MABcn 1, 1891.] 



KNOWLEDGE. 



4.9 



A N ILLUSTRATED 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: MARCH 1, 1894. 



CONTENTS. 



The Tenuity of the Suns Surroundings. By E. Waiter 

 ilArN-I'ER. Sfc Ji.A.s. 



The Mailed Monsters of Argentina. By R. Ltdekker, 

 B.A.Cantali 



Stinging Insects.— II. By E. A. Butier 



The Sacred Water of Mecca. By C. A. Mitohelt., 



B.A.Oxoii '. 



Sewer Gas and Zymotic Disease. By A. C. Rantarb 



The Stocl< Dove. l!y irARRT F. Witheebt 



The Structure of the Mill<y Way. By A. C. Eaxtard 



Notice of BooVc 



Letters :— H. Proctor ; C. Robixsox, B.A. ; W. T. Lynn j 

 S. E. Peat, 



Animal Heat. By Vaughan Cornish, M.Sc, F.C.S. 



The Root-Tubercles of Peas, Beans, and Vetches. — I. 

 By J. Pbntland S.mitii, M.A , B.So 



The Face of the Sky for March. By Herbert Sadler, 



FB.A.S. 



Chess Column. By C. D. LooocK, B.A.Oxon 



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THE TENUITY OF THE SUN'S SURROUNDINGS. 



By E. Walter Maunder, See. R.A.S., Superintendent vf 

 the Plii/siciil Department, Eoyal Obaervatory, Greenwich. 



IN trying to frame a satisfactory theory as to the con- 

 dition of the sun, we meet a two-fold difficulty ; we 

 have so few facts to go upon, and yet, few as they 

 are, it is difficult so to keep them before us as to 

 give them their proper weight. 

 No fact is better known than that of the great distance 

 of the sun from us, but how easy it is to forget the 

 necessary consequence, viz., that the smallest portion of 

 the sun's surface visible by us as a separate entity, even 

 as a mathematical point, is yet really a widely extended 

 area. We habitually use somewhat small apertures to 

 observe the sun, or we cut down our object glasses by 

 diaphragms in order to diminish the heat and glare. We 

 therefore lessen the resolving power of the telescope, and 

 as we are usually forced to content ourselves with low 

 powers on account of the unsteadiness of the air during 

 the daytime, the practical separating power of the 

 telescope when used on the sun is of a lower order than 

 it is under the best conditions upon stars. Bearing in 

 mind, then, that a second of arc on the sun represents four 

 hundred and fifty-five miles, it follows that an object 

 one hundred and fifty miles in diameter is about the 

 minimum visibile even as a mere mathematical point, and 

 that anything that is sufficiently large to give the slightest 

 impression of shape and extension of surface must have 



an area of at least a quarter of a million square miles ; 

 ordinarily speaking, we shall not gather much information 

 about any object that covers less than a million. 



Now this fact has an important bearing on some of our 

 theories. We easily fall into the mistake of supposing 

 that the most dehcate details which we can see really form 

 the ultimate structure of the solar surface ; but it is 

 not possible that they can do so. The finest granule, the 

 smallest pore, as we see it, is only the integration of a vast 

 aggregation of details far too delicate for us to detect ; and 

 the minute speck of brighter or duller material may, and 

 probably does, contain within itself a wide range of bril- 

 liancy, not to speak of varieties of temperature, of pressure, 

 of motion, and of chemical constitution. 



This is the case when we are concerned with areas upon 

 the solar disc ; it is much more serious when we are 

 dealing with sections of the sun's atmosphere — if the term 

 " atmosphere " may be allowed — at and beyond the limb. 

 The chromosphere, some eight seconds in depth, looks a 

 narrow enough tire to the solar wheel, but its vertical depth 

 is three thousand six hundred miles. We think of it as a 

 homogeneous whole ; but if the chromosphere be actually 

 —as it appears to be, and as it has generally been regarded 

 as being — a true solar statical atmosphere, an atmosphere 

 of heated hydrogen, just as our own atmosphere is one of 

 cool nitrogen and oxygen, how vast the range of varying 

 conditions which are summed up to us in the smallest 

 point that we can perceive of it. We know how in the 

 case of our own atmosphere the pressure is reduced to one- 

 half at a height of three and a half miles, to one-fourth 

 at seven miles, to one-eighth at ten and a half miles, and 

 so on. But the force of gravity at the surface of the sun 

 is so much greater than at the surface of the earth, that, 

 other things being equal, the density would double by a 

 descent of a single furlong, and five miles would take us 

 from a level where the density was only one hundred 

 millionth of an atmosphere to where it exceeded that of 

 solid platinum. And from the outside of the chromo- 

 sphere to the level of the photosphere would involve a 

 range, not seven hundred times as great as this, that is to 

 say, in the proportion of the range from unity to seven 

 hundred times a million million, but a range from unity 

 to a million million to the seven hundredth power ! 



Of course it is clear that this is not a conceivable state 

 of things, and the most natural course is to suppose that 

 the heat of the sun is so great as to counteract the efi'ect 

 of the pressure of the upper layers of the atmosphere, and 

 to render the unit of height something very different to 

 that given above. Thus a temperature of 35,000° C. 

 would suffice to bring us from a density of one hundred 

 millionth of an atmosphere at the upper level of the 

 chromosphere to a density no greater than that of mer- 

 cury (at 0° C.) at its base. 



This difl'erence is, however, far too great a one to be 

 accepted ; but adopting it for the moment, what I wish to 

 point out is the cu-cumstance that the thm red circle of 

 the chromosphere, which appears to us to be so truly 

 homogeneous except at its upper surface, which seems 

 to be a mere narrow line, scarcely to be called a band, 

 would embrace atmospheric densities ranging over the 

 whole of these tremendous differences. On the outside a 

 density of only the one hundred millionth of an atmos- 

 phere ; close to the sun a density equal to that of 

 mercury. Yet we look at it and think about it as if it 

 were substantially the same in character throughout. 



Further, close to the apparent limb of the sun we get 

 all these varying densities superimposed. We see the 

 centre of the solar disc through a depth of three thousand 

 six hundred miles of chromosphere ; we see the limb 



