ater
safety of houses protected by the rod, properly mounted, whilst the
British attempt to substitute balls for points has failed. This
question, as to powder magazines, has lately excited much controversy.
Should a rod be attached to the magazine, or should it be placed upon a
post at some distance? This question has been solved by Henry. When an
electrical discharge passes from one body to another, the electricity in
all the bodies in the neighborhood is affected. Henry magnetized a
needle in a long conductor, by the discharge from a cloud, more than a
mile from the conductor. If a discharge passes down a rod, attached to a
powder house, may it not cause a spark to pass from one receptacle for
powder to another, and thus inflame the whole? The electrical plenum,
which Henry supposed, is no doubt disturbed, and to great distances; but
the effect diminishes with the distance. If all the principal conductors
about a building can be connected with a lightning rod, there is no
danger of a discharge; for it is only in leaving or entering a conductor
that electricity produces heating effects; but if not, the rod is safer
at a moderate distance from the building. The rate at which electricity
moved was another of the experiments of Franklin. A wire was led over a
great extent of ground, and a discharge passed through it. No interval
could be perceived between the time of the spark passing to and from the
wire at the two ends. Not long since, Wheatston of England, aided by our
own great mechanic, Saxton, solved the problem. This has induced Arago,
of France, to propose to test the rival theories of light, by similar
means--to measure thus a velocity, to detect which has heretofore
required a motion over the line of the diameter of the earth's orbit.

In galvanism, our countrymen have made many important discoveries. Dr.
Hare invented instruments of such great power as well to deserve the
names of calorimeter and deflagrator. The most refractory substances
yielded to the action of the deflagrator, melting like wax before a
common fire. Even charcoal was supposed to be fused in the experiments
of Hare and Silliman, and the visionary speculated on the possibility of
black as well as white diamonds. Draper, by his most ingenious galvanic
battery, of two metals and two liquids, with one set of elements, in a
glass tube not the size of the little finger, was able to decompose
water. Faraday, of England, discovered the principle, tha