Thirty-first Annual Meeting 207 
greater the danger. No tree is immune but those trees having 
an extensive root system or a deep tap-root are most apt to be 
struck because they are the best grounded and therefore offer 
the least electrical resistance. Then again if one is caught 
out of doors and is exposed to a violent thunderstorm it is 
best so far as danger from lightning is concerned, to let one’s 
clothes get soaking wet, because wet clothes are much better 
conductors and dry clothes much poorer conductors, than the 
human body. It might even be advisable to lie flat on the 
wet ground, undignified as this may be. For any given local- 
ity, the lower the cloud the greater the danger; hence, when 
the humidity is high it is favorable for a dangerous storm, since 
the cloud will form at a low level and the rain is apt to be very 
abundant. For the same reason a winter storm is likely to be 
more dangerous than a summer storm of equal intensity. 
And now how do we account for the thunder—that par- 
ticular feature that gives name to our storm? It has taken 
quite a while to answer this question satisfactorily. Many 
very silly theories still persist. The electrical discharge, the 
“‘lightning,’’ furnishes the key to the explanation. The sudden 
and intense heating of the air along the path of the discharge 
causes it to expand suddenly and violently, sending out from 
every part of its path a steep compression wave, which, as we 
understand it, is the real cause of the thunder. The ‘“‘rumbling’”’ 
that sometimes follows is due, chiefly perhaps, to the inequality 
in the distances from the observer to the various portions of the 
lightning’s path, to the crookedness of the path, to a succession 
of discharges, and to some extent to reflection under favorable 
conditions. The distance to which thunder may be heard 
varies from 7 to 15 miles. 
9. Forecasting the Thunderstorm. 
The forecasting of conditions favorable for the formation 
of thunderstorms one or two days in advance is comparatively 
easy but to say, even a few hours in advance, that a thunder- 
storm will occur at a given place, at or about a given time, is, 
to say the least, a hazardous venture. It is only after the 
storm has actually begun and its direction and rate of move- 
ment have been determined, can one speak with even a small 
degree of assurance. As every one knows, a storm may occur, 
in fact several of them, in sight of the observer and yet not at 
