

the At mot [>lte re over the Ocean. 255 



too that the air over the sea is free from dust, and that 

 recombination of the ions may, therefore, take place more 

 slowly than on shore. Thus the diminution in the activity 

 of the emanation may be more than compensated for o wing- 

 to the slower rate of recombination of ions. 



Hence the wind-borne emanation rising from the land may 

 be sufficient to account for the ionization observed over the 

 ocean. 



There are further possible causes of electrification to be 

 considered. Thus Lenard has shown that drops of water 

 shaken against metal acquire a positive charge, and the 

 surrounding air has a negative charge. The electrification 

 of the atmosphere in the neighbourhood of waterfalls is well 

 known. It is probable that the wind breaking the crest of 

 waves at sea produces a similar result. At the end of this 

 paper (Note III.) I have recorded some effects of fine spray 

 in the neighbourhood of an Ebert's instrument causing a 

 rapid discharge of the electroscope, whether charged posi- 

 tively or negatively. This effect was certainly not due to 

 defective insulation. Holmgren has also proved that when 

 the area of water is rapidly changing, owing to the formation 

 of ripples, the air receives a negative charge. In the present 

 state of our knowledge it is not possible to assign values to 

 these causes. But during the voyage of the "Athenia," in 

 calm weather, when spray was absent, the number of ions 

 was approximately 1000 per c.c, and it therefore seems 

 probable that radioactive matter is the main cause of ioniza- 

 tion, and that the ions recombine at a slow rate. Furthermore, 

 it is likely that the greater part of the emanation at sea is 

 carried by wind from land, whilst a very small quantity arises 

 from the sea. Nevertheless, it must be admitted that the 

 ionization observed in mid-ocean is in excess of that which 

 would be predicted from theoretical considerations, and the 

 question should receive further experimental tests whenever 

 opportunities occur. Measurements should be made in mid- 

 ocean of the quantity of active deposit obtained on a negatively 

 charged wire. Elster and Greitel found low values for the 

 active deposit at Juist, an island in the North Sea. Simpson 

 obtained similar results in Norway when the wind blew from 

 the land towards Hammerfest. These results bear out the 

 view that emanation does not arise freely from sea-water. 

 It must also be remembered that the Ebert's apparatus has 

 limitations, for Langevin has proved that a large number of 

 ions in the atmosphere move so slowly in an electric field 

 that they escape detection by that means. 



T 2 



