there are no brush terminals or aigrettes. The French military authorities
also protect magazines by erecting two or more lightning-rods on poles of
sufficient height placed close to, but not touching, the walls of the
magazine. These conductors are joined below the foundations and earthed as
usual.

In the instructions issued by the Government, it is stated that the
lightning-rods placed upon powder-mills should be of such a height, and so
situated, that no danger is incurred in igniting the powder-dust in the
air by the lightning discharge at the pointed rod. In such a case a fork
or aigrette of five or more points should invariably be used in place of a
single point.

[Illustration: FIG. 4_a_.--GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR
LARGE BUILDINGS.]

[Illustration: FIG. 4_b_.--GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR
SMALL BUILDINGS.]

In Fig. 4 (_a_ and _b_) is shown the Government method for protecting
buildings in which explosives are made or stored. Multiple points or
aigrettes would be better. Lord Kelvin and Professor Melsens favour
points, and it is generally admitted that lightning does not strike
buildings at a single point, but rather in a sheet; hence, in such cases,
or in the event of the globular form being assumed by the lightning, the
aigrette will constitute a much more effective protection than a single
point. As to the spacing of conductors, they may, even on the most
important buildings, be spaced at intervals of 50 feet. There will then be
no point on the building more than 25 feet from the conductor. This
"25-feet rule" can be adhered to with advantage in all overground buildings