1851.] On the Rates of Chronometers, 75 



was materially altered by the ship's local attraction, for which of course 

 due allowance was made in the measurements. Captain Fitzroy after 

 describing the precautions taken to place them near the centre of the 

 ship, and mentioning that the local attraction must always have remain- 

 ed the same, says — p. 320. 



" After the Chronometers had been carefully rated at the Observatory, they 

 were embarked on board H. M. S. Adventure, on the 23rd April, 1826 ; but 

 as the ship was detained at Deptford and Northfleet until the 4th May, an 

 opportunity was offered of ascertaining what change had been produced by the 

 alteration of the place; and it turned out to be no means inconsiderable. Five 

 of the watches had accelerated, and the remaining four had retarded rates. It 

 would be difficult to assign any other reason for this change than the effect of 

 the ship's local attraction." 



So far Captain Fitzroy, but from the sketch chart of the Beagle's 

 voyage prefixed to the volume now quoted, it would appear that leaving 

 England, she first crossed the Magnetic Node on her passage to Bahia 

 and Rio Janeiro. She then increased her variation to 20° or 25° East 

 (which from England was a difference of nearly 50° in the whole 

 though acting in opposite directions) when surveying Terra Del Fuego, 

 and afterwards when approaching Lima reduced it to 5° East ; crossing 

 the Magnetic Equator again. 



From the coast of South America to the Galapagos, and thence to 

 the Society Islands she had not above 5° of variation, and would cross 

 the Magnetic Equator a third time, increasing thence her variation to 

 1 5° at New Zealand ; the line of it being not far from that of the 

 keel, and then rapidly decreasing it, nearly to Zero, at Hobart Town ; 

 and from that port to the Cocos, she would sail in what we may call 

 the great zone of little variation* and then again, like the homeward 

 bound East Indiamen, increase her variation to the Cape, having it, 

 for a time, at right angles with her keel. She then approaches very 

 closely to the magnetic Node in the Atlantic as she proceeds to Rio, 

 and from that port sails back, mostly at about right angles to the 

 magnetic meridian, to the Cape de Verds ; then upon it to the Azores, 



* At the Magnetic Equator in the Eastern Hemisphere there is a zone of at 

 least lOOo of Longitude in which the variation only ranges from 5° West to bo 

 East. 



L 2 



