ss mirror concave in one direction
only, or longitudinally; it is in fact a concave portion of a
cylinder, the breadth of which may be considered that of the mirror. A
person looking at his face in this mirror, in the direction of its
concavity, will see it curiously distorted in a very lengthened
appearance; and by turning the cylindrical mirror a quarter round, his
visage will appear distorted another way, by an apparent increase in
width only. If in a very near situation before it, you put your finger
on the right hand side of your nose, it will appear the same in the
mirror; but if in a distant situation, somewhat beyond the centre of
concavity, you again look at your face in the mirror, your finger will
appear to be removed to the other side of your nose.

_Water colder than Ice._

Put a lump of ice into an equal quantity of water, heated to 176
degrees, the result will be, that the fluid will be no hotter than
water just beginning to freeze; but if a little sea salt be added to
the water, and it be heated only to 166 or 170, a fluid will be
produced _colder than the ice was at first_.

_Exploding Salt._

If a small quantity of powdered charcoal and hyper-oxymuriate of
potash be rubbed together in a mortar, an explosion will be produced,
and the charcoal inflamed. Three parts of this salt, and one of
sulphur, rubbed together in a mortar, produce a violent detonation. If
struck with a hammer on an anvil, there is an explosion like the
report of a pistol.

When concentrated sulphuric acid is poured upon this salt, there is a
considerable explosion; it is thrown about to a great distance,
sometimes with a red flame; and there is exhaled a brown vapour,
accompanied with a strong odour.

_Dioptrical Paradox._

Construct a machine similar to that in Fig. 25. Its effect will be,
that a print, or an ornamented drawing, with any object, such as an
ace of diamonds, &c. in the centre F, will be seen as an ace of clubs
when placed in the machine, and viewed through a single plane glass
only, contained in the tube E. The glass in the tube F, which produces
this surprising change, is somewhat on the principle of the common
multiplying glass, as represented at G, which, by the number of its
inclined surfaces, and from the refractive power of the rays
proceeding from the objects placed before it shows it in a multiplied
state. The only difference is, that the sides of this glass are flat,
and diverge upwards from t