OF THE GONIOMETER. 



The principle of the instrument may be thus ex- 

 plained. 



Let us suppose a b c, fig. 55, to be a crystal, of which 

 one plane only is visible in the figure, attached to a 

 circle, graduated on its edge, and moveable on its 

 axis at o ; and a and b the two planes whose inclina- 

 tion we require to know. 



And let us further suppose the lines o e, og, to be 

 imaginary lines resting on those planes in directions 

 perpendicular to their common edge, and the dots at i 

 and 7z, to be some permanent marks in a line with the 

 centre o. 



Let us suppose the circle in such a position, that 

 the line o e would pass through the dot at h, if ex- 

 tended in that direction as in fig. 55. 



Fig. 56. 



If we now turn round the circle with its attached 

 crystal, as in fig. 56, until the imaginary line o g, is 



