ON EARTH TEEMOES. 



307 



oscillation when the axis is inclined at angles t, and i^ to the vertical, 

 we have the following equations : — 



and 



tan i (t2 + i,)=tan i {i.-i,) ^'' + ^% , 



To=TiV'smi,=T2N/ 



SU) lo 



Thus, T,, T2, and 12— i\ being known, we can find i^, i^, and con- 

 sequently Tfl. 



When the point of light leaves the photographic paper, the pen- 

 dalum has to be readjusted by turning the foot-acrews. This always 



Fig. 9. 



produces a certain change of stress, which it is desirable to avoid, espe- 

 cially as in some places the movement is so great that such corrections are 

 necessary every few days. By the employment of a ' correcting-mirror,' 

 which can be turned by screws, the point of light ca'n be brought back to 

 the middle of the roller by the observer without touching or approaching 

 the instrument. The arrangement will be evident from fig. 9. 



It would be a great advantage if the length of the roller used for the 

 photographic records were increased, and an arrangement devised for 

 displacing the roller in the direction of its axis, so that the same paper 

 could be used during two consecutive rotations. The rapidity of rota- 



X 2 



