256 Prof. Thomson on the Mechanical Action 



indicated successively -2°, —18', —28', —15°. At li> 45n» 

 the rain still fell in different directions, but not at Brussels ; the 

 sun shone at iutei-vals, and the electrometer marked — 6°. 



I ought to remark, that during the showers not a single peal 

 of thunder was heard, and not the smallest flash of lightning 

 was visible. 



The example which I have just cited shows how, during the 

 same shower, according to the instant at which an observation is 

 made, we may obtain either positive or negative electricity ; this 

 electricity is very energetic during the showers. If the obser- 

 vation is made at the moment when the sign changes, it may 

 appear to be nearly null ; these inversions, it may be remarked, 

 are always of short duration. 



XL. On the Mechanical Action of Radiant Heat or Light : On the 

 Power of Animated Creatures over Matter : On the Sources 

 available to Man for the production of Mechanical Effect. By 

 Professor William Thomson.* 



On the Mechanical Action of Radiant Heat or Light. 



IT is assumed in this communication that the undulatory 

 theory of radiant heat and light, according to which light 

 is merely radiant heat, of which the vibrations are performed in 

 periods between certain limits of duration, is true. " The che- 

 mical raj's," beyond the violet end of the spectrum, consist of 

 undulations of which the full vibrations are executed in periods 

 shorter than those of the extreme visible violet light, or than 

 about the eight hundred million millionth of a second. The 

 periods of the vibrations of visible light lie between this limit 

 and anothei', about double as great, corresponding to the ex- 

 treme visible red light. The vibrations of the obscure radiant 

 heat beyond the red end are executed in longer periods than 

 this ; the longest which has yet been experimentally tested being 

 about the eighty million millionth of a second. 



The elevation of temperature produced in a body by the in- 

 cidence of radiant heat upon it is a mechanical effect of the dy- 

 namical kind, since the communication of heat to a body is 

 merely the excitation or the augmentation of certain motions 

 among its particles. According to Pouillet's estimate of heat 

 radiated from the sun in any time, and Joule's mechanical equi- 

 valent of a thermal unit, it appears that the mechanical value of 

 the solar heat incident pei-pendicularly on a square foot above 



* From the Proceedings of the Royal Society of Edinhurgli, Fehruary, 

 1852. Commumcated bv the Author. 



