1826.] Jinding the Longitude at Sea, 25 



results ; but since the computation is tedious, and since some 

 skill in observing, as well as in the use of nautical tables, is 

 required, it is rarely adopted by the masters of merchant vessels: 

 indeed the merchant service is left without any ready means of 

 determining the longitude ; it is deduced from the reckoning 

 which depends upon the log only, and the longitude is erroneous 

 frequently as much as 2°, 3°, or even 4° ; and doubtless these 

 errors occasion the loss of many lives and much property. 



It is to be regretted that the owners of ships do not pay more 

 attention to the instruments which are to guide their property, 

 and many of their fellow creatures, during long and hazardous 

 voyages ; frequently the only instrument carried out, besides 

 the compasses, is an octant, with plain sights ; these are usually 

 BO carelessly fitted up by country workmen, that I have detected 

 a very considerable index error, which was not noticed, nor 

 known to exist, either by the workman or the master of the ship. 

 So long as such instruments are employed, neither can the lunar 

 method nor the chronometer be of any use ; frequently these 

 octants are fitted up without a tangent screw ; and of these, in 

 very common use, there are very many on which reliance cannot 

 be placed nearer than two or three minutes. In the adjustment 

 either in estimating or removing the index error, I have seen the 

 workman observe any object, as a chimney corner, not 50 yards 

 off; and with the great error which so crude an observation 

 introduces, the instrument has been taken to sea as perfect. 



The object of the present paper is to propose a more accurate 

 measure of a lunar distance than can be taken with any sextant ; 

 to register the corrections of every possible distance, and under 

 all circumstances, in tables, whereby all the labour of computa- 

 tion will be taken out of the hands of the master of a merchant 

 ship, and the longitude will be found certainly within 30' of a 

 great circle ; while the more scientific mariner will obtain more 

 accurate results by directly working out every observation. 



The lunar method is never employed in the merchant service, 

 because the computation is long and tedious, and the masters of 

 these vessels are seldom able to apply logarithmic tables : who- 

 ever will take the trouble to work out an example will find it 

 occupy more time than the masters of ships in general can 

 spare, and besides they are seldom possessed of sufficient 

 knowledge. The sextant is so imperfect an instrument, that 

 there is room for very considerable improvement; it is certainly 

 a most convenient and valuable nautical instrument; but when 

 the sun or moon is viewed, the image is fringed, on account of 

 the passage of the light obliquely through the mirrors, whence 

 tw o equally good observers will take different measures of the same 

 angle. The radius of the instrument is too small to admit of 

 any great accuracy; for on account of the principle of the instru- 

 ment, it is equivalent to one which does not depend on tl>e 



