The results of flame tests of a permissible explosive and a test of
black blasting powder, all shot without stemming, are shown on Fig. 2,
Plate IX. In this test, the speed of the drum carrying the black powder
negative was reduced to one sixty-fourth of that for the permissible
explosives, in order that the photograph might come within the limits of
the negative. In other words, the duration of the black powder flame, as
shown, should be multiplied by 64 for comparison with that of the
permissible explosive, which is from 3,500 to 4,000 times quicker.

_Apparatus for Measuring Rate of Detonation._--The rate at which
detonation travels through a given length of an explosive can be
measured by an apparatus installed in and near Building No. 17. Its most
essential feature is a recording device, with an electrical connection,
by which very small time intervals can be measured with great exactness.

The explosive is placed in a sheet-iron tube about 1½ in. in diameter
and 4 ft. long, and suspended by cords in a pit, 11 ft. deep and 16 ft.
in diameter. This pit was once used as the well of a gas tank, Fig. 2,
Plate VIII. In adapting the pit to its new use, the tank was cut in two;
the top half, inverted, was placed in the pit on a bed of saw-dust, and
the space between the tank and the masonry walls of the pit was filled
with saw-dust. The cover of the pit consists of heavy timbers framed
together and overlaid by a 12-in. layer of concrete reinforced by six
I-beams. Four straps extend over the top and down to eight “deadmen”
planted about 8 ft. below the surface of the ground.

The recording device, known as the Mettegang recorder, Fig. 2, Plate
VII, comprises two sparking induction coils and a rapidly revolving
metallic drum driven by a small motor, the periphery of the drum having
a thin coating of lampblack. A vibration tachometer which will indicate