466 On the Source and Maintenance of the Sun's Heat. 



body of the sun. The phenomena of the spots show that the 

 stratum is of great thickness. The movements it undergoes 

 justify the assumption that it is in composition cloud-like — that is 

 to say, that it consists of a congeries of very minute masses, as 

 terrestrial cloud consists of extremely minute globules of water. 

 Now the flow of undulations from the sun, spoken of in the 

 foregoing explanations, must traverse this stratum of molecules; 

 and though the undulations will pass through the molecules, for 

 the same reason that they pass through the substance of the sun, 

 yet in the case of such insulated bodies the passage cannot take 

 place without some degree of disturbance and the generation of 

 secondary undulations. These views being understood, the 

 theory I now propose of the generation of the sun's sensible 

 light and heat is, that it is owing to the collision of the second- 

 ary undulations originating in the cloud-stratum with the pri- 

 mary large undulations emanating from the sun's body. On a 

 like principle I account for the zodiacal light by the collision 

 of the same large undulations with steady gyrations about the 

 sun. (See the articles on the Zodiacal Light in the Philoso- 

 phical Magazine for February arid March 1863.) What seems 

 to confirm this explanation is, that the solar light comes princi- 

 pally from the superior boundary of the stratum, up to which 

 limit the supposed collision must be accumulative, and at this 

 limit also there is a sudden break of continuity of the disturb- 

 ances. The amount of collision between the primary and the 

 resulting secondary waves will depend on the number and com- 

 bination of the latter, and will clearly be greatest where the 

 boundary of the stratum cuts at right angles the directions of 

 the propagation of the primary waves. According to this view 

 the slanting surfaces of a depression, constituting the penumbra 

 of a spot, are less bright than the general surface, because they 

 are inclined to the directions of the sun's radii, and on this ac- 

 count the resultants of the secondary waves come less directly 

 into collision with the primary waves. For the contrary reason, 

 the faculse, or bright streaks, are due to crests and elevations of 

 the cloud-matter. On the same principle the general mottled 

 appearance of the sun's face is readily explained — the brighter 

 parts corresponding to the rounded tops of cumuli, and the 

 shaded parts to their slanting sides. 



In this manner the theory accounts for the sensible light and 

 heat of the sun ; and on the hypothesis that the same operations 

 are going on in the stars as in the sun, the theory at the same 

 time accounts for the sensible light and heat, and for the in- 

 ternal heat, of every body of the universe. 



It may be remarked that in giving the above explanations I 

 have not deviated in the least particular from the physical prin- 



