NO. 6.] INTRODUCTION. LATITUDE AND LOCAL TIME. XI 



Determination of Latitude and Local Time. 



Latitude and local time were always determined by observation of alti- 

 tudes. The altazimuths, especially the larger instrument, were almost inva- 

 riably used for stars, in many cases also for the sun. They were then mounted 

 near the ship on an ice pillar, surmounted by a slab of slate. After levelling, 

 the time of pointing on the star was noted by a watch (generally compared 

 before and after with the standard chronometer) and then the vertical circle 

 and its level read off. With few exceptions every star was observed in the 

 two positions of the instrument, with the object glass to the right and to the 

 left of the observer placed at the end of the horizontal axis. The general 

 rule was to determine the latitude and local time simultaneously by taking 

 two stars in azimuths differing about 90. 



As the zenith point for the vertical circle of the large altazimuth never 

 differed more than some seconds from 0, the circle-reading for a point above 

 the horizon was either between and 90, when the object glass was to 

 the left of the observer, or between 270 and 360, when it was to the right. 

 For the sake of brevity these two cases shall be distinguished by the 

 notation "small numbers" and "great numbers". Supposing the zenith point 

 to be exactly 0' 0", the apparent altitude will be 



Circle-reading 270 0' 0" for great numbers 

 and 90 0' 0" Circle-reading for small numbers, 



when the circle-reading includes the correction for level in the sense right 

 left. When the zenith point differs from 0' 0", the numbers 90 0' 0" and 

 270 0' 0" are subject to the same change. 



For the reduction to true altitude BESSEL'S refraction was used, as given 

 in ALBRECHT'S "Formeln und Hiilfstafeln fur geographische Ortsbestimmungen", 

 with an extension of the temperature table down to 50 C., calculated by 

 Bessel's formula. 



On taking the mean of the two altitudes of the same star, as shown 

 above, the result is free from any error in the assumed zenith point; but as 

 the mean of the altitudes does not always correspond, with sufficient accuracy, 

 to the mean of clock-times, it is necessary to apply a correction on this 

 account, when the observations are treated in this manner. 



