[ 347 J 



LXXXI. Description of an Insirumenl ly which i/ie Monn's 

 Diatancefroyn the Sun or a fixed Star may he cleared from the 

 Effect of Refraction and Parallax; — aha of a new Reflecting 

 Goniometer. By Mr. J. B. Emmei'I', of Trinity College, 

 Cambridge, 



iVj-Y object in describing this instrument is to facilitate, if pos- 

 sible, in some degree the method of finding the longitude by a 

 lunar distance, principally for the use of the masters of our mer- 

 chant-ships, by abridging the calculations. 



1 do not offer the instrument as complete and perfect ; it re- 

 quires a greater number of observations than is desirable, and it- 

 self is not very simple. As^ however, it will most probably receive 

 successive improvements and ultimately assume a more conve- 

 nient form, and hast he principle is rater new, it may not be to- 

 tally iniworthy the attention of the mariner. The principle is this : 



Let HQO (Fig. 1, Plate IV.) represent the plane of the hori- 

 zon, ZMmO a vertical circle passing through the true and ap- 

 parent place of the moon; ZsSQ another vertical circle passing 

 through those of the star ; MS is their true and ms their appa- 

 rent distance : let hcjo represent a circle in the plane of the ho- 

 rizon, ZPO, zhq, circles in the planes of ZM77iO and Z^SQ; 

 join MC, ?«P, SC, Sp : the diameter ho must be supposed eva- 

 nescent compared with HO, .: P, p and C must be supposed 

 coincident. An observer at C will perceive the objects at M 

 and S; but if a prism be placed at P which elevates any ob- 

 ject seen through it in the direciion ZPO, the angle of deviation 

 equal MPot, and another depressing verticallv placed at p, hav- 

 ing an angle of deviation equal S/jS, the rays which reach the 

 observer's eye will be PC, pC, proceeding from the real places 

 M and S, a sextant would then measure their true distance, 

 and its plane would be held in the plane MPpS passing through 

 the observer's eye, and their true, and not wPpS their ajjpa- 

 rent places. It is now very easy to perceive that if two qua- 

 drantal arcs be fixed upon the frame of a sextant capable of 

 moving upon axes which ahvays meet in the centres of these arcr^, 

 such arc having a prism of a proper angle of deviation, the re- 

 fraction taking place in the plane of tliis arc, the ravs from the. 

 moon ])assing through one prism, and those from the star 

 through the other ; by placing the axes of tliese arcs in such a 

 position that the angle between them shall equal the true di- 

 stance known within a few minutes from the longitude by ac- 

 count and estimated from the Nautical Almanac, then elevating 

 the arcs till from the point of intersection the arcs measured to 

 the plane of the instrument equal the true altitudiis of the ob- 

 jects. 



