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 and the cause of the updraught. Rapid evaporation, on the other hand, together with the expansion of air in the updraught, is sufficient to account for the cold air, still further chilled by rain and hail, which finally culminates in the downrush.

Practically, the cumulus is the parent of the thunder-storm, and when it develops into the cumulo-nimbus stage it is essentially a thunder-storm. Even the apparently quiet cloud is always in motion within itself. Rising currents of moist air, chilled by its own expansion, cause condensation of the vapor into cloud matter. The coalescence of cloud matter into mist and droplets results in their fall to a lower level, where they are again vaporized; and the vapor, in turn, rises in the updraught. All this is constantly changing and disturbing the electric potential. When, however, the updraught is strong enough to shatter the drops into mist, the potential becomes so high that the violent discharges constitute the thunder-storm.

In other words, if the updraught is sufficiently strong to hurl the cloud matter to a height where condensation is very rapid, and also to shatter the falling rain-drops, the cumulus develops into a thunder-head at the top and a thunder-storm at the base.

Thunder.—The distance of the discharge may be found approximately by noting the interval between the flash and the thunder, allowing noo feet per second for the velocity of the sound wave. In general, a nearby discharge is followed by an instantaneous report and this in itself indicates that the observer is in the danger zone. It also indicates a probability that the discharge passed between cloud and earth rather than between cloud and cloud. If there is no visible flash, it is likely that the discharge took place between cloud and cloud; and if no thunder follows a discharge, either the discharge occurred at a distance so great that the sound wave became inaudible, or else it was a silent “brush” discharge.

The long-drawn rolling of the thunder may be due to either or both of two causes. If the lightning is a flow or “streak” a mile or more in length, the sound from the farther part requires a proportionately longer time to reach the observer than that for the nearby part. Another factor also must be considered; what appears to be a single discharge may be an