74 On the Rates of Chronometers. [No. 1. 



ing Voyage of H. M. S. Adventure and Beagle, p. 345, Captain 

 Fitzroy says — speaking of his chain of Chronometric measurements 

 round the globe (the italics are mine) that — 



" It ought to be clearly stated, however, that the sum of all the parts which 

 form the chain amounts to more than twenty-four hours, therefore error must 

 exist somewhere ; but what has principally caused the error, or where it may 

 be said to exist, I am unable to determine. The whole chain exceeds twenty- 

 four hours, in about thirty-three seconds of time." 



" It appears very singular, that the more the various links of this chain are 

 examined and compared with other authorities, the more reason there seems to 

 be for believing them correct, at least to within a very small fraction of time ; 

 and even allowing that each link were one or two seconds of time wrong, it 

 does not appear probable that all the errors would lie in one direction, unless 

 some hitherto undetected cause affects Chronometers when carried Westward, 

 which might affect them differently when carried Eastward." 



" It would ill become me to speak of any value which may be attached to 

 these Chronometrical measures; even erroneous as they undoubtedly are in 

 some part, if not to a certain degree almost every where. I can only lay the 

 honestly obtained results before persons who are interested in such matters, and 

 request that they may be compared with those of the best authorities." 

 ****** 



" The only idea I can dwell on, with respect to the cause of this error of 

 thirty-three seconds, is, that Chronometers may be affected by magnetic action 

 in consequence of a ship's head being for a considerable time towards the East 

 or West : yet this is but a conjecture. In the measures between Bahia and 

 Rio de Janeiro, and in those between Rio de Janeiro and Cape Horn, there is 

 no evidence of any permanent cause of error ; but the greater part of those 

 measurements were made with the ship's head usually near the meridian." 



As to the Chronometers of H. M. S. generally, and those of the 

 Adventure and Beagle in this instance, we know that they are rated at 

 the Observatory at Greenwich, where every precaution is of course taken, 

 and where they are free from the influence of any of the London masses 

 of iron. It farther appears that the rate of the Beagle's Chronometers 



or fine weather strandings ; not those through sheer stress of weather. And I do 

 not forget that they are the only two landfalls, excepting St. Helena and Ascension, 

 on the voyage. As an opposite extreme we may take the Western entrance to Bass' 

 Straits which is crossed by the line of no variation, so that here the Terrestrial 

 Magnetism (apart from the dip) has no influence. But the error of the watches 

 was accumulating from off the Cape. 



