Faraday to Harriet Jane Moore   17 September 1863

The Green, Hampton Court: September 17, 1863.

My dear Friend, - Many thanks for your lightninglike letter; like and unlike - for it was rather slow in its progress, having been mis-sent to Southampton, vide enclosure; and yet, in that, like the lightning, which often falls in very unexpected places.

Lightning is a very curious thing; I have often seen the course of the discharge upon trees, beginning suddenly, and ending as unexpectedly as some of those your brother¹ speaks of. We have to remember that the electricity is not always as a vivid, concentrated, explosive flash, throughout the whole of its course. The cloud, or the air over a tree, being highly charged, may induce torrents upon it, but the first progress of the electricity may be, in fact, invisible streams or brushes, which, as they come together, accumulate and break out into one luminous, concentrated, and powerful spark. We can easily produce an effect of this kind by our ordinary electrical machines, when working upon the conversion of sparks and brushes into each other. I have several times seen trees, which, having been struck by lightning, have exhibited afterwards the beginning and the ending of the visible barked place: the beginning having occurred at the angle where one branch separated from another: and the ending or bottom at a larger part of the trunk, lower down. By examining the branches carefully upwards, I have seen reason to believe - 1st, that when the atmospheric electricity first took its course to and through the tree, it has fallen on the leaves and fine stems, chiefly as brushes, or in the non-luminous and brush state. 2nd, that as these have been conducted downwards, they have run together, and made more concentrated streams; that the concentration has resulted in the production of one powerful, luminous, heating, and explosive discharge. 3rd, that as the quantity of electricity was continually diminished by the conductive force of the tree (as the electricity neared the ground, and the mass of the trunk became a larger, and therefore a more effective conductor at the lower parts), it might well be that a discharge of electricity (appearing as a bright flash in one part of the trunk), would before it reached the bottom be altogether conducted; and then would lose its luminous character.

As to the difference between the edges and the middle of the ruptured place, I have not seen the case, therefore have no right to form an opinion; but what say you to these thoughts? the tear occurs in a solid resisting body; it is perhaps an inch wide, and several feet long; the dispersive force is at right angles to the length of the mark. Can we not imagine that the escape of the disturbed particles is easier on the two sides or edges of the course of the explosion, than at the middle, and can that circumstance conduce to the difference?

Ever, my dear friend, yours truly, | M. Faraday