HORIZONTAL PENDULUMS. 127 



rotation of the axis, r, which servies as the "steady line," very 

 effectually prevents friction coming into play between itself and 

 the limbs of the forked arm, m, of the pointer when the 

 pendnlnm is recording earth movements. Both of these details 

 are indispensable in making the Horizontal Pendulum sufficiently 

 sensitive to ^m^iW jyuhatory oscillations. (See § 13). 



7. The record is taken on a smoked smooth paper 

 wrapped round a light wooden drum, v, 942 mm. in circum- 

 ference and 350 mm. in length, whose axis is formed by a 

 brass rod 8 mm in diameter. The drum, which is turned by 

 clock works w, at the rate of one revolution in an hour, has at the 

 same time a slow motion of translation in the direction of its axis, 

 one of the external prolongations of the latter being screw-cut. 



The time-marks on the record- receiver are given by the 

 electric time-ticker, x, connected with a good clock, a signal being 

 made once in a minute. 



The drum, which together with its brass axis weighs about \ 

 kgm. consists of a tliin mantle of wood, strengthened by an inner 

 hoop at the middle of its length. Further, to prevent the defor- 

 mation of the mantle, each end is composed of a frame of the same 

 wood, made up of exactly similar radial pieces, as shown in fig. 1. 



8. The above description of the mechanical details applies 

 equally well to each of the EW and NS component Horizontal 

 Pendulums, which differ from each other only in the following 

 two respects. 



Firstly, the rate of translation of the drum is, for the EW 

 component apparatus, 4 mm. per revolution, while for the NS 

 component apparatus it is 3 mm. per revolution. 



1. Provisionally a pendnlum clock has been adopted. It would unquestionably be much 

 better to employ a motor, which drives on continuously, regulated by a governor. 



