g were absent altogether.

There has been a difference of opinion in regard to the area protected
by lightning-conductors, early notions on this point having been much
exaggerated. Leroy's, in 1788, is the earliest positive statement which
I have met. It is, that a conductor protects a horizontal space around
it equal to somewhat more than three times the height of the metal rod
above the building to which it is attached. The physical section of the
Academy of Sciences of Paris, on being consulted by the Minister of War
in 1823, expressed the opinion that a lightning-conductor protects a
circular space of which the radius is equal to the height of the rod.
Here, apparently, is a wide difference, but possibly the estimates refer
to different elevations. Leroy clearly intended an area at a level with
the top of the building: thus, supposing the rod to be attached to a
chimney six feet in height and to rise a foot and a half above the
chimney, then it would protect a radius of about ten feet on the roof.
The estimate of the Academy of Sciences speaks of the total height of
the rod, and refers to a horizontal area at the base of the measurement,
whether this began at the ground or at the top of the structure to which
the rod is attached. In this view the estimates do not differ so much as
might appear, the latter being about one-third less than that of Leroy.
Other French writers estimate the area protected as having a radius of
double the height of the rod above the highest point of the connected
structure, being twice the radius allowed by the Academy. Later
physicists have been cautious in giving figures, for experience has
shown that estimates of protection are not accurately observed by the
descending bolt. For instance, when Her Majesty's corvette Dido,
furnished with the best system of conductors, was struck by lightning,
the discharge fell in a double or forked current upon the main royal
mast, one of the branches striking the extreme point of the royal yard
arm and passing along to the conductor on the mast, while the other fork
fell on the vane, spindle and truck; which last was split open. As soon
as the discharge reached the conductor all damage ceased.

The practice of the best electricians has now long been to protect all
angles and projections, the latter by a branch of the rod, and the
former by running a line of rod over them, having at every few feet
sharp points of an inch or two in length attached to and