Jan. 27, 1887] 



NA TURE 



>97 



Wolves, Mares, and Foals 



When in The Asturias in 1SS5, I was told of a very curious 

 case of animal instinct, wliich may be worth recording. Wolves 

 are by no means unfrequent in The Asturias, and often attack 

 the young foals which are sent up tu^pasturage with the mares in 

 the mountains. The experienced danger seems to have begotten 

 a precautionary instinct of a very intelligent kind. It is said 

 that, on an alarm of wolves, the mares and foals congregate for 

 mutual protection and common defence. The mares form them- 

 ■.elves into a sort of cordon, heads outwards, surrounding a space 

 inclosing the young foals, and are ready for attacking with their 

 fore-feet the wolves on their approach. 



My informant gave me a graphic account of such an attack, 

 of which he was an eye-witness for nearly an hour, and described 

 to me how the wolves circled round and round the defenders, 

 first at some distance, then gradually approaching nearer and 

 nearer, seeking an opening into the inclosure, till at last they 

 came within striking distance, and he saw one wolf rolled over 

 dead by a blow from the fore-foot of one of the mares. 



The fore-foot is not commonly used for defence by any equine 

 species ; but it is obvious that the more powerful hind-leg blow 

 would be of little service against the spri ig of a wolf from 

 behind, without the directing eye to guide the stroke. What a 

 long experience must this mutual protection have been the result 

 of ! We can scarcely understand it, without councils of war 

 having been held, the dangers discussed, and signals for concerted 

 action arranged ; but now all this instinct may merely be the 

 inheritance of the experience of former generations. 



Benthall, Kenley, Surrey, January 6 George Maw 



THE SUN'S HEAT^ 



FROM human history we know that for several thousand 

 years the Sun has been giving heat and light to the 

 earth as at present ; possibly with some considerable 

 fluctuations, and possibly with some not very small pro- 

 gressive variation. The records of agriculttrre, and the 

 natural history of plants and animals within the time of 

 human history, abound with evidence that there has been 

 no exceedingly great change in the intensity of the Sun's 

 heat and light within the last 3000 years ; but for all that, 

 there may have been variations of quite as much as 5 or 10 

 per cent., as we may judge from considering that the inten- 

 sity of the solar radiation to the earth is 61- percent, greater 

 in January than in July ; and neither at the equator nor 

 in the northern or southern hemispheres has this difference 

 been discovered by experience or general observation of 

 any kind. But as for the mere age of the Sun, irrespective 

 of the question of uniformity, we have proof of something 

 vastly more than 3000 years in geological history, with its 

 irrefragable evidence of continuity of life on the earth in 

 time past for tens of thousands, and probably for millions, 

 of years. 



Here, then, we have a splendid subject for contempla- 

 tion and research in natural philosophy, or physics, the 

 science of dead matter. The sun, a mere piece of matter 

 of the moderate dimensions which we know it to have, 

 bounded all round by cold ether, has been doing 

 work at the rate of four hundred and seventy-six thou- 

 sand milHon million million horse-power for 3000 years, 

 and at possibly more, and certainly not much less, 

 than that for a few million years. How is this to be e.x- 

 plained.'' Natural philosophy cannot evade the question, 

 and no physicist who is not engaged in trying to answer 

 it can have any other justification than that his whole 

 working time is occupied with work on some other subject 

 or subjects of his province by which he has more hope of 

 being able to advance science. 



I suppose I may assume that every person present 

 knows as an established result of scientific inquiry that 

 the sun is not a burning fire, and is merely a fluid mass 

 cooling, with some little accession of fresh energy by 

 meteors occasionally falling in, of very small account 



' Lecture on " The Probable Origin, the Total Amount, and the Possible 

 Duration, of the Sun's Heat," delivered by Sir William Thomson, F.R.S., 

 at the Royal Itistitution, on Friday, the 21st inst. 



in comparison with the whole energy of heat which 

 he gives out from year to year. You are also per- 

 fectly familiar with Helmholtz's form of the meteoric 

 theory, and accept it as having the highest degree 

 of scientific probability that can be assigned to 

 any assumption regarding actions of prehistoric times. 

 You understand, then, that the essential principle of the 

 explanation is this : at some period of time, long past, 

 the sun's initial heat was generated by the collision 

 of pieces of matter gravitationally attracted together from 

 distant space to build up his present mass ; and shrinkage 

 due to cooling gives, through the work done by the mutual 

 gravitation of all parts of the shrinking mass, the vast 

 thermal capacity in virtue of which the cooling has been, 

 and continues to be, so slow. I assume that you have not 

 been misled by any of your teachers who may have told 

 you, or by any of your books in which you may have read, 

 that the sun is becoming hotter because a gaseous mass, 

 shrinking because it is becoming colder, becoines hotter 

 because it shrinks. 



An essential detail of Helmholtz's theory of solar heat 

 is that the sun must be fluid, because even though given 

 at any moment hot enough from the surface to any depth, 

 however great, inwards, to be brilliantly incandescent, the 

 conduction of heat from within through solid matter of 

 even the highest conducting quality known to us would 

 not suffice to maintain the incandescence of the surface 

 for more than a few hours, after which all would be 

 darkness. Observation confirms this conclusion so far as 

 the outward appearance of the sun is concerned, but does 

 not suffice to disprove the idea which prevailed till thirty 

 or forty years ago that the sun is a solid nucleus inclosed 

 in a sheet of violently agitated flame. In reality, the 

 matter of the outer shell of the sun, from which the heat 

 is radiated outwards, must in cooling become denser, and 

 so becoming unstable in its high position, must fall down, 

 and hotter fluid from within must rush up to take its 

 place. The tremendous currents thus continually pro- 

 duced in this great mass of flaming fluid constitute the 

 province of the newly-developed science of solar physics, 

 which, with its marvellous instrument of research — the 

 spectroscope — is yearly and daily giving us more and 

 more knowledge of the actual motions of the difterent 

 ingredients, and of the splendid and all-important result- 

 ing phenomena. 



Now, to form some idea of the amount of the heat 

 which is being continually carried up to the sun's surface 

 and radiated out into space, and of the dynamical rela- 

 tions between it and the solar gravitation, let us first 

 divide that prodigious number (476 X 10'-') of horse-power 

 by the number (61 X 10"') of square metres in the sun's 

 surface, and we find 78,000 horse power as the mechanical 

 value of the radiation per square metre. Imagine, then, 

 the engines of eight ironclads applied to do all their avail- 

 able work of, say, 10,000 horse-power each, in perpetuity , 

 driving one small paddle in a fluid contained in a square 

 metre vat. The same heat will be given out from the 

 square metre surface of the fluid as is given out from 

 every square metre of the sun's surface. 



But now to pass from a practically impossible combina- 

 tion of engines and a physically impossible paddle and 

 fluid and containing vessel, towards a more practical com- 

 bination of matter for producing the same effect : still 

 keep the ideal vat and paddle and fluid, but place the vat 

 on the surface of a cool, solid, homogeneous globe of the 

 same size ('697 X lo'-' metres radius) as the sun, and of 

 density (r4) equal to the sun's density. Instead of using 

 steam-power, let the p.addle be driven by a weight descend- 

 ing in a pit excavated below the vat. As the simplest 

 possible mechanism, take a long vertical shaft, with the 

 paddle mounted on the top of it so as to turn horizontally 

 Let the weight be a nut working on a screw-thread on 

 the vertical shaft, with guides to prevent the nut from 

 turning — the screw and the guides being all absolutely 



