HORIZONTAL PENDULUMS. 129 



The strut, h, is adjusted to a nearly horizontal position ])y 

 means of a screw, /, to which the suspension Avire is attached, 

 and which pnsses normally through a closely fitting hole in a 

 steel trinngular prism, (j. The edge of the knife has all been 

 cut away except two small portions, h and i, on which the prism 

 rests in a V-slot, /, in the inclined hypothenuse face of an isos- 

 celes right-angular iron prism, ^,®'°soldered to^ a large square 

 screw, /. The latter passes through two closely fitting stirrups, m, 

 attached to a thick brass 2:>late, n, and the azimuthal position of 

 the heavy cylinder, a, can be adjusted by appropriate movements 

 of the screw, /, by means of the two nuts, o. For properly 

 adjusting the period of vibration of the horizontal pendulum, the 

 whole system is, in its turn, made to move forward and back\vard 

 on the basal plate, /;, by means of a screw, q. Finally, the two 

 screws, r, fixed in the stirrup, 5, rising from the basal plate, 

 secures the plate, n, to the latter. 



The right-angled prism, h, is so perforated as lo allow the 

 free passage through it of the screw, /. Rectangular cuttings, / 

 and ?/-, have also for the same purpose been made in the two 

 plates, n and ]). 



The basal plate, p, is to be fixed to the top of the cast iron 

 stand, (I, with such an angle of inclination that the suspension 

 wire is at right angles to the knife-edge. 



11. The complete period of vibration of this form of the 

 horizontal pendulum has been set at 20 seconds, but it can be 

 raised much above this value, if desiied, the maximum length of 

 the complete period, with which the pendulum can be kept stable, 

 being about 90 seconds. The comparatively low value of 20 

 seconds has been chosen for the sake of preventing the deviation 

 of the pendulum becoming inconveniently great, the purpose here 



