72 



Mr. W. H. L. Russell on the Mechanical [June 17, 



Mr. Gassiot was present on one occasion, and particularly observed with 

 myself the rapidity with which a series of 12 large Ley den jars arranged 

 in cascade were discharged. The noise was great ; and each time the 

 spark (which was very condensed and brilliant) struck the metallic disk, 

 the latter emitted a ringing sound, as if it had received a sharp blow from 

 a small hammer. 



The discharges were made from a point to a metallic disk ; and when the 

 former was positive the dense spark measured from 18| to 18| inches, and 

 fell to inches when the metallic plate was positive and the point 

 negative. 



A variations of the Leyden-jar experiments was tried, by connecting the 

 coil worked by a quantity battery of 25 + 25 cells with six Ley den jars 

 arranged in cascade ; and the spark obtained measured 8| inches. 



The same six jars connected with the coil when the 50 cells were 

 arranged continuously for intensity gave a spark of 12 inches of very great 

 density and brilliancy. 



Other experiments are being tried with the great coil, the results of 

 which will be duly brought before the Society if thought of sufficient 

 importance. 



XXVIII. " On the Mechanical Description of Curves." 

 By W. H. L. Russell, F.R.S. Received June 17, 1869. 



Let A, B, C be three wheels rolling in one another (fig. 1 ) ; they may 

 of course be supposed to describe simultaneously the angles mO, nd, rd, 

 when m } n, and r are constant. 



Fig. 1. 



Let a, /3, y be three nuts situated on A, B, C respectively, at distances 

 a, by c from their centres. Then if these nuts work in horizontal bars (as 

 exemplified in many sewing-machines), the bars will descend vertically 



