thes upon the danger area than usual, as they are apt to become
impregnated with nitro-glycerine, and thus not very desirable or safe to
wear outside the works. It is also necessary that these clothes should not
contain any pockets, as this lessens the chance of matches or steel
implements being taken upon the danger area. Changing houses, one for the
men, and another for the girls, should also be provided. The tools used
upon the danger area should, whenever the building is in use, or contains
explosives, be made of phosphor bronze or brass, and brass nails or wooden
pegs should be used in the construction of all the buildings.

[Illustration: FIG. 2.--MELSENS SYSTEM OF LIGHTNING CONDUCTORS.]

~Lightning Conductors.~--The Explosive Substances Act, 38 Vict. ch. 17,
clause 10, says, "Every factory magazine and expense magazine in a
factory, and every danger building in a magazine, shall have attached
thereto a sufficient lightning conductor, unless by reason of the
construction by excavation or the position of such magazine or building,
or otherwise, the Secretary of State considers a conductor unnecessary,
and every danger building in a factory shall, if so required by the
Secretary of State, have attached thereto a sufficient lightning
conductor."

The exact form of lightning conductor most suitable for explosive works
and buildings has not yet been definitely settled. Lightning-rod engineers
favour what is known as the Melsens system, due to Professor Melsens, of
Brussels, and Professor Zenger, of Prague, but first suggested by the late
Professor Clerk-Maxwell. In a paper read before the British Association,
Clerk-Maxwell proposed to protect powder-magazines from the effects of
lightning by completely surrounding or encasing them with sheet metal, or
a cage of metallic conductors. There were, however, several objections to
his system as he left it.

Professor Melsens[A] has, while using the idea, made several important
alterations. He has multiplied the terminals, the conductors, and the
earth-connections. His terminals are very numerous, and assume the form of
an aigrette or brush with five or seven points, the central point being a
little higher than the rest, which form with it an angle of 45 deg.. He
employs for the most part galvanised-iron wire. He places all metallic
bodies, if they are of any considerable size, in communication with the
conducting system in such a manner as to form closed metallic circuits.