32 
Froceedings of the Eoyal Society 
which a brass disc a' supports three radiating sheets of brass c 
that are surmounted by a brass dome d, on the top of which there 
is a ring for the line. The plate a ' has an india-rubber ring a 
fixed upon it, and its under surface has two rings for attaching the 
a brass cylinder B, the edge of which rests upon the india-rubber 
ring a when the instrument is closed (fig. 2). 
