Instrument for finding the true Lunar Distance. 63 



Cape Horn is confined to a channel, which is widened towards the 

 south in proportion to the latitudinal breadih of the column, that is 

 rushing to the east. Near the southern borders of this channel, the 

 easterly winds commence, returning in eddies towards the west 

 whence they are again carried eastwardly, in the current that rushes 

 around Cape Horn. 





Art. VI. — Plan of an Instrument for finding the true Lunar Dis- 

 tance; invented by M. F. Maury, Passed Midshipman, U. S. Navy. 



* 



G F H E, is a great circle of brass, standing upon three legs Xy Z; 

 it represents the horizon. A E, A F and B C, are arcs of great cir- 

 cles ; they also are of brass, and their planes pass through O, a com- 

 mon centre. 



The periphery of GFHE, the middle curvature PQ, of AE, 

 the concave circumference of B C, and the convex of A F, have 



equal radii centering in O. 



A E, describes half of a hemisphere on the hinge at A, the axis of 

 which, is that of the zenith and nadir, and of course passes through O. 

 The extremity F, of the arc A F, is fixed in the plane of G F H E, 

 and the extremity E, of A E, revolves in the plane of G F H E, and 

 along its circumference, describing arcs of equal circles, from the 

 centre O, until it stands in the plane of AF, the two then represent 

 a semicircle. By means of the screw S, which presses E to G F H E, 

 E is placed and'fixed at any distance from F. The hinge at A, is of 

 brass turning against steel, which lessens friction. 



AE and AF, are arcs of the azimuth circles, in which the sun or 

 a star and the moon are at the time of taking a lunar observation. 

 Each is 90° and graduated to every 10' or 15' on a slip of finer met- 

 al, let in for that purpose. 



B C, is an arc of the geocentric circle, in the plane of which, the 

 sun and moon appear, when the observation is taken ; it is graduated 

 as the other arcs are, but from 20° to 120°. It is intended first to 

 set off the apparent, and then to measure the true lunar distance. 

 To assist in transferring the altitudes and distance with exactness, from 

 the sextants with which they are taken, to the arcs of the instrument, 

 each of them is provided with a vernier scale, a, 6, c, having a tangent 

 and a fixture screw respectively attached to them. The extremity 

 to, of B C, is hinged to zero of the vernier c ; the axis of this hinge 



