period of oscillation of the pendulum is a disturbing 

 element. 



In order to obviate the discrepancies which must arise in the use of 

 various kinds of pendulums for simultaneous observations in different 

 places, Bertelli and Rossi propose a normal ' tromometer,' of which a 

 drawing is given. The length of the pendulum is 1^ meters, the weight 

 100 grammes, and it makes forty-nine free oscillations in a minute. To 

 the bottom of the pendulum is attached a horizontal disk, on the under- 

 side of which are engraved two fine lines at right-angles to one another. 

 These lines are observed, after total internal reflection in a glass prism 

 placed immediately below the disk, by a horizontal microscope, f-irnished 

 with a micrometer. The azimuth of the deflection of the vertical is 

 observed by a position-circle. 



This paper also contains a description of the author's observatory at 

 Rocca di Papa. It is established in a cave at 700 meters above the sea, on 

 the external slope of the extinct Latian volcano. There is a large central 

 l^endulum hanging from the roof, and there ai'e four others with dif- 

 ferent weights and lengths hanging in tubes cut in the native rock. Only 

 the ends of these pendulums are visible, and they are protected by glass at 

 the visible parts. A great part of this paper is occupied with descriptions 

 of seismometers, and this is outside the scope of the present Report. 



In presenting a pamphlet by Father Bertelli, entitled ' Riassunto delle 

 osservazioni microsismiche, &c.,' to the Academy (' Comptes Rendus,' 

 1877, vol. 84, p. 465), M. d'Abbadie summarises Bertelli's conclusions 

 somewhat as follows : — 



1882. H 



