Mrs. Somerville on Thunder storms 201 



tions and repulsions among themselves than to currents of air, though 

 both are no doubt concerned in these hostile movements. The 

 atmosphere becomes intensely electric on the approach of rain, hail, 

 snow, sleet, and wind ; but it varies afterwards, and the transitions 

 are very rapid on the approach of a thunderstorm. 



" Since dry air is a non-conductor, it does not convey the electricity 

 from the clouds to the earth, but it acquires from them an opposite 

 kind, and when the tension is very great the force of the electricity 

 becomes irresistible, and an interchange takes place between the 

 clouds and the earth ; but so rapid is the motion of lightning, that it 

 is difficult to ascertain whether it goes from the clouds to the earth 

 or shoots upwards from the earth to the clouds, though there can be 

 no doubt that it does both. In a storm that occurred at Manchester 

 in June, 1835, the lightning was observed to issue from various points 

 of a road, attended by explosions as if pistols had been fired out of 

 the ground, and a man seems to have been killed by one of these 

 explosions taking place under his feet. M. Gay Lussac ascertained 

 that a flash of lightning sometimes darts more than three miles in a 

 straight line. A person may be killed by lightning, although the 

 explosion takes place at a distance of twenty miles, by what is called 

 the back stroke. Suppose that the two extremities of a highly charged 

 cloud hang down towards the earth, they will repel the electricity 

 from the earth's surface if it be of the same kind with their own, and 

 will attract the other kind ; and if a discharge should suddenly take 

 place at one end of the cloud, the equilibrium will be instantly 

 restored by a flash at that point of the earth which is under the other. 

 Though the back stroke is often sufficiently powerful to destroy life, 

 it is never so terrible in its effects as the direct stroke, which is often 

 of very great intensity. Instances have occurred where large masses 

 of iron and stone, and even many feet of a stone wall, have been 

 carried to a considerable distance by a stroke of lightning. Rocks 

 and the tops of mountains often bear the marks of fusion from its 

 intense heat. An insulated conductor on the approach of a storm 

 gives out such quantities of sparks that it is dangerous to approach 

 it, as was fatally experienced by Professor Richman at Petersburg, 

 who was struck dead by a globe of fire from the extremity of a con- 

 ductor, while making experiments on atmospheric electricity. Copper 

 conductors afford the best protection, especially if they expose a 

 broad surface, since electricity is conveyed along the surface of bodies. 

 There is no instance of an electric cloud of high tension being dis- 

 pelled by a conductor, yet those invented by Sir William Snow Harris, 

 and universally employed in the navy, afford a complete protection 

 in the most imminent danger. The ' Shannon,' a 50-gun frigate, com- 

 manded by the brave and lamented Sir William Peel, was enveloped 

 in a thunderstorm when about 90 miles to the N.W. of Java. It 



