364 APPENDIX. 



the chronometer (being the opposite of that of the earth, and 

 nearly equivalent in intensity), would be to neutralise the ef- 

 fect of the magnetism of the earth on the chronometer. 



The apparatus I propose for giving uniformity of position 

 to chronometers at sea, so as to obviate the varying action of 

 the magnetism of the earth on magnetic balances in different 

 positions, is represented in Plate XXIII. , where 



A, Is a compass-card (without a needle), having a light slip 

 of brass across the under side, and an agate cap in the 

 centre, for carrying the chronometer B. 



C, A centre of brass pointed with steel, of a needle temper, 



upon which the card and chronometer traverse. 



D, A magnetic rhomboidal compass-needle, on Captain Ra- 



ter's construction, adopted in consequence of its great 

 directive force. This needle is suspended from the 

 card by 4 or 6 small wires (a, a), which serve to keep 

 the card horizontal, and cause it to traverse. 



EE, A cylinder of thin copper or brass, to the bottom of 

 which is fixed the centre C, which passes through the 

 opening in the middle of the rhomboidal needle. 



FF, The centres by which the apparatus is attached to 

 gimbles in the usual form. 



The whole is enclosed in a square wooden box, with both a 

 glass and a wooden lid. 



The chronometer being placed in the centre of the card, 

 and adjusted with its XII o'clock mark always in one direction, 

 will evidently maintain the same position so long as the card 

 continues to traverse. 



XXVI. 



