Improved Geometrical Plotting Quadrant, £f?c. 169 



the distanee measured on the base line {i. e. 50), then the 

 distance of the object from both stations will be shown on 

 the two limbs, as will also the base and perpendicular on 

 the respective lines. If the divisions on the parallel do not 

 agree with the distance measured, the perpendicular must be 

 altered till that division be shown, when the required di- 

 stance will be given. This is a case of our first problem. 



Fourthly. — To level or measure the Altitude of any Object. 

 It is only necessary to set the plane of the instrument 

 vertical instead of horizontal by means of the joint under 

 the instrument, whence it is evident every case may be 

 known as on the horizon ; and to level, it is only requisite 

 to set the spirit-level at the back of the instrument, the base 

 line and every object cut by the same will be level thereto. 



Fifthly. — To take Angles or Altitudes at Sea, where the 

 Instrument cannot be made stationary. 



For this purpose, it is first requisite to change the sight a, 

 fig. 1, and substitute the one b, fig. 2 ; which being firmly 

 fixed and adjusted at right angles with the upper limb, it is 

 evident that when by reflection any object is brought to co- 

 incide on the mirror, at the extremity of the base line, with 

 another object seen in the direction of such base, the angle 

 will then be known, being double what the upper limb de- 

 notes on the arch ; to which true angle, or its double, the 

 lower limb may be fixed, leaving the one with the mirror 

 again at liberty to take another observation and angle at any 

 distant place or time; which being so taken, this limb may 

 be also moved and fixed to double its apparent angle, and 

 the altitude or distance be then determined, bv setting the 

 perpendicular and parallel as in other common cases on 

 land. 



From this mode of determining distances, as the use of 

 calculations and of tables of sines and tangents are super- 

 seded, it is presumed that much convenience will arise to 

 the unlettered who may have occasion to use it, and thereby 

 the error* of calculations will he avoided. 



As well as the btfore-meiu.oned purposes to which the 

 instrument applies, it is presumed there will be found other 



things 



