M A G iN E T I C OBSERVATIONS. 



SECTION III. 



ASTRONOMICAL OBSERVATIONS. 



THE observations contained in this section were all made on the sun, and are for 

 the determination of latitude, local time, and true bearings. The instruments used 

 were a prismatic sextant of six inches radius, by Pistor and Martins ; a mercurial 

 artificial horizon ; and a pocket mean time chronometer, by Fletcher, marked 

 number 906. 



The index correction of the sextant was usually obtained by measuring the 

 diameter of the sun, both on and off the arc. For determining the density of the 

 atmosphere thermometers with Fahrenheit scales, and a mercurial barometer 

 graduated to English inches, were employed. 



The refractions have been computed by means of BESSEL'S tables, as given in 

 LOOMIS' "Practical Astronomy;" from which book the tabular parts of the reduc- 

 tions to the meridian have also been taken. The necessary fundamental data 

 have been obtained from the American Nautical Almanac. 



Observations of circummeridian altitudes of the sun for latitude were made in sets 

 of twelve, so arranged as to eliminate both the sun's semi-diameter, and all errors 

 depending on the roof of the artificial horizon. 



Circummeridian Altitudes of the Sun for Latitude, observed at the south front of Fort Christian, 



St. Thomas, November 



1865. 



