and 071 the Cause of Terrestrial Magnetism. 55 



2 inches deep, and attached to it a pith-ball electrometer, 

 with balls fths of an inch diameter, and threads 5 inches 

 long, and also attached to the pan a metallic wire, the pointed 

 extremity of which was placed about Tj^th of an inch distant 

 from the point of another wire connected with the ground. 

 The iron pan was then filled with cinders, very hot, from a 

 wind-furnace, and on projecting upon them a few ounces of 

 water, steam was evolved with great rapidity, and at the same 

 moment the pith-balls diverged to the distance of an inch, 

 and sparks passed between the metallic wires. This was 

 several times repeated." — {London and Edinburgh Philosophi- 

 cal Magazine, 1840, p. 460.) And the experiment on evapo- 

 ration from insulated and uninsulated vessels (an account of 

 which I submitted to the Ashmolean Society in 1841)* tends 

 to shew that electricity is a necessary agent in evaporation at 

 moderate or low temperatures. 



It is of little consequence, as regards the phenomenon in 

 question, whether the vapour carries off the electricity, or 

 whether (as I have endeavoured, in former papers, to shew) 

 the electricity carries off the vapour ; it is sufficient to know 

 that, during evaporation, positive electricity is carried off, 

 and the water left in a negative state; consequently, from 

 the enormous evaporation which is continually going on in 

 the equatorial regions, the earth there must always be in a 

 negative state unless the whole of the vapour falls again in 

 the same regions ; but I believe it undoubted, that a great 

 portion of the rains which fall in the frigid regions is from 

 vapour carried by the superior trade-winds towards the poles ; 

 and if any more vapour falls in the frigid regions than rises 

 from those parts of the earth, the earth there must be sur- 

 charged with electricity, which surcharge would naturally 

 rush along the earth's surface towards the equatorial or ne- 

 gative parts of the earth. The analytical researches of Pro- 

 fessor Forchhammer, on the quantity of saline matter in 

 sea-water from tropical and frigid regions, appear fully to 



* See note to a paper on this subject in the Edinburgh New Philosophical 

 Journal, vol. xzxviii., p. 50. 



