Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2- Appendix): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

328 APPENDIX. reasons for preferring to give undivided attention to an unbroken series of chronometrical observations, rather than allot any portion of time to independent astronomical observations ; which, to be really- valuable, required what I could not command, namely — time ; a well- placed and good transit instrument ; sldll in its use ; and habits of obsen'ing, which are neither readily nor easily acquired. Besides which, there is always a degree of uncertainty involving the deduc- tions from observations of any celestial phenomena, at a great dis- tance from well-known obsen'atories ; even when the observer and his means are unexceptionable. The causes of this uncertainty are familiar to many, but, as these pages may meet the eye of a reader who is not aware of them, I will mention that the figure of the earth is not yet quite accurately known, that parallax and refraction cannot be allowed for with absolute certainty, that levels and plumb-lines are not everj'where exactly at right angles to, or coincident with, a line di'awn from them to the earth's centre ; and that tables, how- ever excellent, are not perfect. That able and indefatigable astronomer, Mr. Fallows, was along time at the Cape of Good Hope before he could determine its longi- tude ; and, after all his exertions, his successors have adopted a result differing from it half a mile.* There is reason to doubt whether Paramatta Observatory is well determined in longitude. To fix that of St. Helena, and that of the Mauritius, occupied much time and talent, aided by excellent instruments in well-built observa- tories. A great deal of time and pains, and ability, have been employed at Madras ; yet, as far as chronometers can tell, there is a great discordance between the hitherto published longitudes of Madras, the Mauritius, and Paramatta, when \iewed in connection Avith their respective meridian distances ; such, at least, as have yet been measured. Even on the coast of the Baltic, what differences were found by Lieutenant- General Schubert, in 1833, between the received posi- tions of various observatories, and those which he deduced from the results of fifty-six chronometers ; — placed at his disposal, with a steam-boat, by the Emperor of Russia.f But, to return from this digression : — In the Beagle's measure- • Mr. Fallows considered the longitude of the Cape observatory to be Ih. 13m. 53s. E. Mr. Henderson Ih. 13m. 55s. E. t Journal of the Royal Geographical Society. Vol. VI. Part II. ] 836, pp. 413-6.