ON SEISMOLOGICAL IXVESTIGATIOX. 259 



Tock and scoria gi\-e a good idea of the chai-acter of the materials on which 

 Catania and its observatory are founded. 



The observatory, which overlooks the city and the sea, stands on the 

 top of one of the steps indicating the contour of the country now buried by 

 the molten flood of 1669. 



The buildings are rugged, large, and massive, and apparently form 

 portion of an uncompleted church ; and it is in the spacious vaults of this 

 church (Convento dei Benedettini) beneath the astrophysical observatory 

 that the Seismological Laboratory is established. 



The foundations of these buildings, like those of many of the buildings 

 in Catania, follow the very irregular contour of the lava bed from which 

 they rise. 



The Iiidrumeiits. 



The entrance to the Physical Observatory on the north side is a hall 

 at the end of which stone stairs lead to upper storeys. In the open space 

 between these there is a thin metal tube reaching from the roof above, and 

 passing downwards through the floor into the vaults beneath. This tube, 

 which is steadied by horizontal wire ties, protects the supporting wire of 

 the great pendulum, which is about 25-3 metres (83 feet) in length. At 

 its upper end it is supported from a double T-iron beam, and at its lower 

 end it carries a cylindrical mass of metal weighing 300 kilos. The bob 

 hangs freely in a case standing on the floor of a special chamber in the 

 crypt- like vaults below. 



The movements of the pendulum or of the ground relatively to the 

 same are recorded by pens charged with glycerine ink, somewhat similar 

 to the pens employed in the Richard meteorological instruments, upon a 

 band of paper moving at a rate of one cm. per minute. These pens, 

 which are balanced so that they barely touch the recording surface, 

 are attached to the ends of aluminium levers which multiply the relative 

 motion of the pendulum and the ground 12-5 times. The shorter arms of 

 these levers (a c and b c) are slotted, and embrace the wire of the pendulum, 

 which is 6 mm. thick, just above the bob (fig. 10). 



The motion is recorded by suitable levers. At each hour, by means of 

 an electro-magnet in connection with a chronograph, the pens are gently 

 raised from the paper for a period of about six seconds. In this manner 

 time intervals are obtained. 



Fig. 10. 



Inasmuch as the period of the pendulum relatively to that of local earth- 

 quakes is long, it acts as a steady point, and a record of the movements of 

 the ground magnified about 12-5 times is obtained upon the moving band 

 of paper. Two such shocks were recorded on the morning of my arrival in 

 Catania. 



With the long-period earthquakes originating at great distances it is 

 assumed that the pendulum follows the slowly tilting ground. 



s2 



