ure. Mr. Aiken has lately broached a novel theory of the formation of fogs and rain-clouds, which also has an important bearing upon the theory of atmospheric electricity. He believes that dust, or impalpable saline particles, are necessary for the formation of clouds. Without dust, or these particles, we should always have cloudless skies. To prove this theory, he caused water to evaporate in two receivers, the air in one of which had been passed through cotton-wool and other media, to intercept the dust; in the other the air contained dust. The usual fog which is perceived when water is suddenly evaporated under a receiver was absent in the receiver whose air had been deprived of its dust, but was present as usual in the other receiver. Various experiments of the same nature were tried. Fog suddenly formed in the dust-deprived air of a receiver if a bit of wood was burned, and thus caused to throw off small particles; burning sulphur was very active in producing these fogs. The fogs of London are thus partially caused by the consumption of coal and the evolution of sulphurous gas, and the fogs along the sea-coast by the saline dust formed from the spray.
If this theory is a correct one, our views of the electrical charges upon different layers of air must be modified to a considerable extent. Some experiments are now being made in the Physical Laboratory of Harvard University to test the inductive capacity of dust-films, and also to trace the fluctuations in the electric spark when it is caused to pass through air deprived of its dust, and air which contains impalpable matter. In order to add to our knowledge of the electrical conditions of the air, simultaneous observations are needed at a large number of stations. The cost of fitting up such stations would not fall short of six hundred dollars for each station, and the services of experienced observers would be necessary. It is not likely that the United States Government will establish such stations unless it can be shown that observations upon the electrical state of the air are important in predicting the path of storms. In the mean time, the various colleges in the different States might, by some system of co-operation with the Government, undertake such observations, since there are in nearly every college rooms and observers at command. As soon as it became apparent that the observations thus taken were of value, the interests of commerce would demand that they should become a part of the regular work of the Signal Service.