107 
Mr Trougliton’s Nautical Top. 
which cannot be effected but by good workmanship. His next 
experiments were directed to give the instrument a better form ; 
but, after trying different forms, to the amount of eight, he 
found himself completely disappointed ; for, among that num- 
ber, there was not one which performed better than that first 
described. In fact, they were very nearly all alike. 
One of Mr Troughton's Tops was sent out with each of the 
four ships that went last year to the Arctic Regions, and, though 
they did not give such satisfactory results as were expected, yet 
he is perfectly satisfied with the reports of the able officers who 
commanded them. He was never, himself, sanguine of success; 
but he wished, if possible, to give to the mariner a standard in- 
strument ; well aware that, to do this, it must command a de- 
gree of accuracy very nearly, if not quite, equal to that of the 
natural horizon. 
The Nautical Top consists of two separate parts ; the Top 
which is to be spun, and a piece of machinery from which it 
is to receive its rotatory motion. 
The form which Mr Troughton first gave to the top, was 
that of a hollow cylinder of brass, open at the bottom, and ter- 
minated above by a circle of dark glass. The inner diameter of 
the cylinder was inches, the outer diameter 4/^ inches, 
its height It inch, and the diameter of the reflecting glass 4| 
inches. 
Mr Troughton afterwards surrounded the cylinder with a 
solid brass ring, fastened to it by four projecting arms. Ihe 
upper surface of the ring was on a level with the circle of black 
glass which formed the surface of the top ; and the inner curv- 
ed surface of the ring was concentric with the outer curved 
surface of the top. 
The improvement which Mr Troughton has made upon the 
top since it went out with the Arctic Expeditions, consists in 
giving it the form of an inverted frustum of a cone. The base 
or lower surface of the frustum is about 6 inches in diameter, 
the upper surface about 4 inches, and its height about inches. 
The thickness of the metal which forms the cone is | of an 
inch. The reflecting plane which occupies the whole upper 
surface of the conical frustum, rests in a steel cup half an inch 
