Because electricity does not penetrate the _masses_ of _bodies_, but affects generally _their surfaces_. Hence electricity exists in the _surfaces of clouds_, and in its efforts to _find an equilibrium_ it causes the clouds to roll in _heavy masses_, having dark outlines. [Verse: "All ye inhabitants of the world, and dwellers on the earth, see ye, when he lifteth up an ensign on the mountains; and when he bloweth a trumpet, hear ye."--ISAIAH XVIII.] The fact that electricity resides in, and is conducted by, the _surfaces_ of bodies, is well established, and should receive due attention in the protective measures adopted to secure life and property against the effects of lightning. A practical suggestion that arises out of this fact is, that _tubes of copper_ would form far more efficient conductors than _bars_ of the same metal. A _copper tube_, of half an inch diameter, would conduct _nearly double_ the amount of electricity which could be conveyed away by a _bar_ of copper of the same diameter. The upper extremity of the tube should be open obliquely, that the electric current might be induced to pass over _both the inner and outer surfaces_. CHAPTER XXIX. 614. _What is thunder?_ Thunder is the _noise which succeeds the rush_ of the electrical fluid through the air. 615. _Why does noise follow the commotion caused by electricity?_ Because, by the violence of the electric force, vast _fields of air are divided_; great volumes of air are _rarefied_; and _vapours_ are _condensed_, and thrown down as _rain_. Thunder is therefore caused by the _vibrations of the air_, as it collapses, and seeks to restore its own _equilibrium_. 616. _Why is the thunder-peal sometimes loud and continuous?_ Because the electrical discharge takes place near the hearer, and therefore the vibrations of the air are heard in their full power. 617. _Why is the thunder-peal sometimes broken and unequal?_ Because the electrical discharge takes place at a _considerable distance_, and the vibrations are affected in their course by _mountains_ and _valleys_. Because, also, the _forked arms_ of the lightning strike out in different directions, causing the sounds of thunder to reach us from _varying distances_. [Verse: "Lo, these are parts of his ways; but how little a portion is heard of him? but the thunder of his power who can understand?"--JOB XXV.] 618. _Why has the thunder-peal sometimes a low grumbling noise?_ Because the electrical discharges, though violent, take place _far away_, and the vibrations of the air _become subdued_. 619. _Why does the thunder-peal sometimes follow immediately after the flash of lightning?_ Because the discharge of electricity takes place near the hearer. 620. _Why does the thunder-peal sometimes occur several seconds after the flash?_ Because the discharge takes place far away, and _light_ travels with a much greater velocity than _sound_.