An apparatus known as Kundt’s 
tube, inside of which the air is 
set vibrating, and which is 
strewn with lycopodium, or 
other light powder, is one of 
the most familiar for this pur- 
pose. The air set vibrating is 
at one point in rapid motion, 
at another almost at rest, and 
the powder collects at these 
latter, called “‘ nodal”’ points, 
leaving the rest of the tube. 
The number and distance be- 
tween these points will vary 
with the nature of the rod 
which imparts its vibration to 
the air in the tube, and by 
filling the tube with other 
gases instead of air the relative 
speeds of these latter may also 
be ascertained. 
Marvels of the Universe 15! 
‘ Photo bu] aa [S. LZ. Bastin. 
A MAMMILLARIA. 
The tips of the tubercles which bear the spines can be seen covered with 
white hair-like prickles. 
A body of air rotating often forms a vortex ring, and, as such, possesses many of the properties 
of an elastic solid. Vortex rings in air are sometimes accidentally produced from the bowl of a 
pipe, but can be more regularly formed in the following manner. A box is taken, covered at 
one end with a cloth, whilst a round hole is cut at the other end. This box 1s filled with smoke, 
or, better, hydrochloric acid gas and ammonia are introduced through separate apertures. These 
instantly combine and produce dense white fumes of sal ammoniac. A smart tap on the back of 
the box sends a ring through the opening, and this ring persists for some time ; if the experi- 
ment is carefully performed 
it may travel right across a 
room before dissipating. A 
second ring encountering the 
first does not coalesce with it, 
but rebounds, bouncing away 
as though an elastic ball. 
Air-currents and waves have 
been rendered visible by photo- 
graphy, by taking advantage 
of the difference of refraction 
between hot and cold air, a 
phenomenon which is said to 
account for the “‘ fuzziness”’ of 
a heated poker. From every 
aeroplane and other flying 
machine when in motion there 
issues a continual series of 
waves, like those in water from 
the propeller of a ship, but in 
ordinary cases they are not 
Photo by] 2 s [S. Z. Bastin. 
HEDGEHOG CACTUS. 
An example of a numerous family protected by close rosettes of fine spines from 
which longer ones stand out. 
