Mechanical Energies of the Solar System. 419 



strike the sun with the velocity due to half the potential energy 

 of gravitation lost in coming in from an infinite distance to his 

 surface. The other half of this energy goes to generate heat 

 very slowly and diffusely in the resisting medium. Many a 

 meteor, however, we cannot doubt, comes in to the sun at once 

 in the course of a rectilineal or hyperbolic path, without having 

 spent any appreciable energy in the resisting medium; and, 

 consequently, enters the region of ignition at his surface with a 

 velocity due to the descent from its previous state of motion or 

 rest, and there converts both the dynamical effect of the potential 

 energy of gravitation, and the energy of its previous motion, if 

 it had any, into heat, which is instantly radiated off to space. But 

 the reasons stated above make it improbable that more than a 

 very small fraction of the whole solar heat is obtained by meteors 

 coming in thus directly from extra-planetary space. 



In conclusion, then, the source of energy from which solar 

 heat is derived is undoubtedly meteoric. It is not any intrinsic 

 energy in the meteors themselves, either potential, as of mutual 

 gravitation or chemical affinities among their elements ; or actual, 

 as of relative motions among them. It is altogether dependent 

 on mutual relations between those bodies and the sun. A 

 portion of it, although very probably not an appreciable portion, 

 is that of motions relative to the sun, and of independent origin. 

 The principal source, perhaps the sole appreciably efficient source, 

 is in bodies circulating round the sun at present inside the 

 earth^s orbit, and probably seen in the sunlight by us and called 

 ^^ the zodiacal light.''-' The store of energy for future sunlight is 

 at present partly dynamical, that of the motions of these bodies 

 round the sun; and partly potential, that of their gravitation 

 towards the sun. This latter is gradually being spent, half 

 against the resisting medium, and half in causing a continuous 

 increase of the former. Each meteor thus goes on moving faster 

 and faster, and getting nearer and nearer the centre, until some 

 time, very suddenly, it gets so much entangled in the solar 

 atmosphere as to begin to lose velocity. In a few seconds more 

 it is at rest on the sun's surface, and the energy given up is 

 vibrated in a minute or two across the district where it was 

 gathered during so many ages, ultimately to penetrate as light 

 the remotest regions of space. 



Explanation of Tables. 



The following tables exhibit the principal numerical data re- 

 garding the mechanical energies of the solar system. 



In Table I., the mass of the earth is estimated on the assumption 

 that its mean density is five times that of water, and the other 



