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XVIII. De<!criplion of a practical Diagram for oltain'mg in a 

 simple a?id ea^y Manner the Correction of ike Luvar Distances 

 as ol'served by a Sextant. By L/ew^ James Bulkeley, of 

 the Royal Navy. 



To Mr. Tilloch. 



Sir, — 1 TAKE the opportunity of sending you a description of 

 a practical diagram, for obtaining in a simple and easy mauler 

 tlie correction of tlie lunar distances as observed by a sexiant, 

 which is submitted to your approbation and opinion for publica- 

 tion. I am, sir, 



Your obedient servant, 



Huntlev-Hall, Cliea-lle, Staffbrdshiic, JaMES BuLKELEY. 



June G, 1814. 



Description. [Plate II. fig. 1.] 

 AB, a semicircle divided into 180 degrees or equal parts; 

 AC, the semidiameter graduated as a line of sines ; CD, a 

 flat bar moveable round the centre of the instrument, and ending 

 with a nonius or pointer to the semicircle, and divided like the 

 .•semidiameter as a line of sines ; EF, a bar sliding at ri^;Iit angles 

 to the semidiameter, and on a pivot, to enable it to fall either to 

 tlie right or left of the semidiameter, and gra,duat-ed as a line of 

 chords terminating on the semidiameter v.-ifh a nonius. HG, a 

 small bar moving at riglit angles to the moveable radius CD, 

 and cutting witli a silk thread the pcrpencHcular bar EF, as at I. 



Ale f hod of using the Instrument. 

 Suppose the appare/it distance to be 104'^ 30'; the sun's ap- 

 pnrent altitude 43° 20'; and the moon's apparent altitude 

 12 ■ .30' ; horizontal parallax 57' 49": to find the true distance. 

 With the moveable radius CD set off the ajjpar^nt distan(.'e on 

 the graduated semicircle AB 104^ 30' as per figure. Then ^vith 

 the perpendicular EF set off on the semidiameter AC, vv'hich is 

 divided as a line of sines, tlie moon's apparent altitude 12^ 30', 

 as per figure ; and on the radius CD with the sliding perpendi- 

 cular CH set off the sun's apparent altitude 43° 20', as per 

 figure: then will the silk thread on GH cut on the perpendicular 

 EF, (whicli is divided as a line of chords, but the <!egrees reck- 

 oned as minutes, and the minutes as seconds) a number, as 44' OO'' 

 at I per figure ; which if multiplied by the horizontal parallax, 

 and (as the perpendiculars cit to the right of tlic semidiameter) 

 divided by 62, will give a Cfn-rection to be subtracted from the 

 a|)parent distance, to find the true, as 44' OO" x 57' 49" hori- 

 zontal parallax; and -t-G2 = 42' 26"— from apparent distance 



104° 



