A PHOTOGRAPHIC FOUCAULT-PENDULUM. 263 



places, the investigation was carried out by a visual method 

 of some kind or other, which in certain cases at least, lays 

 itself open to criticism. 



During November of last year, while planning details for 

 a quantitative repetition in Sydney, with all the precision 

 possible, of Foucault's experiment, with a view to the 

 verification of his law in the Southern Hemisphere (the 

 observations at Rio Janeiro in 1851 having given unsatis- 

 factory results), it occurred to me that it would be inter- 

 esting to endeavour to obtain also a photographic record of 

 the apparent variation in azimuth of the oscillation-plane 

 of the pendulum. This appeared to have certain advantages 

 from several points of view, not as supplanting, but as 

 supplementing, the time-honoured visual method, which 

 alone I intended following in the first instance. It appeared 

 to me that a small glow-lamp and short-focus lens could 

 be enclosed within the heavy leaden "bob" of the pendulum, 

 and thus a well-defined spot of light could be made to record 

 the change of azimuth of the latter on a sheet of bromide- 

 paper placed below. Professor Pollock, f.r.s., to whom I 

 spoke about the idea, encouraged me to attempt to realise 

 it, which I did. The results so far are, I hope, such as to 

 justify further experiments, to be begun soon. Various 

 difficulties of course soon presented themselves, but they 

 were eventually overcome. Among them was the question 

 of feeding the glow-lamp. Several alternative plans were 

 considered, but finally I decided to adopt an 8-volt supply, 

 derived from four storage-cells symmetrically disposed 

 within the bob itself. The latter was originally designed 

 as a pair of brass hemispheres with bayonet-joint coupling, 

 and filled with lead, except for a cubical space in the centre 

 for the cells and lamp, and two vertical cylindrical spaces, 

 one below, for a draw-tube holding the lens, the other 

 above, to maintain symmetry of mass. This plan was 



