THE SUN. 



temporarily carried downwards, displacing "by its force the strata 

 of luminous matter beneath it (which may be conceived as forming 

 an habitually tranquil limit between the opposite upper and under 

 currents), the upper of course to a greater extent than the lower, 

 and thus wholly or partially denuding the opaque surface of the 

 sun below. Such processes cannot be unaccompanied by vorticose 

 motions, which, left to themselves, die away by degrees, and dis- 

 sipate, with this peculiarity, that their lower portions come to rest 

 more speedily than their upper, by reason of the greater distance 

 below, as well as the remoteness from the point of action, which lies 

 in a higher region, so that their centre (as seen in our water-spouts, 

 which are nothing but small tornadoes) appears to retreat upwards.* 



Sir J. Herschel maintains that all this agrees perfectly with 

 what is observed during the obliteration of the solar spots, which 

 appear as if filled in by the collapse of their sides, the penumbra 

 closing in upon the spot and disappearing afterwards. 



It would have rendered this ingenious hypothesis still more 

 satisfactory, if Sir J. Herschel had assigned a reason why the 

 luminous and subjacent non-luminous atmosphere, both of which 

 are assumed to be gaseous fluids, do not affect, in consequence of 

 the rotation, the same spheroidal form which he ascribes to the 

 superior solar atmosphere. 



26. It has been shown that the intensity of heat on the sun's 

 surface must be seven times as great as that of the vivid ignition 

 of the fuel in the strongest blast furnace. This power of solar light 

 is also proved by the facility with which the calorific rays pass 

 through glass. Herschel found, by experiments made with an 

 actinometer, that 81/6 per cent of the calorific rays of the sun 

 penetrate a sheet of plate-glass 0*12 inch thick, and that 8o - 9 per 

 cent, of the rays which have passed through one such plate will 

 pass through another, f 



27. One of the most difficult questions connected with the phy- 

 sical condition of the sun, is the discovery of the agency to which 

 its heat is due. To the hypothesis of combustion, or any other which 

 involves the supposition of extensive chemical change in the 

 constituents of the surface, there are insuperable difficulties. 

 Conjecture is all that can be offered, in the absence of all data 

 upon which reasoning can be based. Without any chemical 

 change, heat may be indefinitely generated either by friction or 

 by electric currents, and each of these causes have accordingly 

 been suggested as a possible source of solar heat and light. 

 According to the latter hypothesis, the sun would be a great 

 ELECTBIC LIGHT in the centre of the system. 



* Herschel's Cape Observations, p. 43-1. t Ibid., p. 133. 



112 



