We are, however, only dealing at this point with the electro-static energy in the electric field, as we shall deal with the electro-kinetic energy in the following chapter. We have, therefore, to conceive of an electrified body generating electric or electro-magnetic waves, which speed away from the generating source on every side with the velocity of light. Now we have already seen that the aetherial waves which give rise to heat and light possess a repulsive power, that is, they exert a pressure on the body with which they come into contact. If, therefore, in the electric field there is this energy manifested as proved by Maxwell, and that energy takes partly the form of a pressure as stated by Maxwell, then we have in the electro-static energy of the electric field, another indication of that centrifugal force for which we are looking, and whose existence was so satisfactorily demonstrated to Herschel by the phenomena of comets' tails. That there is this pressure in an electric field was conclusively proved by Maxwell, and experimentally demonstrated by Professor Lebedew (Art. 77). Maxwell distinctly states on this point, "that the combined effect of the electro-static and electro-kinetic stresses is a pressure equal to 2 P. in the direction of the propagation of the waves," that is, away from the electrified or charged body. He continues: "Thus, if in strong sunlight the energy of light which falls on one square foot is 83.4 foot-pounds per second, the mean energy in one cubic foot of sunlight is about .0,000,000,882 of a foot-pound, and the mean pressure on a square foot is .0,000,000,882 of a pound weight. A flat body exposed to sunlight would experience this pressure on its illuminated side only, and would therefore be _repelled_ from the side on which the light falls."[31]