we deal with the dynamical value of light. That we do not feel the power and energy of the light waves is due to the well-known fact that their power is broken by the activity of the atmospheric particles, each of which, in their myriads, is ever moving with great velocity, and therefore bombard the light waves, as they endeavour to strike the earth. Thus the aetherial light waves are broken up and shattered, and fall to the earth not with their full energy or power, but in a blended form, or with that reflected energy which we call light. If they were to come unbroken and unchecked upon us, and on the earth, in the same way that they apparently do upon our satellite the moon, we doubtless should experience very different effects of their energy and power due to their enormous velocity. ART. 77. _Dynamical Value of Light._--We have already learned (Art. 68) that heat possesses a dynamical value, such value being measured by Joule, and its equivalent in foot-pounds being exactly ascertained. We have further seen (Art. 69, on the identity of light and heat), that the same aetherial waves which produce heat are also concerned in the production of light. If, therefore, the aetherial waves which give rise to heat possess a dynamical action and equivalent, it follows that light must also possess a dynamical action and equivalent, and such action should be capable of being expressed in terms of foot-pounds. Clerk Maxwell has recorded the exact dynamical equivalent of light. On this matter he writes:[15] "If in strong sunlight the energy of light which falls upon a square foot is 83.4 foot-pounds per second, the mean energy of 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." We have here then the exact dynamical equivalent, according to Maxwell, of a cubic foot of sunlight near the earth's surface, and of the pressure exerted by light on a body with which it comes into contact. Again, Lord Kelvin[16] has measured the exact dynamical equivalent of a cubic mile of sunlight, both near the surface of the sun and then near the surface of the earth, and in a note adds that the relation of the two values is as 46,000 to 1. So that if the dynamical value of a cubic mile of sunlight near the earth's surface be represented by unity, then the value of a cubic mile of sunlight near the sun's surface would be 46,000 times greater, while he further adds that it would take 4140 horse-power every minute, as the amount of work required to generate the energy existing in a cubic kilometre of light near the sun, a kilometre being equal to about 1093 yards.