VOL. XXIV.l PHILOSOPHICAL TRANSACTIONS. 131 



the whole instrument. And that you may more easily see the pole-star through 

 the sight, let the plumb-line be a very fine cat-gut string, or horse hair, &g. 

 And if it be white, or some such light colour, it will be the better seen, with 

 the help of a candle shining on it by night ; which is necessary. 



The sight cd, and opposite plumb-line being thus set in a direct line with the 

 pole-star on the meridian, it is manifest, that the instrnment lies exactly in the 

 meridian, so as to see any star on the meridian to the north. And that you 

 may see the same southerly ; the next day, or when you please, you may hang 

 up the plumb-line ef, on the southern arm cd, so as that the plumb-line may 

 exactly intersect the perforation cd. This may be easily done by moving the top 

 joint, with the plumb-line on its cross-piece, backward and forward, till the 

 plumb-line hangs to your mind. If the sight, with the lesser perforation ab, 

 be not exactly under the northern plumb-line, it must be brought to be so by 

 turning the sight, by help of its joint at i. And then all the instrument is set 

 right, so as to see the sun, moon, or stars come on the meridian towards the 

 south. 



But to see the sun transit the meridian, it is necessary to guard the eye with 

 a coloured glass, or a glass darkened with the smoke of a lamp or candle ; 

 which is done in the following manner: Chuse two pieces of glass, cut into 

 the same size and figure; taking care that they do not refract vitiously ; which 

 may be known by moving the glass before the eye. If the objects you look on 

 seem to dance about, the glasses are false and refract ; but true, if all seems 

 steady. Smoke one of these glasses over the flame of a lamp or candle, till it 

 be obscured enough to take off the sun-rays, but not so as to darken it too 

 much : which may be seen by looking at the sun or candle with it. One of the 

 glasses being thus darkened, lodge them both together, and fasten them in a 

 little case fit for the purpose, with the smoked side innermost, and an edging 

 of card between to keep them asunder, so as that the soot may not be rubbed 

 off or disordered. It is is proper to have two glasses thus prepared ; one for a 

 strong sun ; the other less darkened, for the sun behind a thin cloud, mist, 

 &c. With one of these glasses, held behind or before the sight ab, you may 

 plainly view the sun pass. 



To imitate the aforesaid Instrument on a Journey y or wherever you come. — 

 Instead of an entire instrument, only prepare two sights, as in fig. 22, with 

 perforations as before. Let these sights be nailed or screwed down on the tops 

 of two stakes at ii, so as to turn stifl[ly upon them. The plumb-lines, at least 

 one of them, may be hung up at the end of a house, as at k, or on the bough 

 of a tree, if the wind do not shake it, or any where you see fit. And the 

 sights must be stuck up, so as to bring the pole-star to intersect, and all be 



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