HORi;^ONTAL PENDULUMS. 123 



of long period. But the iriethod is expensive, and, besides, the 

 inertia of the mass of the pendulum is too small to be suitable 

 for observing ordinary earthquakes, which throw the pendulum 

 into motion with its own period. 



3. With regard to the mechanical method of registration, 

 we can render the instruments as sensitive as we please, by 

 making the " steady mass " sufficiently heavy, say 100 or 200 

 kilogrammes in weight, as has been done by several Italian 

 observers ; or, by greatly lessening the friction between the parts 

 of the machine, as in my Horizontal Pendulum, whose heavy 

 mass is only 3 kilogrammes in weight. (See section III). Further, 

 the definition of the diagram can be made quite sharp, and the 

 rate of motion of the smoked paper, which serves as the record- 

 receiver, can, without expense and trouble, be increased to any 

 extent. 



Horizontal Pendulums adapted for mechanical registration 

 have already been successfully tried by Grablovitz^ and by 

 Cancanil In the horizontal pendulum of Grablovitz the vertical 

 height between the points of suspension and of support is 2 m., 

 while the horizontal distance between the vertical axis and the 

 centre of the heavy mass is only 10 cm., the weight of the 

 latter being 12 kgm. The vertical height in Cancani's apparatus 

 is 5.25 m., and the horizontal distance between the upper point 

 of support and the writing pen 2.7 m., the weight of the heavy 

 mass being 25 kgm. and the complete period of vibration of the 

 pendulum 24 seconds. 



1. Giulio Qrabloritz. Peiiduli orizzontali a registrazione lueccanica continua. Bdl. d. Soc. 

 Sism. Ituliana. Vol. IT. 



2. A. Cancani. I Pendoli orizzuntali, etc. BolL d. Soc. Sism. llaliana. Vol. III. 



