Because the electricity, being resisted in its progress by the air, _flies from side to side_, to find the readiest passage. 587. _Why does lightning sometimes appear forked?_ Because, being resisted in its progress by the air, the electricity divides into two or more points, and _seeks a passage in different directions_. 588. _Why is lightning sometimes like a lurid sheet?_ Because the flash is distant, and therefore we see only the _reflection_. [Verse: "He directeth it under the whole heavens, and his lightning unto the ends of the earth."] 589. _When is the flash of lightning straight?_ When the distance between the clouds whose electricities are meeting, is small. 590. _What is the cause of the aurora borealis?_ The _mingling of the electricities_ of the higher regions of the atmosphere. 591. _When does the flash of lightning appear blue?_ When the degree of electrical excitement is intense, and _general throughout the atmosphere_. 592. _Why does lightning sometimes appear red, at others yellow, and at others white?_ Because of the varying humidity, which affects the _refracting power_ of the atmosphere. 593. _Does lightning ever pass upwards from the earth to the clouds?_ Yes; when the earth is charged with a _different electricity_ to that which is in the clouds. 594. _Does lightning ever pass directly from the clouds to the earth?_ Yes; when the electricity of the clouds seeks to combine with the _different electricity_ of the earth. The mingling of the electricities of the earth and the air must be continually going on. But _lightning_ does not attend the phenomena, because all natural bodies, vapours, trees, animals, mountains, houses, rocks, &c., &c., act more or less as conductors between the earth and the air. It is only when there is a great disturbance of the electrical forces, that _terrestrial lightning_ is developed. When lightning strikes the earth with great force, it sometimes produces what are called _fulgurites_ in sandy soils; these are hollow tubes, produced by the melting of the soil. 595. _What is the extent of mechanical force of lightning?_ Lightning has been proved, in one instance, to have struck a church with a force equal to more than 12,000 horse-power. A single horse-power, in mechanical calculations, is equivalent to raising a weight of 32,000 lbs. one foot in a minute. The force of lightning, therefore, has been proved to be equal to the raising of 384,000,000 lbs. one foot in a minute. This is equal to the united power of twelve of our largest steamers, having collectively 24 engines of 500 horse-power each. The velocity of electricity is so great that it would travel round the world _eight times in a minute_. [Verse: "After it a voice roareth: he thundereth with the voice of his excellency; and he will not stay them when his voice is heard."--JOB XXXVII.]