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

Al^PENDIX. 319 One-day box chronometer, No. 3290 designated E One-day 3291 fI in One-day 3292 G [ one box. One-day 3293 H, Z had been going at the Observatory for many months, and had preser\'ed a very regular rate, but all the others Avere quite new, and had scarcely settled to a steady rate when I received them. In addition to the above I was furnished with a pocket chrono- meter. No. 553, by Mr. Murray. This watch had been at the Ob- servatory for several months, and had performed remarkably well : and before I sailed Messrs. Parkinson andFrodsham intrusted to my care for trial a pocket chronometer. No. 1048, that was only completed in time to be sent to me two days before the Expedition sailed from Ply- mouth. Mr. French also lent me a pocket-watch to use for observ- ing with, in order that the rest might not be unnecessarily moved. In the Beagle were three excellent box chronometers. Two by Messrs. Parldnson and Frodsliam, Nos. 254 and 228, which had been used in the Polar Voyages ; and the third, No. 134, made by Mr. M'Cabe. The means, therefore, that were placed at my command to effect this most interesting object were tolerably ample : and the result will prove how admirably these machines are adapted to measure such differences when a great number are emjiloyed ; because the irregularities and errors of individual watches are compensated for by employing the mean of the whole. In the observations for the determination of time, a sextant by Troughton, No. 1140, and an artificial horizon, were the instruments used : and the mode, whenever it could be adopted, was that of cor- responding altitudes. Occasionally, however, absolute altitudes were used, but only in those places where the latitude was correctly ascertained ; — and in some instances the chronometers were rated by a transit instrument. The chronometers were always compared with the 'journeyman' watch before and after the observations, and when corresponding al- titudes were observed all the watches were compared at noon. Their rates were carefully observed before sailing from one port, as well as after the arrival at another ; and in calculating the acceleration or re- tardation of their rate of going, the correction was obtained by inter- polation, upon the supposition of their having changed gradually. Whenever it appeared, by comparing the watches with each other.