62 



ASCERTAINING THE DIP AT SEA, 



XII. 



On the befi Method of afcertaining the Dip at Sea, 

 from Mr. Ezekiel Walker. 



In a Letted 



SIR, 



To Mr. NICHOLSON, 



The dip of the 

 horizon render- 

 ed very uncertain 

 by refraction- 



The back ob- 

 fervaticoi will 

 not correftiy 

 ihew the dip. 



It Is beft found 

 by taking the 

 fun's altitude 

 and its op polite 

 fupplement by 

 two perfons } 



or by one perfon 

 with two inftru- 

 znencs. 



IT Is well known that refradlion varies fo much near th6 

 horizon as to render the depreffion or dip very uncertain, 

 whence it feems delirable, in the prefent improved flate of 

 navigation, to find the dip by obfervation rather than truft to 

 the table. An inlkument fo con(iru6tcd as to meafare an 

 angle of ISC would be very convenient for this purpofe, 

 and the back obfervation in Hadley's quadrant has been piro- 

 pofed ; but the back obfervation is attended with fo many in- 

 conveniences to the obferver, and fo little to be depended 

 upon, as to render it nearly ufelefs. The iniirument which I 

 have propofed for taking any angle lefs than 180"^ feems welf 

 adapted to this purpofe *. The diftance of the two oppofite 

 horizons at fea is = 180° -{- twice the dip; but the motion of 

 the fnip renders this method very troublefome and uncertain j 

 for whatever uncertainty attends taking an altitude of a celef- 

 tial obje6t, that uncertainty is doubled in taking the diftance of 

 the two oppofite horizons. To avoid this inconvenience, 

 let the meridian altitude of the fun's upper or lower limb 

 be taken with a fextant by one obferver, and at the fame 

 time, let another obferver, with an inftrument fo conflruded 

 as to meafure an angle of 180*^ take the diftance of the fame 

 limb of the fun, from the oppofite horizon ; the fum of thefe 

 two angles— 180° is = twice the dip. And as two obferva- 

 tionsof the fun's altitude are thus taken independently of each 

 other, the latitude will be obtained with greater precifion than 

 by a fingle one. 



The obfervations may alfo be taken with two inftruments 

 by one perfon. 



Let4he obferver begin fome time before noon, and take the 

 altitude of the fun's limb. 2. With the other inftrument take 

 the diftance of the fame limb from the oppofite point of the 



* See Philofophical Journal, vol. iv. p. 218. and vol, vi*. p* 219«. 



horizon^ 



