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

'' 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.