434 Description of Crow's Seaman's Octant. 



in light airs and calms, any distance is readily set off, and the 

 corresponding results expeditiously obtained. Now to suit 

 our convenience, we may assume the first 10 on either of the 

 sides, on the common scale, as 1 or unity, and 20 above as 2, 

 30 as 3, &c., then will the first division which is cut up to the 

 second line be y\j, and the longer division -^-q, &c. &c. In like 

 manner may the first 1 on the large scale be assumed as 10, 



2 as 20, 3 as 30, &c. ; then will all the longer divisions from 

 1 to 2 be each 1, or unity, and the subdivisions between these 

 will each become yL. Also may the 1 on the large scale stand 

 for 100, 2 for 200, &c, ; the subdivisions then are 1 each. 

 Now admit the distance run to be 6 miles on a S.W. by S. 

 or 3-point course — place the chamfered edge of the index to 



3 on the arch, then bring the slide to cut 60, which now stands 

 for 6 miles, the dist. run, and it is done. On the limb is 5' 

 for difF. of lat., and on the slide is 3'-3, or a little more, for 

 the dep. 



A variety of examples are given of the use of the instru- 

 ment, of which we shall extract one. 



Tojind the Difference of Longitude by the Octant. 



To find the difF. of long, with the dep. and comp. mid. lat., 

 the proportion is, — As the co-sine mid. lat. is to dep. in miles, 

 so is radius or 90° to the difF. of long, in miles. 



Example I. — Required the difF. of long, between Dover 

 and Ostend. The dep. is 60 miles, and the comp. mid. lat. 

 38° 45'. 



Rule. — Turn the index over the comp. mid. lat. on the 

 arch 38° 45', and shift the dep. side with 60' thereon, to cut 

 the edge of the index, and it is done. Where the division 60 

 intersects, is 96 miles nearly, for the difF. of long, required on 

 the index. 



The long, of Ostend is 2° 55' E. 



Theiong. of Dover Castle 1 19 E. 



Diff. of long, as above 96' = 1 36 



Example II. — What is the diff. of long, between An- 

 tigua and St. Thomas, in the West Indies; the dep. being 

 177 miles. 



Lat. of Antigua, E. point 17° 06' N. Diff. oflat. 2) 1° 12' 1/° 6' 



Lat. of St. Thomas, E. point 18 18 N. Add. i Diff. of lat.... 36 



Is mid. lat 17 42 



And departure 177 miles. 



Rule. — Place the index over the mid. lat. 17° 42' on the 



arch, shift the slide to 177 miles on the limb, and it is done. 



On 



