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Art. XIX . — Description of the Nautical Top^ as constructed 
hy Edward Troughton F. E. and Member of the Aiiie- 
rican Philosophical Society. 
In taking the altitudes of the sun and other celestial bodies at 
Sea^ it is always necessary to perceive the apparent horizon. 
When the horizon is obscured, and when the vessel has httle or 
no motion, the observations may be made vdth an artificial Ao- 
rizon^ consisting either of mercury or tar, placed in a shallow 
vessel, and protected by a glass cover from partaking of the agi- 
tations of the external air. If the ship, however, has the 
least motion, these artificial horizons are of no use, and it be- 
comes impracticable to make the usual observations for the 
latitude. 
Soon after the value of Hadley’s Quadrant became known, 
every one was anxious to have attached to it an apparatus for 
taking altitudes, when the natural horizon could not be seen ; 
and all the varieties of pendulums, plumb-hnes, and levels, were 
tried and rejected. Perhaps, among these Warious contrivan- 
ces, there were fev/ that, under particular circumstances, might not 
have been useful ; yet their discrepancies were such, that any 
thing like a standard instrument never appeared, and perhaps 
none of them answered the purpose better than Astrolabes had 
done before. The class of instruments above alluded to have 
never disappeared ; they have continually been brought forward 
under patents and new forms, but, to a very late date, they have 
succeeded no better than those that preceded them. 
Mr Serson, who was lost on board of his Majesty’s ship the 
Victory about the middle of the last century, had observed^’ 
that, Avlien a top was spun, its upper surface directed itself in 
the course of two minutes after it was set up, in a true horizon- 
tal plane ; — that this plane was not at all disturbed by any mo- 
See Phlloso^Idcai TransarMons 1751, vol. xlvji. p. 352. 
