18 Proceedings of the Royal Society of Victoria. 



thousandth part of the senn-amplitude of the excursions of the 

 pendulum bob. The sutficient stability of the apparatus is 

 amply guaranteed by this result. 



Early in the course of the experiments it was noticed that 

 certain changes in the position of the observer, when near the 

 pendulum stand, caused changes in the zero reading, the 

 maximum displacement being produced by the experimentee's 

 crossing the room from north to south or vice versa. Accordingly 

 the observer who adjusted the weights in the statical experi- 

 ments always retired to a marked position in the room before 

 readings were taken. It appeared worth while to examine the 

 changes in the floor-level thus produced : and the following tests 

 were made for this purpose. 



A lead cylinder, weighing 38^ lbs., was placed at the foot of the 

 south and north pillars of the stand altei'nately; the readings were 



Wt. near South pillar. Wt. near North pillar. 



20-9 ... 



... 21-7 



20-9 ... 



... 21-7 



20-9 



21-8 



Hence the displacement of the weight through a space of five 

 feet on a floor six feet five inches wide, caused a change of level 

 amounting to 0'"8G, which would correspond to a depression by 

 the weight amounting to '00024 inches. Considering the 

 solidity of the flooring, this at first surprised me a good deal ; 

 until I reflected that a load of a couple of tons would produce 

 under the same circumstances a depression of about 0*2 inches : 

 now we have only to watch the bending of the railway track 

 under the wheels of a luggage train — in which a load of a couple 

 of tons per wheel is something like the usual thing — in order to 

 see that the depression thus produced in the solid mass of the 

 track is much greater than 0'2 inches; consequently the experi- 

 ment really demonstrates the exceptional solidity of the floor. 



It is necessary to mention that changes of level of the order of 

 magnitude of those produced in these experiments could not be 

 detected by the spirit levels, and would have no sensible effect 

 on the pendulum observations. 



