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 



