PORTION OF THE COSMOS. VELOCITY OF LIGHT. 77 



Measurements made with conductors 1050 Eng. statute 

 miles, or 968 geographical miles, in length, gave, from 18 

 equations of condition, the rate of "propagation of the hydro- 

 galvanic current at only 18700 statute or 16240 geogra- 

 phical miles in a second ; ( 146 ) i. e. fifteen times slower than 

 the electric current in Wheatstone's rotating disc apparatus ! 

 As in Walker's remarkable experiments two wires were not 

 used, but half the conduction, according to the common ex- 

 pression, took place through the moist body of the earth, it 

 might seem a justifiable supposition that the velocity of the 

 propagation of electricity is dependent on the-nature as well as 

 on the dimensions ( 147 ) of the medium. In the voltaic circuit 

 bad conductors become more heated than good conductors, 

 and electric discharges are very variously complicated phe- 

 nomena, as appears by the latest experiments of Eiess. ( 148 ) 

 The now prevailing views respecting what is commonly called 

 " connection through the Earth" are opposed to the view of 

 linear molecular conduction between the two ends of the wire, 

 and to the conjectures of impediments to conduction, and of 

 accumulation and discharges in a current ; as that which was 

 once regarded as intermediate conduction in the Earth is now 

 supposed to belong only to an equalisation or to a restora- 

 tion of electric tension. 



Although, according to the present limits of exactness in 

 this kind of observation, it is probable that the aberration- 

 constant, and therefore the velocity of light, of all the fixed 

 stars, is the same, yet the possibility has more than once 

 been spoken of, that there may be luminous bodies in space 

 whose light does not reach us because, from their enormous 

 mass, gravitation constrains the luminous particles to return. 

 The emission theory gives to such fancies a scientific 



