621. _Through what distance will the sound of thunder travel?_ Some _twenty or thirty miles_, according to the _direction of the wind_, and the violence of the peal. 622. _Through what distance will the light of lightning travel?_ The _light_ of lightning, and its reflections, will penetrate through a distance of from _a hundred and fifty to two hundred miles_. 623. _How may we calculate the distance at which the electric discharge takes place?_ Sound travels at the rate of _a quarter of a mile in a second_. If, therefore, the peal of thunder is heard _four seconds_ after the flash of lightning, the discharge took place about a mile off. The pulse of an adult person beats about _once in a second_; therefore, guided by the pulse, any person may calculate the probable distance of the storm:-- 2 beats, 1/2 a mile. 3 beats, 3/4 of a mile. 4 beats, 1 mile. 5 beats, 1-1/4 miles. 6 beats, 1-1/2 miles. 7 beats, 1-3/4 miles. 8 beats, 2 miles, &c. Attention should be paid to the _direction and speed of the wind_, and some modifications of the calculation be made accordingly. Persons between 20 and 40 years of age should count _five beats of the pulse to a mile_; under 20, _six beats_. [Verse: "The clouds poured out water; the skies sent out a sound; thine arrows also went abroad."] 624. _Why are electric storms more frequent in hot than in cold weather?_ Because of the _greater evaporation_, as the effect of heat; and also of the _effect of heat_ upon the particles of all bodies. 625. _Why do electric storms frequently occur after a duration of dry weather?_ Because _dry air_, being a bad conductor, prevents the _opposite electricities_ from finding their _equilibrium_. 626. _Why is a flash of lightning generally succeeded by heavy rain?_ Because the electrical discharge destroys the _vescicles_ of the vapours. If a number of _small soap-bubbles_ floating in the air were _suddenly broken_ by a violent commotion of the atmosphere, the _thin films_ of the bubbles would form _drops of water_, and fall _like rain_. 627. _Why is an electrical discharge usually followed by a gust of wind?_ Because the equilibrium of the atmosphere is disturbed by the _heat and velocity of lightning_, and the _condensation of vapour_. Air, therefore, rushes towards those parts where a degree of _vacuity_ or _rarefaction_ has been produced. 628. _What is a thunderbolt?_ The name _thunderbolt_ is applied to an electrical discharge, when the lightning appears to be developed with the greatest intensity around a nucleus, or centre, as though it contained a burning body. But there is, in reality, _no such thing as a thunderbolt_. [Verse: "The voice of the Lord is upon the waters: the God of glory thundereth; the Lord is upon many waters."--PSALM XXIX.] 629. _Why do electric storms purify the air?_