610-612] Mechanical Action 531 



As Maxwell has pointed out*, these formulae enable us to determine the 

 magnitude of the electric and magnetic forces involved in the propagation of 

 light. According to the determination of Langley, the mean energy of sun- 

 light, after allowing for partial absorption by the earth's atmosphere, is 

 43 x 10~ 5 ergs per unit volume. This gives as the maximum value of the 

 electric intensity, 



Y Q = '33 c.G.s. electrostatic units 



= 9'9 volts per centimetre, 

 and, as the maximum value of the magnetic force, 



<y = -033 C.G.s. electromagnetic units, 



which is about one-sixth of the horizontal component of the earth's field in 

 England. 



The Pressure of Radiation. 



612. In virtue of the existence of the electric intensity Y, there is in free 



KY* 

 ether ( 165) a pressure -^ at right angles to the lines of electric force. 



Thus there is a pressure per unit area over each wave-point. Similarly 



the magnetic field results ( 471) in a pressure of amount ^- per unit area. 

 Thus the total pressure per unit area 



- cos 2 K(X at). 



8?r STT 



This is exactly equal to the energy per unit volume as given by expression 

 (599). Thus we see that over every wave-front there ought, on the electro- 

 magnetic theory, to be a pressure of amount per unit area equal to the energy 

 of the wave per unit volume at that point. The existence of this pressure 

 has been demonstrated experimentally by Lebedewf and by Nichols and HullJ, 

 and their results agree quantitatively with those predicted by Maxwell's 

 Theory. 



* Maxwell, Electricity and Magnetism (Third Edition), 793. 



t Annalen der Physik, 6, pp. 433458. 



J Amer. Phys. Soc. Bull. 2, pp. 2527, and Phys. Rev. 13, pp. 307320. 



REFERENCES. 



On the Electromagnetic Theory of Light, and Physical Optics : 



MAXWELL. Electricity and Magnetism, Vol. n, Part iv, Chap. xx. 



SCHUSTER. Theory of Optics (Arnold, London, 1904). 



DRUDE. Theory of Optics (translation by Mann and Millikan) (Longmans, 



Green and Co., 1902). 

 WOOD. Physical Optics (Macmillan, 1905). 



