HORIZONTAL PENDULUMS. 125 



a strut, h, which consists of an iron tube 1.5 cm. in diameter, 

 and of such length that the distance between its end and the 

 axis of the cylinder is 1 m. 



The heavy-bob, a, is hung by a fine steel wire, c, from an 

 inside jorqjection of one of the walls of the " Earthquake-proof 

 House," the lower end of the wire being attached to a stirrup, 

 d, pivoted at e, a little above and in front of the centre of gravity 

 of the bob. The tubular strut, h, is furnished at the end with 

 a sharp conical steel point of about 50°, which is pivoted in a 

 conical steel socket,/, of about 120°, fixed to a step-like projection 

 in the lower part of the wall. 



The upper end of the wire, c, is attached, by means of a 

 screw, g, to a triangular steel prism. A, whose knife-edge, reduced 

 to two small portions, i and _/, works on a steel V-groove, k^ 

 properly perforated to allow the passage of the screw (j. The V- 

 groove is mounted on a stout arm of hard wood bolted to the 

 upper piojection of the wall in such a way that the wii e and the 

 knife-edge, whose intersection determines the upper point of 

 suspension of the pendulum, stand at right angles to each other. 

 The pendulum can thus swing freely about its axis, the vertical 

 distance between the points of suspension and of support being 1\ 

 ra. The vertical adjustment of the heavy-bob, a, is made by the 

 screw, g, while its horizontal adjustment is made by a screw (not 

 shown in the figures) moving the point of support in the direction 

 normal to the pendulum plane. 



As the period of free vibration of the Pendulum is slow, and 

 as the strut, ô, is sufficiently long, the axis of the heavy cylinder 

 may be considered as being '* steady " not merely with respect to 

 infinitesimal and quick vibrations of the ground, but also with 

 respect to considerable pulsatory or very slow undulations, since 



