52 0)1 the Use of the Pocket Box-Sextant. 



any trouble. I shall now subjoin a few observations made with 

 the pocket-sextant, to shew how admirably it is adapted to the 

 variety of purposes to which a traveller may wish to apply it. 



Upon this very small and consequently portable instrument, a 

 much greater reliance may be placed than at first sight appears 

 probable ; or than many persons who have not tried its powers 

 would be willing to believe. 



Its construction secures it from injury, the adjustments of the 

 mirrors being contained in a small box, as also the rackwork by 

 which the index is moved ; so that nothing remains upon the 

 upper plate but the divided limb, the milled head of the rackwork, 

 and a lens to read the divisions. 



The same exterior box which preserves this work upon the 

 upper plate from injury when not in use, serves the purpose of a 

 handle when used. 



Further description is unnecessary, as it is to be seen in the 

 windows of most opticians, who do not make it a very expensive 

 instrument. 



When it is considered that a traveller may have much use for 

 an angular instrument, and that one varying from 2^ to 5 guineas 

 in price is to be had, possessing great accuracy, which he can 

 carry without incumbrance in a waistcoat-pocket, so that he needs 

 scarcely ever be without it, particularly in situations where angular 

 operations are likely to be necessary ; it is presumed that a few 

 observations upon its utility will need no apology. 



It has been proved by experiment, that horizontal distances 

 exceeding 18,000 or 20,000 feet can be ascertained by it within 

 2 or 3 feet of the result given by more expensive instruments ; 

 but in this case great care must be taken, and the distances calcu- 

 lated, whereas in common cases construction of the triangles is 

 generally considered sufficiently correct. The base was 5786 

 feet in length, and the ultimate lengths deduced from it through 

 several triangles, were at a distance of about four miles from the 

 base. Now we may remark, that when two objects are in a plane 

 very oblique to the horizon, a method is easily practised, by which 

 the horizontal angles can be approximated very nearly, and thus 



