♦SS Desa-iptio7i o/" Crow's Seamarks Octant 



in which the angles are given, as they usually are, to every 

 degree. 



(Signed) " E. Riddle, F.R.A.S. 



** Master of the Mathematical School, Greenwich Hospital." 

 "22nd Oct. 1831." 



The arch of the instrument is graduated to 45 degrees, and 

 these are subdivided by a vernier into minutes. The double 

 row of figures to the degrees denotes angles and their comple- 

 ments, in either case ; that is, when the angle is less than 45°, 

 the figures of the lower I'ow become the complement of the 

 given angle, and the contrary — only that the minutes found 

 on the vernier are to be subtracted from 20', and the differ- 

 ence added to the complement will be the degrees and minutes 

 greater than 45°. When the given angle is less than 45°, 

 the index is to be placed over it on the arch, in the usual 

 way. For instance, let the index be placed over 38° 34' on 

 the arch, and it will instantly show its complement, which in 

 this case is 51° 26'; for deducting 14' (which cuts a division 

 on the arch) from 20' on the vernier, and adding the difference 

 6' to 51° 20', gives the complement, as above, 51° 26'*. 



Observe always to count from left to right when the angle 

 is less than 45°, and from right to left when it is more than 45°. 

 The same observation applies to the scale of rhumbs or points, 

 which are immediately below; for if the course be 3 points, it 

 shows at once its complement, 5 points, underneath; and if 5 

 points be the given course, then the diff. of lat. is found on the 

 slide and dep. on the divided limb. Now for the three sides 

 of the triangle : — 



First. The difF. of lat. is on that side upon which the slide 

 traverses; audit is also called the divided limb. Here the 

 difF. of lat. is always to be found or layed off^ if the course is 

 not more than 4 points or 45 degrees. 



Second. The dep. is the slide, and traverses up and down ; 

 upon which the dep. is always to be found or set off, when the 

 course is not more than 4 points or 45 degrees. 



Third. The Distance or Index. On this side of the triangle 

 the dist. is to be found in the angle of intersection with the 

 slide. 



With respect to the diff. of lat. and dep., one general rule 

 must invariably be observed — it is this. That when the given 

 course is more than 4 points or 45 degrees, these sides change 

 names; and so they always do when the difF. of lat. is less than 

 the dep. For if it be a 5-point course, the difF of lat. is then 



• To work and read off by this instrument, it is intended that the heel, 

 or angular point, should be towards the body. 



to 



