Prof. G. C. Foster on some Lecture-experiments in Electricity. 229 



within somewhat more than 420 miles of the equator, supposing 

 the sun to be vertical over the whole area. 



This enormous difference is quite sufficient to account for the 

 lower mean temperature of the southern hemisphere. 



But it may be noticed that although, the return currents at the 

 equator are colder than the direct currents, yet they are not so 

 in the polar regions. The water which leaves the polar seas is 

 much colder than the water which replaces it from the tropical 

 regions. 



The general tendency of the great system of ocean-currents is 

 to cool the equatorial region of the globe and to warm the tem- 

 perate and polar regions. Also, owing to the present distribu- 

 tion of sea and land, and partly to the effects on the trade-winds 

 resulting from the eccentricity of the earth's orbit*, small as that 

 eccentricity is at present, there is a constant transference of heat 

 by means of currents from the southern hemisphere to the 

 northern. Ocean-currents tend to reduce the enormous differ- 

 ence of temperature which, according to theory, ought other- 

 wise to exist between the equator and the poles f . 



On a former occasion it was shown that aerial currents at the 

 equator only tend to cool the equator ; they do not carry heat 

 to higher latitudes. But aerial currents in temperate and polar 

 regions diffuse over the land the heat carried by ocean-currents. 

 It is the ocean and not the air that conveys the heat from the 

 tropics to the temperate and polar regions J. 



XXVII. Description of some Lecture-experiments in Electricity. 

 By Professor G. C. Poster, F.R.$.§ 



THE object of this communication is simply to point out 

 methods, differing somewhat from those commonly de- 

 scribed in the books, of demonstrating two or three familiar 

 truths of electricity. The experiments I am about to describe 

 may probably be well known under one form or another, espe- 

 cially to practical electricians, who often have opportunities of 

 using apparatus and witnessing phenomena which do not fall to 

 the lot of mere scientific students. I do not claim for them any 

 novelty, unless it be as lecture-room illustrations. 



1. Experiments with the Electrophorus. — So far as I am aware, 

 the experiments by which the accepted explanation of the action 

 of the electrophorus is supported refer exclusively to the statical 

 conditions of the instrument, or, in other words, to the states of 



* Phil. Mag. S. 4. vol. xxviii. p. 135; vol. xxxiii. p. 122. 



t Ibid. vol. xxxiii. p. 435; vol. xxxiv. p. 128. 



% Ibid. vol. xxxiii. pp. 127-130. Geological Magazine for April 1869. 



§ Communicated bv the Author. 



