Supposing, however, that the cloud had actually descended to D when the
discharge took place. Then the latter would strike to the nearest point;
and any point within the circumference of the portion of the circle, B
C (whose radius is D B), would be at a less distance from D than either
the point B or the point C.

_Hence a lightning-rod protects a conic space whose height is the length
of the rod, whose base is a circle having its radius equal to the height
of the rod, and whose side is the quadrant of a circle whose radius is
equal to the height of the rod._

I have carefully examined every record of accident that was available,
and I have not yet found one case where damage was inflicted inside this
cone when the building was properly protected. There are many cases
where the pinnacles of the same turret of a church have been struck
where one has had a rod attached to it; but it is clear that the other
pinnacles were outside the cone; and therefore, for protection, each
pinnacle should have had its own rod. It is evident also that every
prominent point of a building should have its rod, and that the higher
the rod the greater is the space protected.

* * * * *




PHOTO-ELECTRICITY OF FLUOR-SPAR CRYSTALS.