32 Rev. Mr. Emmett onjinding the Longitude at Sea. [Jan. 



index to 0°, 0', 0'', and bring the index mirror G H into the 

 same plane with the fixed one, 1 K. Hold the plane of the 

 instrument perpendicularly to the horizon, and in such a posi- 

 tion that the horizon of the sea is received through the telescope. 

 Let M' be the moon, or a star; move the index till this object is 

 seen through the telescope. As in the common sextant, the 

 rays from both objects pass down the axis of the telescope, and 

 therefore the images are in contact. The advantages are that 

 the telescope is not limited in size ; the mirrors being very near 

 the object lens, less light is lost than when more remote, as in 

 the sextants in common use ; the images do not suffer the 

 excessive loss occasioned by two reflexions ; the images are 

 very bright and very distinct ; the oscillation is half that of the 

 sextants in common use. There is far less difficulty in finding 

 small objects than might be supposed ; and stars of even the 

 sixth magnitude are easily observed. If the circle be completed 

 it makes an excellent repeating circle ; it is infinitely easier to 

 find a star with it than with the common repeaters, because 

 each of the objects must alternately suffer two reflections; and 

 upon the proposed plan, the instrument does not require alter- 

 nate inversions. If a crystal be substituted for one mirror, it 

 makes an excellent goniometer. Should this form of the sextant 

 be not approved, the Newtonian may be recommended as having 

 some great advantages ; a larger and more powerful telescope 

 maybe employed than canbe applied to those in common use, and 

 can be more permanently secured ; the mirrors are much nearer 

 to each other and to the telescope : on this account, it gives 

 brighter and better defined images. 



1 shall have great pleasure in presenting the requisite appara- 

 tus to any officer in the navy, and to one in the East India 

 Company's service, who will take the trouble of observing with 

 it during a voyjige; requisite calculations and tables will be 

 provided for their use ; requesting that in return these gentlemen 

 will furnish me with the observations they make, and compara- 

 tive results from the chronometer, and the lunars, taken on board 

 their respective vessels. 



I remain, Gentlemen, yours very truly, 



J. B. Emmett. 



