302 ON THE PROBABLE CAUSES 



the positive electricity, and the water the negative ; and this hypo- 

 thesis, with some limitations, has been very generally admitted by 

 physicists. On the other hand, M. de la E-ive 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 evaporation acts merely by 

 transporting one of the separated electricities, and carrying it into 

 the higher regions of the atmosphere. But whatever 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. It follows from this, that the effect pro- 

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

 and will be greatest, cceteris paribus, where they come into juxta- 

 position along 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 position alone. 



Now this seems to be 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 /row 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 Van Diemen's Land, 

 the same influence appears 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 cur- 

 rents 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 maximum of current intensity takes place 

 between 10 A.M. and 11 A.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 



