After Mr. Harrison’s watch was tried, some watches by Earnshaw, Mudge, and others, were rated and examined by the Astronomer-Royal.
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 |
Till February 4 the watch was exposed to the external air outside a north window; from February 5 to March 4 it was placed in the chamber of a stove heated by gas to a moderate temperature; and from April 29 to May 20 it was placed in the chamber when heated to a high temperature.
The advance in making chronometers since Harrison’s celebrated watch was tried at the Royal Observatory, more than ninety years since, may be judged by comparing its rates with those above.
GEOMETRY OF SHELLS.
There is a mechanical uniformity observable in the description of shells of the same species which at once suggests the probability that the generating figure of each increases, and that the spiral chamber of each expands itself, according to some simple geometrical law common to all. To the determination of this law the operculum lends itself, in certain classes of shells, with remarkable facility. Continually enlarged by the animal, as the construction of its shell advances so as to fill up its mouth, the operculum measures the progressive widening of the spiral chamber by the progressive stages of its growth.
* * * * *
The animal, as he advances in the construction of his shell, increases continually his operculum, so as to adjust it to his mouth. He increases it, however, not by additions made at the same time all round its margin, but by additions made only on one side of it at once. One edge of the operculum thus remains unaltered as it is advanced into each new position, and placed in a newly-formed section of the chamber similar to the last but greater than it.