Natural PMlosophy^ — Optics. 411 
dianism is preserved from tarnishing, by its not being exposed 
to the external air. 
3d, Its motions are conveyed through slender tubes, and hol- 
low pillars and arms, in such a manner as to conceal from view 
the clumsy appendages of large wheels, and almost every other 
part of the apparatus, except the heavenly bodies themselves 
moving in order round their respective centres. 
Mil, The diurnal revolutions and those of the satellites can 
be discontinued at pleasure, in order to exhibit a more rapid 
circulation of the planets in the ecliptic in their true proportion- 
al periods, whereby even the slow motion of the Georgium Sidus 
is rendered visible to the eye. 
Bill, When such exhibitions are not required to be made, the 
whole may be kept in continual motion by a small time-piece, 
and thus the orrery becomes a perpetual ephemeris, represent- 
ing on inspection the true positions of the planets, the phenome- 
na of their seasons, occasioned by the inclination and parallelism 
of their axes, so far as known, the lunar phases, eclipses, &c. 
OPTICS. 
3. Curious instance of unusual Refraction . — When Captain 
Colby was ranging over the coast of Caithness, with the tele- 
scope of the great Theodolite, on the 21st June, at eight o’clock 
P. M., from Corryhabbie Hill, near Mortlich, in Banffshire, he 
observed a brig over the land, sailing to the westward, in the 
Pentland Frith, between the Dunnet and Duncansby Heads. 
Having satisfied himself as to the fact, he requested his assist- 
ants, Lieutenants Robe and Dawson of the Royal Engineers, to 
look through the telescope, which they immediately did, and ob- 
served the brig likewise. It was very distinctly visible for seve- 
ral minutes, whilst the party continued to look at it, and to sa- 
tisfy themselves as to its position. The brig could not have 
been less than from 90 to 100 miles distant ; and as the station 
on Corryhabbie is not above 2550 feet above the sea, the pheno- 
menon is interesting. The thermometer was at 44°. The night 
and day preceding the sight of the brig had been continually 
rainy and misty, and it was not till seven o’clock of the even- 
ing of the 21st that the clouds cleared off the hill. 
