SURVEYING INSTRUMENTS. 69 



of the glass, but by reflection in the silvered part ; if the 

 object in the silvered part exactly meets, and forms one con- 

 tinued line with that seen through the unsilvered part, then 

 is the instrument said to be adjusted, and the horizon glass 

 to be parallel to the index glass ; but if the objects do not 

 coincide, then loosen the screw on the under side of the 

 quadrant, and turn the horizon glass on its axis, by means 

 of its adjusting lever, till you have made them perfectly 

 coincide. This adjustment ought to be examined before 

 every important observation. 



3. To adjust the Horizon Glass perpendicular to the Plane 

 of the Quadrant. Incline the quadrant on one side as much 

 as possible, provided the distant object continues to be seen 

 in both parts of the glass at the same time. If, when the 

 instrument is thus inclined, the object continues to form an 

 unbroken line, the quadrant is perfectly adjusted ; but if the 

 reflected object be separated from that seen by direct vision, 

 the glass is not perpendicular to the plane of the quadrant ; 

 and if the observer is inclined to the right, with the face of 

 the quadrant upward, and the reflected object appears higher 

 than the real object, you must slacken the screw before the 

 horizon glass, and tighten that which is behind it ; but if the 

 reflected object appears lower, the contrary must be per- 

 formed. Care must be taken in these adjustments to loosen 

 one screw before the other is screwed up, and to leave the 

 adjusting screws tight, or so as to draw with a moderate 

 force against each other. 



Price $14.00 to $18.00. 



Sextant. (Fig. 96, page 70.) The annexed figure re- 

 presents a sextant of Trough ton's construction, having a 

 double frame, A A, connected by pillars, a a t &c., thus 

 uniting strength with lightness. The arc, B C, is generally 

 graduated to 10' of a degree, commencing near the end, C, 

 and it is numbered towards B. The divisions are also con- 

 tinued on the other side of zero, towards C, forming what 

 is called the arc of excess, which is useful in determining 

 the index error of the instrument, as will be explained here- 

 after. The limb is subdivided by the vernier, E, into 10", 

 the half of which (or 5") can be easily estimated : this 

 small quantity is easily distinguishable by the aid of micro- 

 scope, H, and its reflector, b, which are connected by an arm 

 with the index, I E, at the point, c, round which it turns as 



