ht the waterways would be trackless and chaos
would reign.

The distress signals of a vessel are rockets, but any burning flame also
serves if rockets are unavailable. Fireworks were known many centuries
ago and doubtless the possibilities of signaling by means of rockets
have long been recognized. An early instance of scientific interest in
rockets and their usefulness is that of Benjamin Robins in 1749. While
he was witnessing a display of fireworks in London it occurred to him
that it would be of interest to measure the height to which the rockets
ascended and to determine the ranges at which they were visible. His
measurements indicated that the rockets ascended usually to a height of
440 yards, but some of them attained altitudes as high as 615 yards. He
then had some special ones made and despatched letters to friends in
three different localities, at distances as great as 50 miles, asking
them to observe at a certain time, when the rockets were to be sent up
in the outskirts of London. Some of these rockets rose to altitudes as
great as 600 yards and were distinctly seen by observers 38 miles away.
Later he made rockets which ascended as high as 1200 yards and concluded
that this was a practical means of signaling. Since that time and
especially during the recent war, rockets have served well in signaling
messages.

The self-propelled rockets have not been altered in essential features
since the remote centuries when the Chinese first used them in
celebrations. A cylindrical shell is mounted on a wooden stick and when
the powder in the shell burns the hot gases are ejected so violently
downward that the reaction drives the shell upward. At a certain point
in the air, various signals burst forth, which vary in character and
color. One of the advantages of the rocket is that it contains within
itself the force of propulsion; that is, no gun is necessary to project
it. The illuminating compounds and various details are similar to those
of the illuminating shells described in another chapter.

At present the rocket is not scientifically designed to obtain the
greatest efficiency of propulsion, but its simplicity in this respect is
one of its chief advantages. If the self-propelled rocket becomes the
projectile of the future, as some have ventured to predict, much
consideration must be given to the design of the orifice through which
the gases violently escape in order that the best efficiency of
propulsion may be