OF THE GONIOMETER. 29 



i 



brought into the same position as the line o e is in 

 fig. 55, we may observe that the No. 120 will stand 

 opposite the dot at t. 



This is the number of degrees at which the planes 

 a and b incline to each other. For if we suppose the 

 line og, extended in the direction oz, as in fig. 56, it 

 is obvious that the lines oe, oz, which are perpen- 

 dicular to the common edge of the planes a and 6, 

 would intercept exactly 120 of the circle. 



Hence an instrument constructed upon the principle 

 of these diagrams, is capable of giving with accuracy 

 the mutual inclination of any two planes, if the means 

 can be found for placing them successively in the 

 relative positions shewn in the two preceding figures. 



Fig. 57. 



When the planes are sufficiently brilliant, this pur- 

 pose is effected by causing an object, as the line at /w, 

 fig. 57, to be reflected from the two planes , and &, 

 successively, at the same angle. 



It is well known that the images of objects are 

 reflected from bright planes at the same angle as that 

 at which their rays fall on those planes; and that 

 when the image of an object reflected from a horizon- 

 tal plane is observed, that image appears as much 



