EXTENSION OF THE SIGNAL SERVICE. en 



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 impalpa- 

 ble saline particles, are necessary for the formation of clouds. With- 

 out 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 spai-k when it is caused 

 to pass through air deprived of its dust, and air which contains im- 

 palpable 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 im- 

 portant in predicting the path of storms. In the mean time, the vari- 

 ous colleges in the different States might, by some system of co-opera- 

 tion 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 be- 

 come a part of the regular work of the Signal Service. 



