119 



steel plate, this being mounted as in the preceding experi- 

 ments. Our observations were taken, during the earlier ex- 

 periments, at intervals of five minutes, which Ave afterwards 

 changed to ten minutes ; and having thus determined the im- 

 portant points, we watched these continuously, using an interval 

 of twenty minutes for the other portions of the motion. 



" The two experiments with hemp fibres occupied a time 

 exceeding sixteen hours each. The experiments with the 

 wire suspension occupied periods of twenty hours each, and 

 the experiments with the silk fibre suspension occupied fifteen 

 and nineteen hours respectively. At the close of these periods 

 the motion was distinctly visible, and being almost rectili- 

 near, the direction of vibration could be determined with con- 

 siderable accuracy. 



" The results of these and subsequent experiments will 

 shortly be offered to the Academy in detail ; at present we 

 shall confine ourselves to an abstract of those results, and 

 to a brief comparison of the theory of M. Foucault with obser- 

 vation. 



" The facts which we observed, without making any allow- 

 ance for instrumental error, apsidal motion due to the dif- 

 ference between the arc of vibration and its sine, or disturb- 

 ances produced by the air, may be expressed by the following 

 statements, the initial motion being in the meridian : 



" I. The azimuthal velocity diminished from the com- 

 mencement of the motion, attaining a minimum in the position 

 between E. 30° N. and E. 



" II. The azimuthal velocity increased after it had passed 

 the east, and attained a maximum, which appeared to lie between 

 S. 40° E. and S. 



" III. The motion of the azimuthal plane near the south and 

 south-west appeared very unsteady, as if greatly affected by 

 disturbing causes; but indicated a general tendency to increase 

 its rate from S. to S. W. 



