

A PHOTOGRAPHIC FOUCAULT-PENDULUM. 



!67 



a 



fij.4- 



ing to the pendulum-length, so as to secure uniform 

 definition in the photograph (Fig. 3). 



By the end of January last the ap- 

 paratus was ready, and some trial 

 photographs were obtained, one of 

 which is reproduced in Plate XX, (A). 

 Each of the experiments was made 

 during the quietest portion of the 

 night, some time between 1 a.m. and 

 4*30 a.m., and of course using red light. 

 One of the two observers, while making* 

 the exposures (in pairs, North and 

 South, or East and West) every five 

 minutes, gave a signal of the instant 

 of maximum excursion of the spot of 

 light, the other observer noting the 

 time with a half-second chronometer* 

 In Plate XX, the alteration in azimuth 

 during the five-minute intervals 

 between the exposures is evident, and 

 agrees fairly well with the theoretical 

 value for the latitude of Sydney, 8° 21' per siderial hour. 

 But several slight perturbing influences were present, and 

 are now to be eliminated. The elliptical path sooner or 

 later always followed by the bob is also clearly visible, 

 and capable of quantitative measurement. Obviously it is 

 quite too rapidly generated, in this trial photograph; but 

 in the next series of exposures, now about to be undertaken, 

 I have reason to hope it will be greatly reduced, as well 

 as some minor defects, with the introduction of a number 

 of refinements in the apparatus and in the modus operandi. 



Thus, as I have endeavoured to show, the main feature 

 of this "photographic Foucault-pendulum," (which for con- 

 venience I have ventured to name " geogyrograph," — not 



Cardan suspension: 

 a, outer ring, screwed 

 on b, (fig. 3); b, inner 

 ring, on knife-edges ; 

 c, clamp for suspend- 

 ing wire (d), on knife- 

 edges; h, knife-edges. 



