Much of the electricity of the air is caused by the rubbing of moist air against dry air. A great deal of moisture is made by the sun or wind turning into vapor or mist the salt water of the ocean. More water is turned into vapor during the heat of summer and autumn than in winter and for this reason there is more lightning in warm weather than in cold. The electricity in the air in clear weather is generally positive, but during fogs, rains, or snows it tends to change to negative. Sometimes it happens that two clouds, one charged with positive electricity and the other with negative electricity, come near each other. The two kinds of electricity then rush together and we see a flash of lightning and hear thunder. Lightning is the same thing as a spark from an electrical machine, the only difference being that a flash of lightning is sometimes several miles long and the spark only a few inches,