328 Dr. J. Kerr's Experiments on the 



with particular care. Width of pillar and of each wing about 

 J inch, length of slit about 2 J inches. 



Fig. 3. 



>— 1 P— " 



The screw-press is a rectangular frame of metal, with two 

 strong and fine-threaded screws passing through the top, and 

 bearing on a massive sliding piece of metal which is guided 

 by the two sides of the frame. The winged plate stands 

 vertically between the plane base of the frame and the sliding 

 piece, being separated from the base by a dozen folds of tin 

 foil, and from the sliding piece by plates of wood and india- 

 rubber. When the whole piece is placed in a polariscope, 

 and the screws are worked forward with proper care, there is 

 an intense effect of pure vertical compression obtained in the 

 central pillar, without a trace of effect in either wing. Pillar 

 and wings act as parts of one unbroken plate, which differ 

 only in regard to strain. 



The first use made of this piece, and of a similar plate 

 arranged roughly for vertical tension, was to revise the proofs 

 of propositions I. and II. Both propositions were fully verified, 

 the effects being sensibly neither better nor worse than 

 those obtained already with the bent plate. Details are 

 omitted as unnecessary. 



Proposition IY. 



Things being still arranged as in propositions I. and II., it is 

 required to measure the strain-generated retardations of the 

 two pencils, and to show that, for the same strain, the greater 

 of these effects is exactly or very nearly tivice the less. 



8. My earliest results in this direction were obtained by 

 some rough measurements with the bent plate and the 

 refractor. The plate was mounted as in the diagram of 



