At the Royal Observatory, Greenwich, there are frequently above 100 chronometers being rated, and there have been as many as 170 at one time. They are rated daily by two observers, the process being as follows. At a certain time every day two assistants in charge repair to the chronometer-room, where is a time-piece set to true time; one winds up each with its own key, and the second follows after some little time and verifies the fact that each is wound. One assistant then looks at each watch in succession, counting the beats of the clock whilst he compares the chronometer by the eye; and in the course of a few seconds he calls out the second shown by the chronometer when the clock is at a whole minute. This number is entered in a book by the other assistant, and so on till all the chronometers are compared. Then the assistants change places, the second comparing and the first writing down. From these daily comparisons the daily rates are deduced, by which the goodness of the watch is determined. The errors are of two classes--that of general bad workmanship, and that of over or under correction for temperature. In the room is an apparatus in which the watch may be continually kept at temperatures exceeding 100° by artificial heat; and outside the window of the room is an iron cage, in which they are subjected to low temperatures. The very great care taken with all chronometers sent to the Royal Observatory, as well as the perfect impartiality of the examination which each receives, afford encouragement to their manufacture, and are of the utmost importance to the safety and perfection of navigation. We have before us now the Report of the Astronomer-Royal on the Rates of Chronometers in the year 1854, in which the following are the successive weekly sums of the daily rates of the first there mentioned: Week ending secs. Jan. 21, loss in the week 2·2 ” 28 ” 4·0 Feb. 4 ” 1·1 ” 11 ” 5·0 ” 18 ” 4·9 ” 25 ” 5·5 Mar. 4 ” 6·0 ” 11 ” 6·0 ” 18 ” 1·5 ” 25 ” 4·5 Apr. 1 ” 4·0 ” 8 ” 1·5 ” 15, gain in the week 0·4 Apr. 22, ” 2·6 ” 29, loss in the week 1·4 May 6 ” 2·1 ” 13 ” 3·0 ” 20 ” 5·1 ” 27 ” 3·3 June 3 ” 2·8 ” 10 ” 1·8 ” 17 ” 2·0 ” 24 ” 3·0 July 1 ” 2·5 ” 8 ” 1·2