90 Lieut 0. S. Clarke, On an optical method [Dec, 3, 



In the Bangor and Carnarvon district there was, he thought, 

 much reason for suspecting that the conglomerate seen near and 

 pierced by the east shaft to the railway tunnel was transgressive 

 across the outcrops of the uuderlying rocks. Moreover it con- 

 tained fragments of all the underlying rocks and of others which 

 had not yet been identified. 



He considered therefore that the conglomerate should be 

 taken as the base of the Cambrian Rocks — that the Lingula 

 Flags and Harlech Grits were thinning out to the North but 

 would still be recognised, and that the Carnarvon and Bangor 

 rocks below the cono-lomerate were a volcanic series. 



December 3, 1877. 

 Prof. Liveing, Phesident, in the chair. 



(1) The following communication was made to the Society by 



Lieut. G. S. Clarke, B.E., On an 02:)tical method for in- 

 vestigating Rotary Motion. 



If a number of dots at equal intervals are viewed in a mirror, 

 or through a lens attached to a tuning-fork, then, in virtue of 

 the retention of their images on the retina, the dots will appear 

 as straight lines when the fork is set in vibration. Thus the 

 images of the dots shewn in Fig. 2 will appear drawn out into 

 the lines shewn in Fig. 3 if the fork is so placed that the motion 

 of the images is at right angles to a line passing through the 

 dots. 



If now a motion is given to the dots at right angles to the 

 direction of their images, the two combined rectilinear motions 

 produce the appearance of a sinuous line, or wave form. 



The height of this wave will depend on the amplitude of 

 vibration of the fork, while the wave length will depend on the 

 relation of the speed of the dots to the period of the fork. 



If certain exact ratios obtain between the velocity of the 

 dots and the period of the fork, the waves formed will be ab- 

 solutely stationary. If the velocity of the dots is slightly greater, 

 or less than that required for the fulfilment of these ratios, the 

 wave will be the same in form as that which the exact ratio 

 would give, but it will have a slow progressive motion. This 

 progressive motion will be in the same direction as that in which 

 the dots move if the velocity of the latter is too great, and in 

 the reverse direction if it is too small for the exact ratios. 



