24 



The ichnographical plan and position of both the mirrors 

 is represented in fig. u. as they are fixed on the head of the 

 index. 



jB is the great mirror, and b is the cock supporting it, with 

 its case ; D the wing or adjusting lever of the cock ; /and g 

 the screws for erecting it perpendicular to the plane of the 

 instrument in the usual manner; and e e are the steady pins 

 in the index fastening the cock ; d d are two pins on which 

 the edge of the mirror rests. 



A is a round brass plate, with a milled edge of the same 

 size with the head of the index, and with the fig, both being 

 three inches in diameter, and screwed fast to it, concentric 

 with the index, by the four screws s ss s screwed into the in- 

 dex. C is the little mirror screwed to the plate by the screw h 

 passing through the Aving IL of its cock c : it is erected and 

 fastened perpendicular by means of the screws h and i, in the 

 same manner as the mirror B is by the screws y and g: m is a 

 steady pin fastened in the cock c, inserted into a hole in the 

 plate A ', and k another strong steady pin rivetted in the plate, 

 the upper part of which, being cylindrical passes up through 

 a hole in the strong bar or wing E of the cock c, which hole it 

 exactly fills, but allowing the cock to be elevated or depressed 

 a little for adjustment of the mirror, without any angular mo- 



to take altitudes or angles exceeding 45 degrees, to find the latitude, &c., for whicli 

 the back horizon glass must be used. It is also desirable for the interests of science 

 and navigation, that quadrauts of snfRcieot performance should be made capable of 

 being fabricated in different places. 



