NO. 6.] INTRODUCTION. CHRONOMETERS. XXIII 



azimuth. For the determination of the apparent altitudes the refraction corres- 

 ponding to the meteorological conditions was of course used; while an error 

 affecting the true and the apparent altitude alike has only an insensible effect 

 on the calculation of the true distance (being multiplied by the Sine of the 

 difference between true and apparent altitude) an error in the refraction or 

 parallax would affect the true distance by a quantity of the same order as 

 the error itself. 



The results are not satisfactory. In some cases when the observations 

 have been taken with intervals of a few days or weeks, the chronometers are 

 unanimous in protesting against the deduced Greenwich times. As the tempe- 

 rature during these observations was only once (1896 April 22) as high as 

 16 C., and on all the other occasions between 27 and 43, it is prob- 

 able that the sextant was affected with errors that would not have been of 

 great importance for ordinary altitudes of the Sun, but which proved fatal 

 to the delicate operation of determining the longitude by Lunar Distances. It 

 is also to be remarked that the index error was not determined on each occa- 

 sion but for some time considered as constant, because a determination in 

 August 1893 in the Barents Sea and another off the mouth of Lena shortly 

 before the enclosure in the ice had given identical results. 



It would of course have been better to use the altazimuth for determining 

 the difference of azimuth between the Moon and a star or the Sun, and thence 

 deduce the Moon's right ascension. But as it happened that the planet Jupiter 

 was circumpolar during all the 3 years of enclosure in the ice and so was 

 always at hand when the Sun was absent, it was found to be a much more 

 ready means of getting an approximate longitude to observe the eclipses of 

 Jupiter's Satellites and compare with the predicted times in the Nautical Al- 

 manac. 



The results of the Lunar Distances are included in a table below (Tab. c) 

 containing the results of these Eclipses. 



Eclipses of Jupiter's Satellites. 



The observed moment of commencement or end of an eclipse of a Satel- 

 lite is dependent on many circumstances, the aperture of the telescope being 

 perhaps the most important. As the predicted times are sometimes seriously 



