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

86 MEMORANDUM. " The local attraction of the Beagle will of course have been ascertained before she leaves England ; but when favour- able opportunities occur, it will be satisfactory to swing her again in different latitudes, and under large differences of variation. " No day should pass at sea without a series of azimuths, and no port should be quitted without having ascertained not only the magnetic angle, but the dip, intensity, and diurnal variation. If these observations should have been well made in the same places before, we shall at once obtain the annual change ; and by multiplying them in new places, we shall have the means of inferring the magnetic curves. " The Commander has been so accustomed to the manage- ment of chronometers, that there is no doubt, with proper pre- cautions and with proper formulaj for determining their rates, that he will succeed in obtaining good results in reasonably short intervals of time and in gradual changes of temperature ; but after long periods, and sudden changes of heat and cold, it will be absolutely necessary to check them by astronomical means. " Eclipses, occultations, lunar distances, and moon-cul- minating stars, will furnish those means in abundance : of all these, the last can be obtained with the greatest regularity and certainty ; they have become part of the current business at the establishments of the Cape of Good Hope, Paramatta, and St. Helena, in the southern hemisphere ; probably at Madras, and in many of the European observatories, and it will there- fore be scarcely possible that there should not be corresponding observations for all such as he may have made. " The eclipses of Jupiter's third and fourth satellites should also be sedulously observed whenever both immersion and emersion can be seen, as the different powers of the telescopes employed by the observers do not in that case affect the results. " There are also some remarkable phenomena, which will be announced in the Nautical Almanacks, and which will occur during the Beagle's voyage. Some of these will be highly