sun-spots and famines. 



129 



a sea liquid, fiery, and so transparent that round 

 the bases of these solar hills the shallower por- 

 tions of the molten ocean might be detected. 



This announcement gave a tone to subsequent 

 work. To Sir William Herschel, who outstripped 

 even De la Lande-in imaginative power, the spots 

 were parts of a cool, habitable globe. We are 

 told of mountainous countries with peaks six 

 hundred miles high, and the outer shining enve- 

 lope, according to him, was so constituted that, 

 while it gave light and heat to all the members 

 of the solar family, its brilliance was tempered 

 in such a manner to the inhabitants of the cool 

 solid sun beneath as to render life possible. 



The science of the nineteenth century has 

 swept away these beautiful dreams. In such in- 

 quiries the telescope has given place to the spec- 

 troscope, and no fact is now more certain than 

 that the sun is a huge incandescent globe, the 

 very coolest visible portion of which is glowing 

 with a heat which transcends all our earthly fires. 



This is no vague statement put forth without 

 evidence, or in the absence of ascertained facts. 

 The chemical composition of the exterior of this 

 vast furnace is now to a great extent known, and 

 the physical astronomer can easily detect when 

 a fresh supply of the vapor, now of iron, or now 

 of magnesium, is shot up from below to recruit 

 the glow of the exterior. 



We have called the sun a furnace, but this 

 word must be used with a qualification. The 

 heat of the sun is due, not to combustion as in 

 I our ordinary fires, but to the vivid incandescence 

 of each particle brought about by the original 

 contraction of the vaporous globe, or by causes 

 I even more remote and unknown. But this we 

 know, that the energies at work on the sun are 

 not always constant. At times, there are spots 

 on its surface of such enormous magnitude that 

 they are visible to the naked eye ; at others, it is 

 apparently as spotless as the most eager of Gal- 

 ileo's adversaries, who had the dictum of Aris- 

 totle to defend, could have desired. At times, 

 again, glowing vapors rush up from its bowels 

 with such persistence that the careful observer 

 is sure to catch a sight of their eruptions when- 

 ever he looks for them. At other times they 

 are invisible for months together. 



Strange forms are also seen, exquisite in color, 

 fantastic beyond description in outline, and of 

 stupendous magnitude. These are the solar 

 prominences or red flames, the existence of which 

 was formerly revealed to us in eclipses only. 

 Like the spots, and like the eruptions, they wax 

 and wane. At one time a dozen may be visible 

 45 



round the edge of the sun, some of them a hun- 

 dred thousand miles high ; at other times there 

 is scarcely the most feeble indication of this form 

 of solar activity. The sun, then, may not only 

 be likened to a furnace the heat of which is be- 

 yond expression ; but it may be likened to a fur- 

 nace the intensity of which is apparently variable. 



The next point is that the apparent variation 

 in activity is not irregular and therefore unpre- 

 dictable, but that it is regular and predictable, 

 at all events within certain limits. The variation 

 is in fact periodic, and the solar phenomena to 

 which we have referred vary together ; that is, 

 when we have the greatest number of uprushes 

 of heated matter from below, we have the great- 

 est number of spots and the greatest number of 

 prominences. 



All these phenomena ebb and flow once in 

 eleven years. So that every eleven years we have 

 the greatest activity in the production of up- 

 rushes, spots, and prominences ; and between the 

 periods of maximum we have a period of mini- 

 mum, when such manifestations are almost en- 

 tirely wanting. In fact, the spots may be taken 

 as a rough index of solar energy, just as the rain- 

 fall may be taken as a convenient indication of 

 terrestrial climate. They are an index, but not 

 a measure of solar activity; and their absence 

 indicates a reduction, not the cessation, of the 

 sun's energy. Whether this reduction means one 

 in a hundred or one in a thousand, we do not know. 



If we now pass from the sun, the great reser- 

 voir of energy in our planetary system, to our own 

 earth, we find a very different order of things. 

 The incandescence of our planet is a thing of the 

 past ; and the loss by radiation of its internal 

 heat is now so small and varies so slightly in a 

 long period of time that, as compared with a 

 period of eleven years, we may regard this heat 

 as a constant quantity. 



It was, perhaps, scarcely necessary thus to 

 clear the ground for the general statement, now 

 an accepted fact of science, that, with the excep- 

 tion of tide-work, all our terrestrial energies come 

 from the sun. In the great modern principle of 

 the conservation of energy, we have not only 

 proof that the actual energy stored up in our 

 planet is constant, but that the solar energy is 

 the great prime mover of all the changeable phe- 

 nomena with which we are here familiar, espe- 

 cially in the inorganic world. 



That energy gives us our meteorology by fall- 

 ing at different times on different points of the 

 aerial and aqueous envelopes of our planet, there- 

 by producing ocean and air currents, while, by 



