and of studies on coal dust, this bulletin discusses such factors as the
restraining influence of shale dust and dampness on coal-dust
explosions. It then takes up practical considerations as to the danger
of explosions, including the relative inflammability of old and fresh
coal dust. The problems involved are undergoing further investigation
and elaboration, in the light of information already gathered.

_Permissible Explosives._--The most important progress in these tests
and investigations has been made in those relating to the various
explosives used in getting coal from mines. Immediately upon the
enactment of the first legislation, in the spring of 1908, arrangements
were perfected whereby the lower portion of the old Arsenal grounds
belonging to the War Department and adjacent to the Pennsylvania
Railroad, on the Alleghany River, at 40th and Butler Streets, Pittsburg,
Pa., were transferred to the Interior Department for use in these
investigations. Meantime, in anticipation of the appropriation, Mr.
Clarence Hall, an engineer experienced in the manufacture and use of
explosives, was sent to Europe to study the methods of testing
explosives practiced at the Government stations in Great Britain,
Germany, Belgium, and France. Mr. Joseph A. Holmes also visited Europe
for the purpose of studying methods of ameliorating conditions in the
mines. Three foreign mining experts, the chiefs of investigating bureaus
in Belgium, Germany, and England, spent three months studying conditions
in the United States at the invitation of the Secretary of the Interior,
to whom they submitted a valuable report.[5]

Under the supervision of the writer, Chief Engineer of these
investigations, detailed plans and specifications had been prepared in
advance for the necessary apparatus and the transformation of the
buildings at Pittsburg to the purposes of this work. It was possible,