PREFACE. 



I SHALL preface this manuscript, by giving an account of 

 the instruments with which the expedition was tarnished, for 

 making observations. 



1st. A bell-metal sextant of seven inches radius, divided 

 by the assistance of the vernier to 15" on a silver arch, ex- 

 ecuted by Spencer, Browning and Rust, London. 



2d. Another sextant of brass, five inches radius, divided 

 on a silver arch by the assistance of the vernier to 30" ; made 

 by Gary, London. It is a convenient and portable instru- 

 ment: the handle, the supporter of the telescope, and the 

 compound microscope for reading off angles, may be de- 

 tached readily, and packed in the case containing the instru- 

 ment when travelling. 



3d. A circle of reflection, of brass, divided on a brass 

 limb by the assistance of the vernier to 30", which may be 

 subdivided by the eye to the nearest 15". The graduated 

 limb is twelve inches in diameter, made by Spencer, Brown- 

 ing and Rust, London. It is not constructed with the greatest 

 nicety, and the larger sextant was preferred to it for Lunar 

 distances. 



4th. An artificial horizon of mercury with a glass roof, the 

 surfaces of which were ground parallel, and then polished 

 smooth, to prevent errors from an irregular refraction. The 

 mercury is carried in a wooden box ; and the cup for holding 

 it, with the roof, are secured in a box together, when tra- 

 velling. 



5th. Another smaller roof which I had made at Engineer 

 Cantonment, after the same plan, out of two speculum 

 glasses for a sextant, which were provided in Philadelphia. 

 I made use of this roof with a mercurial horizon, for observ- 

 ing the double altitudes of the sun and stars after I separated 

 from Major Long at Engineer Cantonment, and found that 

 by being cautious to bring the direct and reflected images of 

 the object whose double altitude was observed, in contact, in 

 the middle of the cup of mercury, it answered as well as the 

 larger horizon and roof. 



