232 



Things not generally Known. 



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 tem- 

 peratures 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 chro- 

 nometers 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 sees. 



Jan. 21, loss in the week 2-2 

 40 



Feb. 4 

 11 

 18 

 25 



Mar. 4 

 ., 11 

 ., 18 

 .. 25 



Apr. 1 

 it 8 



1-1 

 50 

 49 

 55 

 60 

 60 

 1-5 

 4-5 

 4-0 

 1-5 



Week ending sees. 



Apr. 22, gain in the week 26 

 29, loss in the week 1-4 

 May 6 , 2-1 



18 



20 

 27 



June 3 

 10 

 17 

 24 



July 1 

 8 



3-0 

 51 

 3-3 

 2-8 

 1-8 

 2-0 

 3-0 

 2-5 

 1-2 



15, gain in the week 4 



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 cele- 

 brated watch was tried at the Royal Observatory, more than 

 ninety years since, may be judged by comparing its rates with 

 those above. 



GEOMETEY OF SHELLS. 



There is a mechanical uniformity observable in the descrip- 

 tion 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 



