'' APPENDIX. 327 



to a solid substance, as on board the Adventure, they feel the vi- 

 brations caused by people running on the decks, by shocks, or by 

 a chain cable running out. Cushions, hair, wool, or any such sub- 

 stance, is preferable to a solid bed ; but, perhaps, there is nothing' 

 better than coarse dry savs^-dust. 



Some chronometrical measurements have erred, and caused much 

 perplexity, in the following manner. The chronometers were rated 

 in air whose average temperature was — let us suppose, for example, 

 '70. They were then carried through air either considerably hotter, 

 or considerably colder, and again rated in a temperature nearly 

 equal to that specified. The rates were not found to differ much, 

 and it was supposed that the chronometers had been going extremely 

 well ; though, in truth, the rates of most of the watches had differed 

 extremely (from those found in port) during the voyage ; but they 

 had returned nearly to the old rates upon reaching nearly equal tem- 

 perature. And this has happened, more or less, to every ship carry- 

 ing chronometers across the Equator ; especially when going to Rio 

 de Janeiro with the sun to the northward of the Line. 



How far, or in what manner, magnetism, or electrical influence, 

 may affect chronometers, is hitherto unknown : but there is sufficient 

 reason for suspecting considerable effects, under certain conditions, 

 from one or both of these causes. 



The Beagle's chronometers were all wound daily, at nine (except 

 the eight-day watches, which were wound every Sunday morning), 

 and compared at noon. Whatever other comparisons might be made, 

 for equal or corresponding altitudes, sights for time, &c., the noon 

 comparison was regularly made and forthwith examined, in order 

 that any change might be at once detected. Whether at sea, or in har- 

 bour, this same method was punctually and accurately executed by 

 one person only, under the inspection of Mr. Stokes and myself. 

 This person, Mr. G. J. Stebbing, of Portsmouth — who was en- 

 gaged for the purpose, as well as to keep our instruments in repair, 

 take care of our collection of books,* assist in magnetic, and other 

 observations, and write for me — was of invaluable assistance ; and, I 

 may well say, contributed largely to whatever was obtained by the 

 Beagle's voyage. 



In Images 74 and 75 of the second volume, I have mentioned a few 



* Our books, which were not a few, considering the small size of the 

 vessel, were collected in one cabin, under Mr. Stebbing's charge, and lent 

 to the officers, without reserve, under certain regulations. 



