packed in metal cases, and hence would present a line
of smaller electrical resistance than the surrounding earth would offer to
the lightning. The conductor should be arranged on the same system as for
overground buildings, but be applied to the surface of the ground over the
magazines.

In all situations where several conductors are joined in one system, the
vertical conductors should be connected both at the top and near the
ground line. The angles and the prominent portions of a building being the
most liable to be struck, the conductors should be carried over and along
these projections, and therefore along the ridges of the roof. The
conductors should be connected to any outside metal on the roofs and
walls, and specially to the foot of rain-water pipes.

All the lightning conductors should be periodically tested, to see that
they are in working condition, at least every three months, according to
Mr Richard Anderson. The object of the test is to determine the resistance
of the earth-connection, and to localise any defective joints or parts in
the conductors. The best system of testing the conductors is to balance
the resistance of each of the earths against the remainder of the system,
from which the state of the earths may be inferred with sufficient
accuracy for all practical purposes.

Captain Bucknill, R.E., has designed an instrument to test resistance
which is based on the Post Office pattern resistance coil, and is capable
of testing to approximate accuracy up to 200 ohms, and to measure roughly
up to 2,000 ohms. Mr R. Anderson's apparatus is also very handy,
consisting of a case containing three Leclanche cells, and a galvanometer
with a "tangent" scale and certain standard resistances. Some useful
articles on the protection of buildings from lightning will be found in
_Arms and Explosives_, July, August, and September 1892, and by Mr
Anderson, Brit. Assoc., 1878-80.

~Nitro-Glycerine.~--One of the most powerful of modern explosive agents is
nitro-glycerine. It is the explosive contained in dynamite, and forms the
greater part of the various forms of blasting gelatines, such as gelatine
dynamite and gelignite, both of which substances consist of a mixture of
gun-cotton dissolved in nitro-glycerine, with the addition of varying
proportions of wood-pulp and saltpetre, the latter substances acting as
absorbing materials for the viscid gelatine. Nitro-glycerine is also
largely used in the manufacture of s