187 



The speculations respecting the origin of atmospheric and terrestrial 

 electricity are various. Thus, De Saussure believed that this electricity 

 was developed by evaporation, the vapour taking the positive electricity, 

 and the vrater the negative ; and this hypothesis, with some limitations, 

 has been very generally admitted by physicists. On the other hand, 

 M. de la Eive is of opinion that the origin of this electricity is to be 

 sought in the chemical actions which he supposes to be going on in the 

 interior of the solidified crust of the earth ; and he thinks that evapo- 

 ration acts merely by transporting one of the separated electri cities, and 

 carrying it into the higher regions of the atmosphere. But what- 

 ever be the correct view as to the force which develops the electricity, 

 it seems to be granted that the separation of the two electricities, 

 in the earth and the atmosphere, is the consequence of evaporation, the 

 vapour carrying with it the positive electricity, and the vaporizing 

 body retaining the negative. isTow, it follows from this, that the effect 

 produced will vary greatly with the distribution of land and water, and 

 will be greatest, cceteris paribus, where they come into juxtaposition at 

 the coasts of the great continents, especially where the coast-lines are in, 

 or near, the meridian. The evaporation from the surface of the sea being 

 much greater than from the land, the electricity will be most deficient 

 at the former. Hence there will be a flow of electricity from land to sea, 

 which will combine with, and often mask, that due to the sun's posi- 

 tion alone. 



l^ow this is precisely what happens. The most marked instance of 

 the phenomenon which we possess is that afforded by the diurnal changes 

 of the currents at St. Helena. There the currents (as I have already 

 shown) flow from the coast of Africa during the hottest portion of the 

 day, and towards it during the night. The influence of the form of the 

 coast seems to be shown in the diurnal curve of the Cape of Good Hope, 

 by the existence of three maxima, of which the principal is directed 

 from the land, and the two subordinate along the lines of coast. At 

 Hobarton, in Yan Diemen's Land, the same influence is shown in the 

 extension of the southern lobe of the curve, which is there nearly equal to 

 the northern. 



I have since calculated the direction and intensity of the currents at 

 the Indian stations, and I find that the curves follow nearly the type of 

 the St. Helena curve. Thus, at Singapore, for which place we possess 

 the results of observation during the three years 1843-1845, the maxi- 

 mum of current intensity takes place between 10 a. it. and 11a. m., and 

 its direction is S. 80° W. At Madras, so far as may be inferred from 

 the observations of a single month, the maximum takes place at noon ; 

 and the direction of the current is then nearly the same as at Singapore, 

 viz. S. 78° W. At Simla, in the Himalaya, the maximum occurs also 

 at noon ; but the direction of the current of greatest intensity is more 

 southerly, its mean yearly direction being S. 47° W. This is pre- 

 cisely what should happen according to the hypothesis, this being 



