for ascertaining the Deviation of the Magnetic Needle. 361 



Method of using the Instrument for ascertaining the Variation 

 of the Needle by Inspection. {See Plate I.) 



Suppose the platform H I to be screwed upon the binnacle, 

 or other convenient part of the ship, in such a position that 

 the line H I, which is drawn upon the platform, may coincide 

 exactly in a longitudinal direction with a line immediately over 

 or parallel with the vessel's keel : — now, if the hemisphere C be 

 elevated upon its axes or pivots c, until the divisions upon the 

 arc g h are brought to read against the vei-nier k at the de- 

 gree of the latitude of the place, it will be evident that the dial 

 plate A B will be situated parallel to the plane of the equator, 

 or perpendicular to the earth's polar axis. In this situation, 

 if the hand or index E be set to the apparent time, say three 

 hours, or forty-five degrees from the meridian, as shown in the 

 figure, and the rod 5 be elevated in the tube o to the division 

 corresponding with the tangent of the sun's declination, the 

 compass plate, F G, which has the instrument mounted upon 

 it, is then to be turned round upon its axis until the shadow of 

 the point of intersection of the cross wires O may fall upon the 

 surface of the ivory q, so as that the said point of intersection 

 may coincide with the intersection of the cross lines upon the 

 surface q. The line A B or F G will then be in the plane of 

 the true meridian, and the variation of the magnetic compass 

 in the binnacle, or any other magnetic compass which is pa- 

 rallel to it, may be ascertained by inspection. In whatever 

 course or direction the vessel may be proceeding, if the com- 

 pass plate F G is turned upon its axis until the vernief I is 

 brought to coincide with the point or degree corresponding 

 with that course, the shadow of the intersection of the cross 

 wires O should always coincide with the intersection of the 

 lines upon the ivoi-y q. 



The vessel will then proceed in its course without deviation, 

 the instrument serving as a constant and immediate check to 

 the irregularities of the needle as long as any one of the hea- 

 venly bodies is visible. 



Method of using the Instrument for steerimt -diithout Magnetic 

 Aid. (&'^' Plate I.) 



The dial jilate A B is provided with watch-work, by which 

 the hand E is moved round once in twenty-four hours. 



Having fixetl the platfonn H I over or parallel to the ves- 

 sel's keel, and the hand E being set to the time, and the hemi- 

 sphere C being set to the latitude, as above described, the 

 compass plate J'" G, which has Uie instrument mounted upon 

 it, is to be turned round upon its axis, until the zero of the 



Vol. Gfj. No. 325. May 1 H2F>. Z z vernier 



