402 



NA TURE 



\Feb. 25, 1886 



still another day, and then we find that the rotation takes 27J 

 days. 



Several distinguished men have endeavoured to formulate a 

 law, a mathematical statement, by which, given the movement 

 in one latitude, we may determine it for all other latitudes, and 

 several of them have very nearly succeeded in doing it ; but they 

 all confess that it does not amount to much at the end of the 

 chapter ; by wliich I mean that the formula after all contains no 

 physical basis. It is what is termed an empirical formula. 



The formulje to which I have referred may be given in this 

 place. In them all x = daily motion in minutes of solar longi- 

 tude, and / = latitude. They are as foUows : — 



Carrington 



Faye 



Spbrer 



Zbllner ... 



.(- = S65' - 165' sin J / 

 X = 862' — 1S6' sin - / 

 X = ion' - 203' sin (41° 13' + /) 

 _ 863' - 619' sin ^ / 

 cos / 



Certainly we have here, as I thinlc I shall be able to show 

 you by and by, one of the points in the mechanism of the sun 

 which it behoves those interested in solar physics to work at 

 with the utmost diligence. 



We have now got the fact that the sun, like the earth, rotates 

 on its axis ; that the inclination of its axis to the plane of the 

 ecliptic is much less than the inclination of our axis ; that its 

 node lies in a different longitude ; and that the photosphere, 

 instead of being a solid thing like the .surface of our earth, is a 

 something which makes its journey round the sun's centre in 2?. 

 days less time in the central portions than it does half-way 

 between the equator and the poles. 



T/w S:in's Density 



Now then let us come to .another point. We are accustomed, 

 in dealing with the earth and comparing it with other planets, 

 to refer to the density of the various bodies. We say, for in- 

 stance, that the density of the earth is 54 times greater than the 

 density of water ; that is to say, that the earth put in one scale 

 would weigh down 5^ earths of the same size, if they were made 

 of water, put in the other. And we say, further, that the density 

 of the earth is about the same as the density of Venus and of 

 Mars ; but the density of the other planets is very much less. 

 We know on the earth that water is less dense, for instance, 

 than mercury. We know that spirit is less dense than water. 

 We can, indeed, put water in a tumbler, and by proper means 

 add the spirit so that it will float on the top of the water. 

 We do not generally do that. Again, we put' lead into water, 

 and it sinks. We put a cork into water, and it floats. All 

 these represent different orders of density. The same thing 

 happens v/ith regard to gases. We know that hydrogen is less 

 dense than oxygen and nitrogen, and so on. 



Now, what is the density of the sun ? Is the sun denser than 

 the earth ? No ; according to the books it is just about a quarter 

 as dense as the earth, so that it is a little denser than water. In 

 fact, if we take water as our unit of density, if water equals i, 

 the density of the sun is I '444. If we take the density of the 

 earth as I, then the value is aljout o'25 — practically, a quarter. 



Now, these are the values given in the books, but I think that 

 possibly we must call them in question. They have been deter- 

 mined by taking the volume of the sun as given by the diameter 

 of the photosphere — S6o,ooo miles. Now, we have had to cm- 

 cede 100,000 miles for the height of one atmosphere above the 

 photosphere, and 1,000,000 miles for another, and it is not fair 

 that those atmospheres should be left out of consideration. If 

 we include these atmospheres, though wedo not alter the mass, we 

 alter the volume. If we put the same mass into a bigger volume, 

 we naturally reduce the density. Now, if we take the atmo- 

 sphere of the sun as extending to 100,000 miles above the photo- 

 sphere, that will give us a radius of 530,0^ miles, instead of 

 430,000 miles, and we shall, as nearly as may be, double the 

 sun's volume. Therefore we shall have halved the density. 

 Instead of being a quarter as dense as the earth, it will 

 only be one-eighth as dense ; and, instead of being just 

 denser than water, it will be a little over half the density of 

 water. For my own part, I think that this 100,000 miles is not 

 suflicient. I think that it is the minimum. I think that most 

 students of solar physics would agree that a height for this pur- 

 pose of 500,000 miles above the photosphere would be probably 

 nearer the mark. That will give us exactly ten times the 

 volume of the sun bounded by the photosphere, so that the 



densities will be reduced to the tenth ; we shall get a density 

 then of about one-eighth of water. This, of course, is the aver- 

 age density ; it is the density of the whole volume in which 

 the mass is supposed to be diffused — the mass which is a fact 

 which we cannot get out of, and which has a definite relation to 

 the mass of our own earth. Now if these arguments are of any 

 value we must concede that the density of the sun is very low 

 indeed, much lower than that of any planet or satellite with 

 which we are acquainted ; so that we are perfectly justified in 

 saying that it is an enormous globe of gas, by which I do 

 not mean that it is absolutely and completely gaseous to the 

 core. The gases of the centre — gases under very great pressure 

 — may put on the appearance, if they do not put on all the 

 physical properties, of liquids ; but be this as it may, in any 

 region that we can get at, unfortunately limited to something 

 like 400,000 miles away from the centre, we are undoubtedly 

 dealing with masses of gas. 



The Sim's Heat 

 Another point in which we find an enormous difference between 

 the sun and any other body that we investigate in the solar system 

 is this — that the sun is an intensely heated globe of gas. It is of 

 no use to use any adverbs to tell you how hot it is, and, unfortu- 

 nately, tl^ere are veiy few available facts ; so that I must ask you 

 to give your imagination play, and to believe that it is very, very 

 hot. The values that have been suggested by various men 

 of science vary between 18,000,000° and 3000°. You m.ay 

 take your choice. The fact is, I think, that we are not yet in a 

 position to find out the very best method of determining the 

 solar temperature and then marking it down in an absolutely 

 perfect manner, for the reason that the more one knows about 

 the problem, the more one sees how terribly complicated it is. 



No doubt we have here a field of work of the very highest 

 interest. Of course, when men of science have stated that the 

 temperature of the sun is 18,000,000° or 3000°, they have re- 

 ferred to the temperature of that part of the sun which is avail- 

 able to our observation, and to the hottest parts of it. Naturally, 

 if the sun be a heated globe of gas, on the outside it must be 

 cool, so that they do not mean that this globe of gas is equally 

 heated throughout, but that the hottest part of it — the part which 

 sends us the effective heat which we try to measure — is at that 

 temperature. 



There is one other very interesting question connected with 

 the remark that the atmosphere must cool to the outside. This 

 time last century the idea was that the sun was a habitable globe 

 just like the earth. An intense heat and light were granted to 

 an exterior envelope, but it was imagined that there wa> a 

 reflecting stratum inside which sent all the heat away earthward, 

 and planet-ward, and star-ward with redoubled energy, while at 

 the same time it shielded the inhabitants who were below 

 this reflector from the direct light and heat of this enve- 

 lope. That was Sir William Herschel's idea. We know 

 now that these things cannot be so. If the walls, and ceiling, 

 and floor of this lecture theatre were incandescent, you may 

 depend upon it that, in spite of any number of reflectors we 

 should soon be incandescent too. According to what is now 

 known as Prevost's theory of exchanges, anything inside a 

 heated chamber must, if you give it time, get to the temperature 

 of the walls of that chamber, for the reason that the walls would 

 be giving heat to the object inside, and the object would be 

 sending the heat back again if it had a surplus of it, and you 

 would get this exchange going on until the temperature of every- 

 thing inside would be the same as the temperature of the 

 envelope ; so that we are now perfectly certain that, if the tem- 

 perature of the photosphere of the sun, let uS say, be 3000', or 

 30,000°, or 3,000,000°, the temperature of the internal part of the 

 sun will not be less. It may be much more. So that we have 

 to give up all that beautiful idea of the habitability of the sun 

 by creatures like ourselves. 



Now, if this mass of gas, a million and a half of miles in 

 diameter, let us say, is coolest outside, and hottest at the centre 

 — which I think you will gr.int — there must be a gradual in- 

 crease both of temperature and of pressure towards the centre. 

 The observations which have been made during eclipses indicate 

 with sufficient definiteness that there is an undoubted increase 

 of temperature towards the centre, and that the various 

 appearances which we get at the photospheric level really mean 

 that at this point, where the pressure is greater than at any 

 superior level — as the pressure in London is greater than the 

 pressure on the top of Mont Blanc — the temperature also is 

 higher, as is indicated by the extreme brightness of the objects 



