Notes. 



589= 



I have much pleasure in offering this instrument to the Society 

 for its Cabinet. 



As the evolution of the prism bar has been alluded to above, it 

 might not be out of place K? "?pend a diagram of the section of the 

 various bars. 



Fig. 144. 



Fig. 145. 



Fig. 146. 



Fig. 147. 



Fig. 14S. 



1. Benjamin Martin, with internal rack, 1770, fig. 144. 



2. Eoss- Valentine, angles slightly truncated, 1831, fig. 145. 



3. Powell's, a cylindrical bar with three faces planed off, 1833 r 

 fig. 146. 



4. Powell's next form was merely an enlargement of fig. 145 y 

 1843. 



5. Eoss abandons the triangular for a rectangular parallelopiped, 

 1851, fig. 147. 



6. Powell's truncated prism, now in use, 1861, fig. 148. 



In the above figures, the shaded portion in each case represents 

 the rack. 



Oct. 21st, 1908 



2 Q 



