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

326 APPENDIX. rose ratlier above the centre of gravity of the box and watch ; so that they could not be displaced unless the ship were upset. The shelves, on which the sawdust and boxes were thus secured, were between decks, low down, and as near the vessel's centre of motion as could be contrived. Placed in this manner, neither the running of men upon deck, nor firing guns,* nor the running out of chain-cables, caused the slightest vibration in the chronometers, as I often proved by scattering powder upon their glasses and watching it with a mao-nifying glass, while the vessel herself was %'ibrating to some jar or shock. All the watches were in one small cabin, into which no person entered, except to compare or wind them, and in which nothing else was kept. The greater number were never moved fi-om their first places, after being secured there in 1831, until finally landed at Greenwich in 1836. Durino- eight years' observation of the movements of chronometers, I have become gradually convinced that the ordinary motions of a ship, such as pitching and rollmg moderately, do not affect tolerably good timekeepers, which are fixed in one place, and defended from vibration as well as concussion. Frequently employing chronometers in boats, and in very small vessels, has strengthened my conviction that temperature is the chief, if not the only cause (generally speak- ing) of marked changes of rate. ITie balances of but few watches are so well compensated as to be proof against a long continuance of higher or lower temperature. It often happens that the air in port, or near the land, is at a temperature very different from that over the open sea — in the vicinity ; and hence the difference sometimes found between harbour and sea rates. The changes so frequently noticed to take place in the rates of chronometers moved from the shore to the ship, and the reverse, are well known to be caused partly by change of temperature and partly by change of situation.! In the Beagle we never found the watches go better than when their boxes were bedded in saw-dust, and they themselves were moving freely in good gimbals. Suspending chronometers, as on board the Chanticleer, not only alters their rate, but makes them go less regularly ; and when fixed » The Beagle's guns were long six and long nine pounders, of brass : tbey were only fired from the foremost ports, t This may be connected with magnetism.