Some years ago, when the Postal Telegraph Company first began operations between New York and Chicago, the writer made observations twice a day for some time of the temperature and direction of the earth-current. The first two wires constructed gave only two ohms resistance to the mile, which facilitated the experiments. I found that in almost every instance the current flowed from the point of higher temperature to the lower. If the temperature in New York were higher at the time of observations than in Chicago the current would flow westward, and if the conditions were reversed the current would be reversed also. CHAPTER VIII. ATMOSPHERIC ELECTRICITY. Nature has another mode of generating electricity, called atmospheric. The normal conditions of potential between the earth and the upper atmosphere seem to be that the atmosphere is positively electrified and the earth negatively. These conditions change, apparently from local causes, for short periods during storms. In some way the sun's rays have the power directly or indirectly to give the globules of moisture in the air a potential different from that of the earth. In clear weather we find the air near to the earth in a neutral condition, but gradually assuming the condition of a positive charge as we ascend; so that the upper air and the earth are oppositely charged like the two sides of a Leyden jar or two leaves of a condenser. This condition is intensified and localized when a thunder-cloud passes over the earth. The moisture globules have been charged with potential energy by the power of the sun's rays when evaporation took place; but in this state the energy is neither heat nor electricity, but a state of strain like a bent bow or a wound-up spring. When these moisture globules condense into drops of water the potential energy is set free and becomes active either as heat or electricity. The cloud gathers up the energy into a condensed form, and when the tension gets too great a discharge takes place between the cloud and the earth or from one cloud to another, which to an extent equalizes the energy.